Synthesis and Performance of Lightweight Geopolymer ...

Synthesis and Performance of Lightweight

Geopolymer Concrete

Jiting Xie

A thesis submitted in fulfilment of the requirements for the degree of

Doctor of Philosophy

School of Engineering and Information Technology

UNSW Canberra

August 2015

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Thesis/Dissertation Sheet

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award of any other degree or diploma at UNSW or any other educational

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contribution made to the research by others, with whom I have worked at

UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that

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Acknowledgements

The successful completion of this thesis is largely due to the great supports of

many people. I am pleased to express my acknowledgement to them here.

I give my primary appreciation to my main supervisor, A/Prof Obada Kayali, for

his enlightenment, instruction, support and encouragement in the past four

years, which have been critical and beneficial for completing this research

project.

Also, I am very thankful to my co-supervisor, Prof Evgeny Morozov, for his kind

support of my PhD study all the time.

Meanwhile, I would like to give many thanks to the China Scholarship Council

(CSC), South China University of Technology and UNSW Australia. Owing to

CSC’s policy and the cooperation program of above two universities, I have

been able to complete this research project with sufficient funding support.

Also, I am really appreciative for the technical assistance that I received during

the four-year long PhD study, including that of Mr Qingyong Ren, Dr Barry Gray,

Dr Raiden Acosta, Dr Wayne Hutchison and Prof Hans Riesen from the school

of physical, environmental and mathematical sciences in UNSW Canberra and

for their guidance and support on XRD, XRF and FTIR experiments; Dr Ulrike

Troitzsch in Australian National University for XRD experiments; Mr Jim Baxter,

Mr David Sharp, Mr Matthew Barrett and Mr Thomas Thomson for laboratory

technical support; and Dr Juan Pablo Escobedo-Diaz for his instruction and

support on optical microscopy tests.

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Besides, I thank my colleagues, Dr Chang Lin, Dr Yuan Fang, Dr Muhammad

Junaid, Dr Mohammad Shakhaout Hossain Khan and Ms Yifei Cui, for the

knowledge acquired from the discussions with them.

And, I’d like to thank Prof. Xiaofang Hu in South China University of Technology,

who used to be my supervisor in China, and instructed me in relevant chemical

engineering skills that have been utilised in this project.

Moreover, I hope to express my sincere appreciation to Ms Denise Russel, who

volunteered to revise the English language of my thesis in the last two months.

Her job was brilliant, efficient and absolutely admirable.

Finally, I hope to thank my parents, Jing Wang and Haitian Xie, and also the

other relatives of the family, for their love, care and support which helped me go

through this PhD study. Thanks to my girlfriend, Dr Xue Gong, and other friends

in Australia, for their accompanying me in the past four year journey of my life.

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Abstract

A lightweight concrete (LWC) displays physical, mechanical and structural

features preferred in contemporary concrete industry. A major synthetic method

for manufacturing LWCs is to use lightweight aggregates (LWAs). A fly ash-

based LWA (FA-LWA) has been claimed as a promising material because of its

capability to produce a high-performance LWC and because of its relevant

ecological benefits. However, its manufacture requires high-powered

mechanical and thermal treatments to ensure its quality, which incurs high

energy consumption and production costs. Also, this process consumes a large

amount of fossil fuels which is likely to aggravate current resources and

environmental burdens, and does not fit with the desire for a sustainable and

eco-friendly industry. Concrete is a successful and very popular building

material because it can be used to construct strong and durable structures at a

relatively inexpensive cost. Therefore, there is a considerable concern for the

cost and quality of LWC when it is suggested to replace traditional normal

weight concrete. There has been mounting criticism directed to the cement and

concrete industries because of the large amounts of CO2 emitted by the various

activities of these industries. Therefore, it would be difficult to introduce another

high-energy LWC that could aggravate this situation whereas a low-cost, eco-

friendly LWC would likely be more acceptable. Hence, the significance of this

study is that it investigated a LWC synthesis which consumed less energy but

still possessed the desired features.

This research explored the employment of a fly ash-derived geopolymer

technology capable of generating lightweight and high-strength materials

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consisting of covalent-bonded aluminosilicate molecules called ‘geopolymers’

under mild-temperature (40-80oC) conditions. As this technology has been

proven to be capable of producing high-quality construction materials, such as

the well-known ‘geopolymer binder’, to replace Portland cement for

environmental reasons, it is suggested that it may be a workable solution to

reduce LWCs’ high energy consumption.

In this study, in addition to geopolymer binder, this geopolymer technology was

used to create high-quality FA-LWAs, referred to as geopolymer aggregates

(GAs), to replace the traditional high-energy methods. This way, it is expected

to realise a high-performance low-cost, eco-friendly LWC by increasing its

geopolymer material content, such as by 60-70% if only using GAs, or even by

100% using a combination of GAs and geopolymer binder.

Firstly, this study investigated whether synthesising GAs based on geopolymer

technology was practical. It examined the mechanism of geopolymerisation to

confirm that the synthesized geopolymer material was suitable for the

manufacture of GAs. The characteristics of fly ash, such as its density, particle

size and shape, carbon content, and chemical compositions, were identified

using various techniques that included electron microscopy, XRF and XRD. The

effects of the identified characteristics on fresh geopolymers were later

evaluated from a combination of workability tests, based on which a consistent

fly ash source was selected. Then, a mix design approach and procedure which

used the criteria of SiO2/Na2O, H2O/Na2O and W/G have been proposed. This

has been done so as to determine the effect of each chemical component that

was proposed and allow the final mix design to be calculated. Also, an

appropriate curing regime was researched and correlated with the geopolymer’s

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density, strength and microstructural development. Heat curing at 80oC for 3

days was applied to GAs and, moreover, prolonged room-temperature curing

under a low-moisture condition studied and used for a cold-bonded GA (called

RTGA). A feasible manufacturing procedure based on the outcomes of these

investigations was later designed to produce both coarse and fine GA products.

The performance of this GA was evaluated based on its grading, density, water

absorption capacity, crushing value and rebound properties which indicated that

it had adequate qualities for LWC manufacture according to the relevant

engineering standards and comparison with other FA-LWAs. Then, its porous

microstructure, which is composed of geopolymer molecules, as identified by

infrared (IR) spectroscopy, was studied based on stereo and optical microscopy

techniques. The results indicated that geopolymerisation could be a new

method for creating homogeneously distributed pores, with most less than 50

µm in size, which would facilitate the formation of the lightweight as well as

strong structure desired for GAs.

Then, the performance of LWCs after the application of GAs was examined.

The OPC-based LWC made of both coarse and fine GAs demonstrated

advanced performance with a high strength and high strength/mass ratio. Also,

the LWCs made entirely from geopolymer materials (GAs and geopolymer

binder) resulted in a medium-strength performance sufficient for structural LWC

applications. These outcomes confirmed the possibility of producing a low-cost,

eco-friendly structural LWC based on the application of the GAs created in this

research. Moreover, the fine GAs produced as a new type of artificial sand

resulted in high performance lightweight mortar materials in both OPC-based

and geopolymer binder-based systems. Furthermore, the synthesised coarse

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and fine RTGAs were of sufficient quality for structural LWC applications, even

though they were not as good as their heated counterparts.

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List of Publications

Journal Paper

Jiting Xie, Obada Kayali, 2014, Effect of initial water content and curing

moisture conditions on the development of fly ash-based geopolymers in heat

and ambient temperature, Construction and Building Materials, Volume 67, Part

A, Pages 20-28.

Jiting Xie, Obada Kayali, Qingyong Ren, Wayne D. Hutchison, Yifei Cui, 2015,

Effects of variation in fly ash properties on workability and strength of

geopolymer mixes, Cement and Concrete Composites. Review process, paper

No. CCC-D-15-00238.

Conference Paper

Jiting Xie, Obada Kayali, 2013, Effect of water content on the development of fly

ash-based geopolymers in heat and ambient curing conditions, the 3rd

international conference on sustainable construction materials and technologies

(SCMT3), Kyoto, Japan.

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Table of Contents

Thesis/Dissertation Sheet .................................................................................... i

Statements .......................................................................................................... ii

Acknowledgements ............................................................................................ iv

Abstract .............................................................................................................. vi

List of Publications .............................................................................................. x

Table of Contents ............................................................................................... xi

List of Figures ................................................................................................... xx

List of Tables...................................................................................................xxvi

List of Abbreviations ........................................................................................xxxi

Chapter 1 Introduction and Literature Review ..................................................... 1

1.1 Background to Fly Ash-based Lightweight Aggregate (FA-LWA) .......... 2

1.1.1 Role of Aggregates in Concrete ...................................................... 2

1.1.2 Need for Artificial Aggregates .......................................................... 4

1.1.3 Lightweight Aggregates (LWAs) and Lightweight Concrete (LWC) . 6

1.1.4 Coal Fly Ash (CFA) ......................................................................... 9

1.1.5 FA-LWA ......................................................................................... 13

1.1.6 Problems of Current Manufacturing Method .................................. 17

1.2 Geopolymer Technology ...................................................................... 18

1.2.1 Brief Introduction to Geopolymer Technology ............................... 18

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1.2.2 Advantages of Geopolymer Technology for FA-LWA Manufacture

…………………………………………………………………………..24

1.2.3 Previous Trials using Geopolymer Technology for LWA

Manufacture ............................................................................................... 27

1.3 Scope of Thesis ................................................................................... 29

1.4 Techniques used in research ............................................................... 30

1.5 Structure of Thesis ............................................................................... 31

Chapter 2 Fly Ash and its Influence on Fresh Properties of Geopolymer ......... 38

2.1 Introduction .......................................................................................... 38

2.2 Selection and Characterisation of Fly Ash ........................................... 40

2.2.1 Selection of Fly Ash Samples ........................................................ 40

2.2.2 Density, Particle Size and Specific Surface Area .......................... 40

2.2.3 Loss on Ignition (LOI) .................................................................... 43

2.2.4 Chemical Oxide Composition (based on XRF) .............................. 44

2.2.5 Crystalline and Amorphous Contents (based on XRD) ................. 44

2.3 Geopolymer Mixes ............................................................................... 49

2.4 Fly Ash’s Influence on Fresh Properties .............................................. 51

2.4.1 Slump and Mini-flow Method ......................................................... 52

2.4.2 Vicat Plunger Penetration.............................................................. 59

2.4.3 Evaluation on Fly Ash’s Characteristics for Fresh Properties ........ 61

2.5 Fly Ash’s Effect on Hardened Properties ............................................. 65

2.6 Conclusions ......................................................................................... 71

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Chapter 3 Geopolymer Mix Design ................................................................... 73

3.1 Introduction .......................................................................................... 73

3.2 Materials and Mix ................................................................................. 73

3.3 Terminology ......................................................................................... 75

3.3.1 Fly Ash/Activator Mass Ratio (F/A) ................................................ 75

3.3.2 Activator Composition Na2O·xSiO2·yH2O ...................................... 76

3.3.3 Water-to-geopolymer Solids (W/G) Ratio ...................................... 76

3.4 Mix Design Procedure.......................................................................... 77

3.5 Molarity of NaOH and Mass Ratio Na2SiO3/NaOH .............................. 89

3.6 An Alternative Mix Design Method – using NaOH Solution instead of

NaOH Flakes ................................................................................................. 94

3.7 Mix Designs used in this Thesis ......................................................... 103

3.8 Conclusions ....................................................................................... 104

Chapter 4 Curing Regime for Geopolymer Synthesis ..................................... 106

4.1 Introduction ........................................................................................ 106

4.2 Previous Research on Curing of Geopolymer Synthesis ................... 106

4.3 Curing Temperature ........................................................................... 108

4.3.1 Geopolymer Mix .......................................................................... 109

4.3.2 Curing Regime ............................................................................ 110

4.3.3 Strength ....................................................................................... 111

4.4 Curing Moisture Condition ................................................................. 112

4.4.1 Geopolymer Mix .......................................................................... 113

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4.4.2 Curing Regime ............................................................................ 114

4.4.3 Strength....................................................................................... 115

4.5 Curing Period (Short-term) ................................................................ 117

4.5.1 Geopolymer Mix .......................................................................... 118

4.5.2 Curing Regime ............................................................................ 118

4.5.3 Density and Strength ................................................................... 119

4.5.4 Scanning Electron Microscopy .................................................... 123

4.6 Curing Period (Long-term) ................................................................. 126

4.6.1 Geopolymer Mix .......................................................................... 127

4.6.2 Curing Regime ............................................................................ 127


4.7 Conclusions ....................................................................................... 131

Chapter 5 Geopolymer Aggregate (GA).......................................................... 132

5.1 Introduction ........................................................................................ 132

5.2 Mix Design used for Geopolymer Aggregate ..................................... 133

5.3 Manufacturing Procedure .................................................................. 136

5.3.1 Mixing and Casting ...................................................................... 137

5.3.2 Curing ......................................................................................... 139

5.3.3 Washing and Air Drying .............................................................. 140

5.3.4 Mechanical Crushing, Sieving and Storage................................. 142

5.4 Appearance ....................................................................................... 145

5.5 Grading .............................................................................................. 147

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5.5.1 Original Geopolymer Aggregate .................................................. 147

5.5.2 Refined Geopolymer Aggregate .................................................. 150

5.6 Density and Water Absorption ........................................................... 152

5.7 Internal Pores .................................................................................... 157

5.8 Crushing Value .................................................................................. 163

5.9 Rebound Test by Schmidt Hammer ................................................... 165

5.10 Infrared (IR) Spectroscopy.............................................................. 167

5.11 Evaluation of Quality of Geopolymer Aggregate ............................. 169

5.11.1 Compatibility with Standards .................................................... 170

5.11.2 Comparison with Other FA-LWAs ............................................ 172

5.12 Conclusions .................................................................................... 174

Chapter 6 OPC-based Concrete and Mortar using Geopolymer Aggregate ... 175

6.1 Introduction ........................................................................................ 175

6.2 NWC .................................................................................................. 176

6.2.1 Materials ...................................................................................... 176

6.2.2 Mix Design .................................................................................. 179

6.2.3 Concrete Preparation .................................................................. 180

6.2.4 Workability ................................................................................... 182


6.3 OPC-GA Concrete ............................................................................. 184

6.3.1 Materials ...................................................................................... 184

6.3.2 Mix Design .................................................................................. 184

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6.3.4 Workability ................................................................................... 189


6.3.6 Fracture Surface ......................................................................... 192

6.4 Comparison of OPC-GA Concrete and Other LWCs ......................... 194

6.4.1 LWC using FA-LWA .................................................................... 194

6.4.2 LWC using Geopolymer-assisted LWA ....................................... 198

6.5 OPC-GA Mortar ................................................................................. 202

6.5.1 Materials...................................................................................... 202

6.5.2 Mix Design .................................................................................. 202

6.5.3 Mortar Preparation ...................................................................... 203

6.5.4 Workability ................................................................................... 205


6.6 Conclusions ....................................................................................... 209

Chapter 7 Geopolymer Binder-based Concrete and Mortar using Geopolymer

Aggregate ....................................................................................................... 211

7.1 Introduction ........................................................................................ 211

7.2 Mix Design Procedure for Geopolymer Binder-based Concretes ...... 212

7.2.1 Materials...................................................................................... 212

7.2.2 Geo-GA Concrete ....................................................................... 213

7.2.3 Geopolymer Concrete (GC) ........................................................ 216

7.3 Geopolymer Concrete (GC) ............................................................... 217

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7.3.2 Workability ................................................................................... 220


7.4 Geo-GA Concrete .............................................................................. 223



7.4.3 Fracture Surface.......................................................................... 227

7.5 Comparison of Geo-GA Concrete and Other LWCs .......................... 229

7.5.1 Geo-GA versus OPC-GA ............................................................ 229

7.5.2 Geo-GA versus FA-LWA Geopolymer Concrete ......................... 231

7.6 Geopolymer Binder-based Mortar ...................................................... 233

7.6.1 Materials ...................................................................................... 233

7.6.2 Mix Design .................................................................................. 234

7.6.3 Mortar Preparation ...................................................................... 235

7.6.4 Workability ................................................................................... 236


7.7 Conclusions ....................................................................................... 239

Chapter 8 Room Temperature-cured Geopolymer Aggregate (RTGA) and its

Application in Concrete ................................................................................... 241

8.1 Introduction ........................................................................................ 241

8.2 Manufacturing Procedure of RTGA .................................................... 242

8.3 Characteristics of RTGA .................................................................... 246

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8.3.1 Appearance ................................................................................. 246

8.3.2 Grading ....................................................................................... 246

8.3.3 Density and Water Absorption ..................................................... 248

8.3.4 Infrared Spectroscopy ................................................................. 251

8.4 RTGA in Concrete Manufacture......................................................... 253

8.4.1 OPC-based Concrete .................................................................. 254

8.4.2 Geopolymer Binder-based Concrete ........................................... 258

8.5 Conclusion ......................................................................................... 261

Chapter 9 Conclusions and Recommendations for Future Research ............. 263

9.1 Research on Geopolymer for Manufacture of Geopolymer Aggregate

………………………………………………………………………………265

9.2 Geopolymer Aggregate ...................................................................... 268

9.3 OPC-based Concrete and Mortar using Geopolymer Aggregate ....... 270

9.4 Geopolymer Binder-based Concrete and Mortar using Geopolymer

Aggregate .................................................................................................... 272

9.5 Room Temperature-cured Geopolymer Aggregate (RTGA) .............. 274

9.6 Recommendations for Future Research ............................................ 275

Appendix A Calculation on Crystalline and Amorphous Contents in Fly Ash

based on Rietveld Quantification Data ............................................................ 279

Appendix B Design of Geopolymer Binder for Geopolymer Binder-based

Concretes and Mortars ................................................................................... 283

Bibliography .................................................................................................... 298

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List of Figures

Figure 1.1: Image of aggregate particles in concrete matrix ............................... 2

Figure 1.2: Schematic of relationship between aggregates and cement matrix .. 3

Figure 1.3: SEM image of fly ash (adapted from Fang, 2013) ............................ 9

Figure 1.4: Main flowchart of processes in fly ash aggregate manufacture

(based on Chandra and Berntsson, 2002 and Bijen, 1986) .............................. 14

Figure 1.5: Images of Lytag aggregate (left) and Flashag aggregate (right, from

Kayali, 2008) ..................................................................................................... 16

Figure 1.6: Schematic of principle of geopolymer binder in concrete mix (drew

by the author based on the literature Kong, 2007, Rangan, 2007 and Bakharev,

2005a) ............................................................................................................... 20

Figure 1.7: 4-coordinated configuration of Si and Al tetrahedra in geopolymer

(Davidovits, 1976) ............................................................................................. 21

Figure 1.8: Three-dimensional network of geopolymer structure (adapted from

models proposed by Rowles et al., 2007 and Barbosa et al., 2000) ................. 22

Figure 1.9: Scheme for synthesis of fly ash-based geopolymer ........................ 22

Figure 1.10: Conceptual models for geopolymerisation proposed by Provis

(2005) (left) and Duxson (2007) (right).............................................................. 24

Figure 1.11: Schematic of relationships among thesis chapters ....................... 33

Figure 2.1: Malvern Mastersizer 2000 laser diffraction particle size analyser ... 41

Figure 2.2: Particle size distribution curves of fly ash samples ......................... 43

Figure 2.3: PANalytical Empyrean X-ray diffractometer .................................... 46

Figure 2.4: X-ray diffraction patterns and corresponding Rietveld refinement

results for fly ash samples FA1, FA2 and FA3 .................................................. 47

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Figure 2.5: Preparation steps (1) Hobart mixer, (2) well-mixed fresh geopolymer,

(3) steel moulds with polyethylene cover, (4) well-compact geopolymer, (5)

geopolymer after demoulding and (6) geopolymer after heating ....................... 52

Figure 2.6: Schematic of mini-sized slump cone for slump and mini-flow testings

.......................................................................................................................... 54

Figure 2.7: Scheme of spread measurement using the mini-flow method ........ 54

Figure 2.8: Images of geopolymer flows before (left column) and once after

(right column) vibration in mini-flow test ............................................................ 56

Figure 2.9: Spread area increase by mechanical vibration of geopolymer paste

.......................................................................................................................... 58

Figure 2.10: Standard Vicat plunger for consistency testing ............................. 60

Figure 2.11: Compressive strength testing on a 3000 KN TECNOTEST machine

.......................................................................................................................... 67

Figure 3.1: Mixing procedure for fly ash-based geopolymer ............................. 75

Figure 3.2: Flowchart of procedure for geopolymer mix designs....................... 79

Figure 3.3: A modified mixing procedure for fly ash-based geopolymer ........... 95

Figure 3.4: Flowchart of procedure for an alternative geopolymer mix design

method .............................................................................................................. 97

Figure 4.1: 7-day compressive strengths of geopolymers under different curing

regimes (corresponding to the mix in Table 4.1) ............................................. 112

Figure 4.2: Compressive strengths of 14-day ambient-cured geopolymers coded

as A-14d, AS-14d and AD-14d with different W/G values ............................... 116

Figure 4.3: Density and compressive strength results for heated geopolymer

samples .......................................................................................................... 120

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Figure 4.4: Density and compressive strength results of ambient-cured

geopolymer samples ....................................................................................... 121

Figure 4.5: SEM images of geopolymers with W/G=0.22: (a) A-7d, (b) A-14d

and (e) H-4h; and with W/G=0.26: (c) A-7d and (d) A-14d .............................. 124

Figure 4.6: Variations in specific density versus long-term curing period at room

temperature of fly ash-based geopolymers produced ..................................... 128

Figure 4.7: Compressive strength versus long-term curing period at room

temperature of fly ash-based geopolymer whose mix design is in Table 4.1 .. 129

Figure 4.8: Comparison of compressive strengths of geopolymers under

different curing regimes (where all samples are made from the mix design

shown in Table 4.1) ........................................................................................ 130

Figure 5.1: 7-day compressive strengths of geopolymers with different W/G

values ............................................................................................................. 135

Figure 5.2: Flowchart of manufacturing procedure for geopolymer aggregate 137

Figure 5.3: Fresh geopolymer chunks in plastic container .............................. 139

Figure 5.4: Soaking of hardened geopolymer chunks in water tank ............... 140

Figure 5.5: pH value after every day’s washing .............................................. 141

Figure 5.6: Jaw crusher used for manufacture of aggregate particles ............ 143

Figure 5.7: Grading curves of geopolymer aggregate samples after each jaw-

crushing cycle ................................................................................................. 144

Figure 5.8: Coarse (>4.75mm) (left) and fine (<4.75mm) (right) geopolymer

aggregates in air-dried condition ..................................................................... 145

Figure 5.9: Lynx stereo inspection microscope for observation of surface of

geopolymer aggregate .................................................................................... 146

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Figure 5.10: Enlarged image (40 times) of particle surface of geopolymer

aggregate (length approximately 3mm) .......................................................... 146

Figure 5.11: Grading curves of geopolymer aggregates and regularly used

crushed stone and river sand .......................................................................... 149

Figure 5.12: Grading curves of river sand, and original and refined fine

geopolymer aggregates .................................................................................. 152

Figure 5.13: Polishing procedures (1) mixing resin and hardener, (2) and (3)

soaking sample in resin, (4) covering sample in hardened resin, (5) grinder and

(6) polisher ...................................................................................................... 159

Figure 5.14: Bright field image of geopolymer aggregate sample ................... 160

Figure 5.15: Original captured image (left) and converted black-and-white image

(right) of geopolymer aggregate sample ......................................................... 161

Figure 5.16: Size distribution of detected pores in geopolymer aggregates as

obtained using an optical microscopy method ................................................ 161

Figure 5.17: Distribution of areas occupied by detected pores in geopolymer

aggregates ...................................................................................................... 162

Figure 5.18: Crushing value test of geopolymer aggregate sample ................ 164

Figure 5.19: Appearance of geopolymer aggregate sample after crushing value

test .................................................................................................................. 165

Figure 5.20: IR spectra of fly ash and geopolymer aggregate ........................ 169

Figure 6.1: Grading curves of crushed stone and river sand .......................... 177

Figure 6.2: Flowchart of preparation of OPC-based concrete ......................... 180

Figure 6.3: Steps in preparation of Con-N-OPC: (1) mixing, (2) casting, (3)

immediately after demoulding and (4) after 28-day curing .............................. 181

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Figure 6.4: Steps in preparation of concrete Con-GA-OPC (1) mixing, (2)

casting, (3) immediately after demoulding and (4) after 28-day curing ........... 188

Figure 6.5: Enlarged images (40x) of fracture surface of Con-N-OPC (left) and

Con-GA-OPC (right) (the image represents a length of approximately 3 mm) 193

Figure 6.6: Images of fresh mortar mixtures: (1) Mor-S-OPC and (2) Mor-GA-

OPC ................................................................................................................ 204

Figure 6.7: Images of 28-day cured Mor-S-OPC and Mor-GA-OPC samples . 204

Figure 6.8: Increases in spread areas of fresh Mor-S-OPC and Mor-GA-OPC

caused by mechanical vibration ...................................................................... 206

Figure 7.1: Flowchart of preparation of fly ash-based geopolymer concrete... 218

Figure 7.2: Steps in preparation of concrete Con-N-Geo (1) mixing, (2) casting,

(3) after heating and demoulding and (4) after 28-day curing ......................... 219

Figure 7.3: Steps in preparation of concrete Con-GA-Geo (1) mixing, (2) casting,

(3) after heating and demoulding and (4) after 28-day curing ......................... 224

Figure 7.4: Enlarged image (40x) of fracture surface of Con-GA-Geo (the image

represents a length of approximately 3 mm) ................................................... 228

Figure 7.5: Images on fresh mortar mixtures: (1) Mor-S-Geo and (2) Mor-GA-

Geo ................................................................................................................. 235

Figure 7.6: Images on 28-day cured Mor-S-Geo and Mor-GA-Geo samples .. 236

Figure 7.7: Increases in spread areas of fresh Mor-S-Geo and Mor-GA-Geo

caused by mechanical vibration ...................................................................... 237

Figure 8.1: Flowchart of manufacturing procedure for RTGA ......................... 243

Figure 8.2: pH value after every day’s washing .............................................. 245

Figure 8.3: Coarse (>4.75mm) (left) and fine (<4.75mm) (right) RTGAs in air-

dried condition ................................................................................................ 246

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Figure 8.4: Grading curves of RTGAs and heated GAs .................................. 248

Figure 8.5: IR spectra of fly ash, heated GA and RTGA ................................. 252

Figure 8.6: Steps in preparation of Con-RTGA-OPC (1) mixing, (2) casting, (3)

immediately after demoulding and (4) after 28-day curing .............................. 256

Figure 8.7: Steps in preparation of Con-RTGA-Geo (1) mixing, (2) casting, (3)

after heating and demoulding and (4) after 28-day curing .............................. 260

xxvi

List of Tables

Table 1.1: Bulk chemical compositions of fly ashes from different coal types

(adapted from Ahmaruzzaman, 2010) .............................................................. 11

Table 1.2: Characteristics of Lytag and Flashag aggregates ............................ 16

Table 1.3: Comparison of traditional method and geopolymer technology for FA-

LWA manufacture ............................................................................................. 25

Table 2.1: Relative densities of fly ash.............................................................. 41

Table 2.2: Results of particle size distribution of the chosen fly ash batches ... 42

Table 2.3: LOI values of different fly ash samples ............................................ 43

Table 2.4: Chemical composition of each fly ash sample tested by XRF .......... 45

Table 2.5: Quantification on crystals and amorphous content in fly ash ........... 48

Table 2.6: Mixes of fly ash-based geopolymer with different fly ashes ............. 50

Table 2.7: Workability of produced fresh geopolymer mixes ............................. 55

Table 2.8: Penetration distance of Vicat plunger of fresh geopolymer mixes.... 60

Table 2.9: Air dry density and compressive strength of produced geopolymers

.......................................................................................................................... 67

Table 2.10: Quantification results on crystals and amorphous content before

and after geopolymerisation .............................................................................. 70

Table 3.1: Final mix design of the geopolymer mix example (kg/m3) ................ 89

Table 3.2: Molarity of NaOH solution and mass ratio of Na2SiO3/NaOH for

different geopolymer mix designs ..................................................................... 93

Table 3.3: Final mix design using a 16M NaOH ingredient of the geopolymer

mix example (kg/m3) ....................................................................................... 101

Table 3.4: Comparison of the molarity of NaOH solution between different mix

designs ........................................................................................................... 102

xxvii

Table 3.5: Mix designs for geopolymers used in this thesis ............................ 104

Table 4.1: Mix design of geopolymers for curing regime research .................. 109

Table 4.2: Curing regimes investigated for optimum geopolymer growth ....... 110

Table 4.3: Mix designs for geopolymer mixes with different initial water contents

........................................................................................................................ 114

Table 4.4: Specified curing moisture conditions for produced geopolymers ... 115

Table 4.5: Specified curing procedures for heat-cured and ambient-cured

geopolymers ................................................................................................... 119

Table 5.1: Geopolymer mix designs with different W/G values ....................... 134

Table 5.2: Geopolymer mix design for geopolymer aggregate manufacture ... 138

Table 5.3: Typical sieving analysis of coarse and fine geopolymer aggregates

........................................................................................................................ 148

Table 5.4: Grading requirements regulated in ASTM C330 for lightweight

concretes (printed in ASTM, 2009) ................................................................. 150

Table 5.5: Refined values of fine geopolymer aggregates based on ASTM C330

........................................................................................................................ 151

Table 5.6: Density and water absorption capacity of geopolymer aggregate .. 153

Table 5.7: Various mix designs and curing regimes for geopolymer aggregate

manufacture .................................................................................................... 155

Table 5.8: Density and water absorption values of geopolymer aggregate

samples made from various mix designs and curing regimes ......................... 156

Table 5.9: Crushing value of geopolymer aggregate sample tested ............... 165

Table 5.10: Results of Schmidt hammer rebound test .................................... 166

Table 5.11: Density values of geopolymer aggregates in relation to relevant

LWA standards ............................................................................................... 170

xxviii

Table 5.12: Grading of geopolymer aggregates in relation to relevant LWA

standards ........................................................................................................ 171

Table 5.13: Properties of geopolymer aggregates (GA), Lytag, Flashag and

NWAs (crushed stone (coarse) and river sand (fine)) ..................................... 173

Table 6.1: Typical sieving analysis of crushed stone and river sand .............. 176

Table 6.2: Characteristics of crushed stone and river sand ............................ 178

Table 6.3: Mix design of Con-N-OPC sample (SSD aggregates condition) .... 179

Table 6.4: Slump result for Con-N-OPC measured on a standard slump cone

........................................................................................................................ 182

Table 6.5: Density and strength properties of Con-N-OPC ............................. 183

Table 6.6: Masses and volumes of ingredients in mix designs of Con-N-OPC

and Con-GA-OPC (SSD aggregates condition) .............................................. 185

Table 6.7: Final mix design of Con-GA-OPC (SSD aggregates condition) ..... 187

Table 6.8: Density and strength properties of Con-GA-OPC .......................... 189

Table 6.9: Mix designs of Con-N-OPC, Con-GA-OPC, and Con-LY-OPC ...... 195

Table 6.10: Properties of fresh and hardened Con-N-OPC, Con-GA-OPC, and

Con-LY-OPC samples .................................................................................... 196

Table 6.11: Properties of Con-GA-OPC and Con-FS-OPC (Kayali, 2008)

samples .......................................................................................................... 198

Table 6.12: Mix designs of Con-GA-OPC and LWCs using geopolymer-assisted

LWAs reported in literature (adapted from Jo, 2007 and Bui, 2012) ............... 199

Table 6.13: Properties of concretes using different LWA products (some data

from Jo, 2007 and Bui, 2012) .......................................................................... 201

Table 6.14: Masses and volumes of ingredients in mix designs of Mor-S-OPC

and Mor-GA-OPC (SSD aggregates condition) .............................................. 203

xxix

Table 6.15: Slump and spread values of fresh Mor-S-OPC and Mor-GA-OPC

........................................................................................................................ 205

Table 6.16: Density and strength properties of Mor-S-OPC and Mor-GA-OPC

........................................................................................................................ 207

Table 7.1: Unit weight and ingredient proportions of mix designs of Con-GA-

OPC and Con-GA-Geo (SSD aggregates condition) ...................................... 214

Table 7.2: Mix designs of Con-GA-Geo samples (SSD aggregates condition)

........................................................................................................................ 215

Table 7.3: Mix designs of Con-N-Geo samples (SSD aggregates condition) .. 216

Table 7.4: Density and strength properties of Con-N-Geo .............................. 222

Table 7.5: Density and strength properties of Con-GA-Geo ........................... 225

Table 7.6: Density and strength properties of Con-GA-OPC and Con-GA-Geo

samples .......................................................................................................... 230

Table 7.7: Mix designs of Con-GA-Geo and Con-LY-Geo (SSD aggregates

condition) ........................................................................................................ 231

Table 7.8: Properties of Con-GA-Geo and Con-LY-Geo ................................. 232

Table 7.9: Mix designs of Mor-S-Geo and Mor-GA-Geo (SSD aggregates

condition) ........................................................................................................ 234

Table 7.10: Slump and spread values of fresh Mor-S-Geo and Mor-GA-Geo . 236

Table 7.11: Density and strength properties of Mor-S-Geo and Mor-GA-Geo 239

Table 8.1: Geopolymer mix design for RTGA manufacture ............................ 243

Table 8.2: Typical sieving analysis of coarse and fine RTGAs ....................... 247

Table 8.3: Densities and water absorption capacities of RTGA and heated GA

........................................................................................................................ 249

xxx

Table 8.4: Mix designs of Con-RTGA-OPC and Con-GA-OPC (SSD aggregates

condition) ........................................................................................................ 254

Table 8.5: Density and strength properties of Con-RTGA-OPC and Con-GA-

OPC ................................................................................................................ 257

Table 8.6: Mix designs of Con-RTGA-Geo and Con-GA-Geo (SSD aggregates

condition) ........................................................................................................ 259

Table 8.7: Density and strength properties of Con-RTGA-Geo and Con-GA-Geo

........................................................................................................................ 261

Table A.1: Rietveld quantification preliminary results (which served as the data

for the consequent calculations) of the tested samples .................................. 279

Table A.2: Quantification of crystalline and amorphous contents in the three fly

ash batches .................................................................................................... 282

Table B.1: Ingredient proportions of mix designs of OPC-GA and Geo-GA

concretes (SSD aggregates condition) ........................................................... 283

Table B.2: Densities of ingredients for binding material of Geo-GA concrete . 286

Table B.3: Final mix design of Geo-GA example (SSD aggregates condition)289

Table B.4: Final mix design of GC example (SSD aggregates condition) ....... 291

Table B.5: Ingredient proportions of mix designs of OPC-GA and Geo-GA

mortars (SSD aggregates condition) ............................................................... 292

Table B.6: Final mix design of Geo-GA mortar example (SSD aggregates

condition) ........................................................................................................ 294

Table B.7: Mix design of Geo-GA mortar ........................................................ 296

Table B.8: Mix design of geopolymer mortar .................................................. 296

xxxi

List of Abbreviations

Abbreviation Meaning

FA-LWA Fly ash-based lightweight aggregate

GA Geopolymer aggregate

CDW Construction and demolition waste

AAR Alkali-aggregate reaction

LWA Lightweight aggregate

LWC Lightweight concrete

NWA Normal weight aggregate

NWC Normal weight concrete

GC Geopolymer concrete

OPC Ordinary Portland cement

RTGA Room temperature-cured geopolymer aggregate

CFA Coal fly ash

SEM Scanning electron microscope

Mt Million tonnes

AAC Alkali-activated cement

GGBFS Ground-granulated blast furnace slag

OM Optical microscopy

XRF X-ray fluorescence

XRD X-ray diffraction

IR Infrared

ATR Attenuated total reflectance

xxxii

W/G Water-to-geopolymer solids ratio

LOI Loss on ignition

ER Controlled environment room

RH Relative humidity

SCM Supplementary cementitious material

F/A Fly ash/activator mass ratio

SSD Saturated surface dry condition

OD Oven dry condition

AD Air dry condition

MIP Mercury intrusion porosimetry

1

Chapter 1

Introduction and Literature Review

Fly ash-based lightweight aggregate (FA-LWA) has been claimed as a

promising artificial material in the concrete industry because of its advanced

lightweight property, its capability to supplement the current depletion of natural

aggregates and its efficient recycling of redundant fly ash waste. However, it is

usually considered that the high power consumption of the traditional FA-LWA

manufacturing process will significantly affect costs, resources and the

environment, and thus hinder the promotion of FA-LWA. To better address this

problem, this PhD research explores a new method for the manufacture of FA-

LWA, called ‘geopolymerisation’, to replace the traditional technique. The

aggregate produced is referred to as ‘geopolymer aggregate’ and simplified as

‘GA’. It is designed to be of the same high quality as usual FA-LWAs but

consume much less energy and, be sufficiently workable to produce a high-

performance lightweight concrete.

This chapter introduces the motivation, principle, objectives and outcomes of

this PhD research. It evaluates the need for and benefits of FA-LWA in the

concrete industry; the current development and issues to be further addressed

of FA-LWA; and the principle of geopolymer technology and its practicality in

FA-LWA manufacture. Also, the scope, techniques and structure of this thesis

are presented.

2 1. Introduction and literature review

1.1 Background to Fly Ash-based Lightweight Aggregate (FA-

LWA)

1.1.1 Role of Aggregates in Concrete

In the construction field, ‘aggregates’ refer to the particulate solids that serve as

fillers in composite materials such as concrete. Normally, they are classified as

either coarse or fine depending on their particle sizes. The presence of

aggregates in concrete is shown in Figure 1.1.

Figure 1.1: Image of aggregate particles in concrete matrix

Generally, aggregates are sourced from natural rocks, gravels and sand (CCAA,

2010) and called natural aggregates, with a systematic classification of them

presented in British standard BS 812 (Neville, 1996). Apart from natural sources,

aggregates can also come from construction and demolition waste (CDW)

(Behera et al., 2014), and industrial by-products such as blast furnace slag and

coal ash (Clarke, 1993).

Coarse aggregates

Fine aggregates

Cement matrix

3

The initial motive for using aggregates in concrete is economic as natural

aggregates are cheaper than cement. Concrete in which aggregates occupy

around three-quarters of its volume can still be well bonded by one-quarter of

cement. Therefore, using aggregates has been seen as an efficient way of

consuming less cement and, thus, reducing costs.

Figure 1.2: Schematic of relationship between aggregates and cement matrix

The role of aggregates has been further understood from developments in the

concrete industry. A schematic of the relationship between aggregates and

cement is presented in Figure 1.2. As, in a concrete structure, aggregates

provide the main framework, they should determine the concrete’s primary

mechanical properties while a fresh cement matrix fills the spaces between the

aggregate particles to bond them. Since a natural aggregate is normally harder

than cement, using it is helpful for improving the strength of concrete. Also, as it

has a better shape and volume stability that hardly transforms under severe

conditions, it is preferred for enhancing the durability of produced concrete

(Neville, 1996).

An aggregate’s characteristics, such as density, strength, hardness and

durability, are valuable to the properties of final concrete. As these

Aggregates

Cement matrix


characteristics are mainly determined by the nature of the rocks from where the

aggregates came, strong and durable rocks are normally selected. Moreover,

an aggregate’s characteristics of shape, surface texture and open porosity,

which are created mainly by mechanical treatment during manufacture, also

play an important role in concrete since they can affect its bonding with the

cement matrix (Kayali, 2005, Zhang and Gjørv, 1990). Also, the grading of an

aggregate, which indicates its particle size distribution, can be significant. A

well-graded aggregate portion usually results in a more compact accumulation

of its particles in a concrete mix which reinforces the structure.

Sometimes, the chemical properties of aggregates will also influence the

concrete, with the alkali-aggregate reaction (AAR) being a well-known one. This

may occur in a high-silica aggregate, whereby its reactive silica contents react

with alkalis to form an alkali-silicate gel that easily expands in a hydrated

system and, subsequently, raises the internal pressure which causes harmful

swelling, cracking and disruption (Dent Glasser and Kataoka, 1981).

1.1.2 Need for Artificial Aggregates

Normally, natural aggregates of good quality and low cost are preferred in large-

scale real constructions and are still the dominant construction material in the

current market. However, artificial aggregates, which are not naturally present

but man-made, are increasingly being demanded.

One reason for this is the shortage of natural aggregates to satisfy the rising

demand for construction material by the world’s increasing population and

expanding urbanisation. Regional depletions of natural aggregates have

occurred in some places in Australia, especially the more developed cities, after

5

decades of mining (IQA, 2001), which is also reflected in continuous rises in the

prices of natural aggregates (Wu et al., 2005). A similar situation occurs in other

parts of the world, where depletions of natural resources and declines in the

amount of available quarrying land barely satisfy the increasing demand for raw

materials for construction (UEPG, 2010, Rao et al., 2007, Chen and Fang,

2005). Therefore, the application of an alternative material to natural aggregates

is being considered.

Also, artificial aggregates usually have more advanced properties in terms of

physical performance, durability and uniformity than natural ones (Bijen, 1986,

Chandra and Berntsson, 2002). They can be intentionally designed to have a

lighter or heavier specific gravity than normal natural aggregates. The

lightweight artificial aggregate with porous structure has better thermal and

acoustic insulation, and fire resistance, which is preferred for buildings in severe

environmental conditions (Clarke, 1993). Moreover, the one with anti-corrosive

property is highly required for coastal buildings (Kayali, 2005). Multi-function

artificial aggregate products are already available in the construction market

and comprise a considerable percentage of total aggregate use in Australia and

Europe (CCAA, 2008, UEPG, 2010).

Like natural aggregates, artificial ones can be sourced from natural rocks

(Clarke, 1993). Another category of artificial aggregates is the so-called

recycled ones that come from CDW, most of which are debris from concrete,

masonry and asphalt materials (Behera et al., 2014). Although they have

promising characteristics for concrete manufacture if there is a suitable concrete

design, they may have defects in strength, permeability or shrinkage compared

with natural aggregates (Rao et al., 2007, Zega and Di Maio, 2011, CCAA, 2008,


Rakshvir and Barai, 2006). Artificial aggregates can also come from industrial

by-products, such as furnace clinker, blast furnace slag and pulverised coal ash

(Clarke, 1993, Short and Kinniburgh, 1978).

1.1.3 Lightweight Aggregates (LWAs) and Lightweight Concrete (LWC)

Lightweight aggregates (LWAs) belong to a special group of aggregates which

have a low specific gravity. They can come directly from rocks that have a

naturally low specific gravity, mostly volcanic porous ones such as pumice,

scoria and volcanic cinders but, as natural LWAs are usually limited to certain

regions, it is difficult to promote them widely. Therefore, most current LWA

products are artificially manufactured from natural rocks, such as clay, shale,

slate, vermiculite and perlite, or industrial by-products, such as furnace clinker,

blast furnace slag and pulverised coal ash (Clarke, 1993, Short and Kinniburgh,

1978).

Regularly used natural aggregates have similar densities within a narrow range

referred to as the normal weight aggregate (NWA) (Neville, 1996). However,

LWA is unique as it has a particularly low density compared with the NWA,

appropriate ranges of which are regulated in relevant engineering standards

and manuals. In Australia, LWAs must have particle dry densities of 500-2100

kg/m3 according to Australian standard AS 2758 (AS, 1998a) while the US

standard ASTM C330/C330M-09 regulates that the maximum bulk densities of

coarse and fine aggregates should be 880 kg/m3 and 1120 kg/m3 respectively

(ASTM, 2009). According to the new European standard BS EN 13055, LWAs

should have either a particle density of less than 2000 kg/m3 or loose bulk

density of less than 1200 kg/m3 (BS, 2002).

7

The low specific gravity of LWA generates different characteristics from those of

NWA. Firstly, although LWA usually has less strength than NWA, which could

weaken the mechanical performance of the subsequent concrete (Bremner and

Holm, 1986), it has a higher strength/weight ratio and better tensile strain

capacity (Topçu and Işikdaǧ, 2007, Al-Khaiat and Haque, 1998). Also, the open

pores on its surface can result in its enhanced interlocking with the cement

matrix which strengthens the interfacial zone between them, producing high-

performance concretes (Kayali, 2008, Zhang and Gjørv, 1990, Lo et al., 2004,

Wasserman and Bentur, 1996).

Also, LWA normally has high porosity which is likely to lead to a much higher

water absorption capacity than NWA (Neville, 1996). Based on the results from

a 24-hour soaking test, the general absorption capacity of a LWA ranges from

5-25% by mass of the dry aggregate (ACI, 2003). As this can cause its high

absorption during a concrete mix, special care, such as pre-wetting, is usually

suggested (ACI, 1998). And, it is better for the absorption capacity of a

structural LWA to be less than 15% (Neville, 1996).

Moreover, the unique high-porosity system in a LWA can result in the high

insulation of heat and sound in the resultant concrete products (Chandra and

Berntsson, 2002, Topçu and Işikdaǧ, 2007, Short and Kinniburgh, 1978, Lieu et

al., 2003, Al-Khaiat and Haque, 1998). The pores in a LWA may even resist

damage from external vibrations such as earthquakes (Huang et al., 2007, Kılıç

et al., 2003, Kayali, 2005). The porous system of LWA also affects the impact

on concrete of severe conditions such as freezing and thawing (ACI, 1994,

Klieger and Hanson, 1961).


A major application of LWA is the manufacture of lightweight concrete (LWC)

which has a lower density than normal weight concrete (NWC). NWC, which is

normally made of conventional natural rocks and sand, and ordinary Portland

cement (OPC), lies within a narrow density range of 2200-2600 kg/m3 due to

the relatively consistent densities of above ingredients. However, LWC always

has a much lower density which is normally within the range of 300-2000 kg/m3

(Clarke, 1993, Neville, 1996). Using a LWA instead of NWA can effectively

reduce the weight of the aggregate proportion in a concrete which is 70-80% of

the concrete’s volume, and thus realise LWC.

Like NWC, LWC can be used in buildings to withstand external loadings and is

called structural LWC. In American standard ACI 213R-03, it is regulated that a

structural LWC made from LWAs should have an air-dry density within the

range of 1120-1920 kg/m3 and a minimum 28-day compressive strength of 17

MPa (ACI, 2003). A similar regulation in Australian standard AS 3600 states

that a LWC should have a saturated surface-dry density in the range of 1800-

2100 kg/m3 (AS, 2009).

LWC displays several advanced features due mainly to its application of LWA in

its manufacture. It has a light self-weight to reduce the material to be required

and, thereby, production costs are lowered. It also decreases the dead load,

foundation base and weight to be handled which simplifies the practical work. A

LWC also has special strength and strain behaviours (Lydon and Balendran,

1980, Zhang and Gjørv, 1991), and better resistance to heat, sound, chemicals

and earthquake devastation (Huang et al., 2007, Short and Kinniburgh, 1978,

Chandra and Berntsson, 2002).

9

1.1.4 Coal Fly Ash (CFA)

Coal fly ash (CFA), simply called fly ash, is a major source for the manufacture

of LWAs (Bijen, 1986, Kayali, 2008, Swamy and Lambert, 1981). Based on a

scanning electron microscope (SEM), it has been observed that fly ash is

mainly composed of solid spherical particles, and also a small proportion of

angular particles, as shown in Figure 1.3 (Fang, 2013).

(This figure has been removed due to copyright restrictions)

Figure 1.3: SEM image of fly ash (adapted from Fang, 2013)

Fly ash is a fine ash, part of the coal ash residue from pulverised coal

combustion, that can rise with the flue gas in contrast to the bottom ash that

does not rise (Page et al., 1979). During combustion, the combustible carbon in

the original coal is mostly burned out while the other contents, mainly

incombustible minerals such as SiO2, Al2O3, Fe2O3 and CaO, form the residual

coal ash which, after combustion, is released into the ambient temperature flue

with the flue gas. This subsequent rapid drop in temperature causes the coal

ash to condense into solids referred to as fly ash particles which are then

collected by an electrostatic precipitator or filter bag (Tomeczek and Palugniok,


2002, Page et al., 1979). Due to the unique heating and cooling processes of

coal combustion, most fly ash particles are spherical shapes with small amounts

of irregular carbon particles. The particle size distribution of fly ash is generally

from 0.5 µm to 300 µm (Blissett and Rowson, 2012, Kutchko and Kim, 2006).

The annual production of fly ash is quite considerable since coal is the world’s

dominant energy resource. In Australia and New Zealand, approximately 13

million tonnes (Mt) of CFA were created in 2010 (ADAA, 2010) and a more

recent estimation has reported that the global annual production must be

around 750 Mt considering the rapid growth of developing countries in the last

10 years (Blissett and Rowson, 2012). However, a great deal of fly ash has not

been efficiently recycled. Current statistical data show that the utilisation of fly

ash is approximately 46% in Australia and New Zealand (ADAA, 2010), 39% in

the US and 47% in Europe (ECCPA, 2008, Blissett and Rowson, 2012). This

means that, every year, a large proportion of fly ash is unused and disposed of

as useless waste which may cause further issues in terms of environmental

pollution and land resource wastage (Kockal and Ozturan, 2010, Babbitt and

Lindner, 2005). Therefore, research on using fly ash for aggregate manufacture

could provide an efficient way of recycling and consuming this massive amount

of waste. Also, storage of abundant amounts of fly ash would guarantee

supplies of raw materials for artificial aggregate manufacture which could solve

the problem of the current depletion of natural resources.

Normally, fly ash has a very complex chemical composition which includes SiO2,

Al2O3, Fe2O3, CaO, MgO, K2O, Na2O and TiO2, with those from different

sources possibly being quite different due to the components present in the

original coal from which they were obtained (Vassilev and Vassileva, 2005).

11

This is usually more obvious for fly ashes from different coal types, as indicated

in Table 1.1 (Ahmaruzzaman, 2010) but may also occur among fly ash samples

from different places (Blissett and Rowson, 2012). Therefore, fly ash is usually

classified in several different categories for better reference (ASTM, 2012a, EN.,

2001).

Table 1.1: Bulk chemical compositions of fly ashes from different coal types (adapted from Ahmaruzzaman, 2010)

(This table has been removed due to copyright restrictions)

Apart from its chemical composition, fly ash also has various mineral contents

of which there are three major groups, carbon, an amorphous (glass) phase and

crystals. Carbon comes from incomplete coal combustion which can always be

found in fly ash although in small amounts (Benezet et al., 2008, Kutchko and

Kim, 2006, Tomeczek and Palugniok, 2002). Generally, carbon’s maximum

proportion in fly ash for construction has always been regulated (ASTM, 2012a),

because porous carbon has a high absorption rate which may be harmful to a

concrete’s durability in freezing and thawing conditions or require a more costly


cement for workability (Pedersen et al., 2008, Raask and Bhaskar, 1975, Diaz

et al., 2010).

The amorphous phase and crystals are two different mineral phases and are

the major components in fly ash. The former is the dominant amount (Kutchko

and Kim, 2006, Ward and French, 2006), with crystals, such as quartz, mullite

and some iron oxides like hematite, magnetite and maghemite, always present

(Davidovits, 2008). These two phases are determined mainly by the

composition of the original coal (Tomeczek and Palugniok, 2002, Raask, 1985)

and the combustion and cooling processes experienced during its combustion

(Haugsten and Gustavson, 2000, Henry et al., 2004).

Generally, the amorphous phase in fly ash is very reactive and considered the

main reason for fly ash’s chemical activity. Amorphous silica (SiO2) and alumina

(Al2O3), the two dominant amorphous minerals in terms of proportion, react with

the calcium oxide in solution at ordinary temperatures to generate cementitious

products which is referred to as the fly ash’s pozzolanic property (Raask and

Bhaskar, 1975). This property has been well applied through replacing part of

the cement by fly ash to achieve a low cost and prevent AAR in order to

improve durability in concrete manufacture (Neville, 1996). The chemical activity

in the amorphous phase has also been made use of for producing new cement

materials in recent decades (Davidovits, 2008). In contrast, crystals are much

less reactive (Diaz et al., 2010, Chen-Tan et al., 2009). This thesis will

investigate the possibility of taking advantage of the mineral contents in fly ash

to initiate the geopolymerisation that is necessary to produce geopolymer

aggregates.

13

1.1.5 FA-LWA

The production of FA-LWA is normally conducted based on a series of

mechanical and thermal processes, as illustrated in the basic flowchart in Figure

1.4.

Basically, conventional FA-LWA manufacture consists of granulation and

sintering processes. Firstly, a mechanical process is carried out to bond

adjacent loose, fine fly ash particles together into consolidated large objects

appropriate for aggregate use and is normally referred to as the agglomeration

step (Bijen, 1986). To facilitate this, a bonding agent (mostly water) is usually

needed. This process is normally completed by a granulation technique based

on relevant apparatus, such as a pelletiser, which forms pellets which are still

not sufficiently hard and are referred to as ‘green pellets’ (Bijen, 1986).

Then, further hardening is required to tightly bind the agglomerated particles

and build a strong structure that can sustain an external load. This step is

traditionally completed by sintering which fuses the fly ash particles together at

an elevated temperature of >1000oC. Further mechanical crushing may also be

needed to generate particles of the right sizes for the particular aggregate

application. The above traditional manufacturing steps are discussed in relevant

literature (Chandra and Berntsson, 2002, Bijen, 1986).



Figure 1.4: Main flowchart of processes in fly ash aggregate manufacture (based on Chandra and Berntsson, 2002 and Bijen, 1986)

With developments in the area of FA-LWA, some other methods for completing

the above procedures have been proposed. As shown in Figure 1.4, some

materials which have a binding/cementitious function, such as OPC, lime and

water glass, are used with water which adds an advanced binding action to

enhance the effect of agglomeration (Bijen, 1986). Other agents that can initiate

chemical activation for agglomeration have also been discussed (Fansuri et al.,

2012, Geetha and Ramamurthy, 2010, Gesoğlu et al., 2007).

15

Mechanical compaction is also used to enhance the agglomeration of fly ash

particles by compressing these particles using an external force. Common

compaction methods are briquetting using a piston-type or roll press, pellet

milling and extrusion (Bijen, 1986).

For hardening, apart from elevated-temperature sintering, agglomerated fly ash

particles can also be hardened by an autoclaving process or cold-bonding in a

normal environment (Bijen, 1986, Gesoğlu et al., 2007). This raises the

possibility of removing elevated-temperature heat treatments from FA-LWA

manufacture. However, these methods weaken the properties of the final

aggregate products, and extra costs will be incurred in trying to reach

comparable quality (Bijen, 1986). Therefore, current mainstream FA-LWA

production still requires elevated-temperature sintering to achieve the desired

performance.

FA-LWA products are already commercially available and have been

successfully used in construction. One well-known example is the ‘Lytag’

aggregate (Figure 1.5) which has been widely promoted since its appearance in

the UK in the 1960s (Kayali, 2005, Swamy and Lambert, 1981). In its

manufacture, water is used as the bonding agent to adhere the loose fly ash

particles. A pelletisation process is applied for agglomeration and a sintering

process at a temperature of 1100-1300oC is used to harden the fly ash in order

to obtain the final aggregate product (Clarke, 1993, Swamy and Lambert, 1981).

The common characteristics of a Lytag aggregate are shown in Table 1.2.



Figure 1.5: Images of Lytag aggregate (left) and Flashag aggregate (right, from Kayali, 2008)

Table 1.2: Characteristics of Lytag and Flashag aggregates

(Data of latter from Kayali, 2008)

Properties Lytag Flashag Coarse Fine Coarse & Fine

Specific gravity (SSD) 1.60 1.76 1.69

Specific gravity (OD) 1.41 1.60 1.61

Dry loose bulk density (kg/m3)

805 1067 848

Water absorption (in 24 hours) 13.2% 9.7% 3.4%

SSD: saturated surface dry condition; OD: oven dry condition

Apart from the Lytag aggregate, another FA-LWA called ‘Flashag’ (Kayali, 2005,

Kayali, 2008) was recently proposed, with an image of it presented in Figure 1.5

and its characteristics in Table 1.2. Its manufacturing procedure was modified

from that of the Lytag aggregate by: adding a superplasticiser to reduce the

volume of water required and obtain advanced bonding in the agglomeration

step (Kayali and Shaw, 2004); casting fly ash briquettes for sintering; and

17

granulating the aggregate particles by crushing after, instead of pelletisation

before, sintering (Kayali, 2005). The Flashag aggregate produced is more

simply manufactured and has better properties than conventional Lytag

products, and results in a high-performance concrete (Kayali, 2005). Both Lytag

and Flashag aggregates are qualified for use in structural LWC.

1.1.6 Problems of Current Manufacturing Method

As previously stated, FA-LWA is very promising as a solution to the current

depletion of natural aggregates, for utilisation of its enhanced properties and the

consumption of large scale fly ash waste which should be reasons for it to be

applied in real construction applications. However, it is not very well promoted

because of some confronting issues. For its manufacture, it requires high-power

mechanical and thermal treatments that consume large amounts of energy to

ensure its quality. This could result in extremely high costs for its production

which, if it is used to replace natural aggregates, would add a financial burden

to the construction industry. Therefore, such a FA-LWA can only be used if its

unique features are required in a special application.

Also, as the high energy consumed in FA-LWA manufacture expends large

amounts of fossil fuels, the current depletion of fuel resources will be

aggravated despite mineral resources being saved by using recycled fly ash.

Moreover, the burning of fossil fuels could create new burdens of industrial

waste, environmental pollution and CO2 emissions.

Thus, it is necessary for establishing a firm basis that justifies the use of FA-

LWA, that the above mentioned issues be addressed. Furthermore, the author

examines the hypothesis that geopolymer technology is a promising suitable


mechanism that may be adopted to manufacture lightweight aggregates that

could be suitable for utilisation in construction. This hypothesis, together with

the performance of such geopolymer aggregates are therefore, the subject of

this thesis.

1.2 Geopolymer Technology

1.2.1 Brief Introduction to Geopolymer Technology

It has been argued that research on ‘alkali-activated cement’ (AAC), which

originated in Europe in the 1930s, should be considered the precursor to

geopolymer technology (Roy, 1999). At first, that research concentrated on the

cementitious property that emerged in synthetic slag-based materials under the

activation of alkaline hydroxide (Roy, 1999, Komnitsas and Zaharaki, 2007,

Pacheco-Torgal et al., 2011). Later, Glukhovsky et al. (1980) investigated AAC

reaction mechanisms since the 1950s, indicating that the essential uniqueness

of such a material is its formation of amorphous alumina and silicate structures,

different from OPC materials which are composed of calcium hydrates

(Glukhovsky et al., 1980, Krivenko, 1986). This seemed to realise an improved

durability for cementitious materials (Komnitsas and Zaharaki, 2007). Although

there are still debates about the role of AAC in the development of geopolymer

since these two cementitious materials are not exactly alike, it should be

appropriate to admit that AAC is the most similar to the geopolymer in terms of

property and application.

Nowadays, the term ‘geopolymer’ specifically refers to the synthetic inorganic

polymer material developed by Davidovits since the 1970s (Davidovits, 1991).

The original geopolymer material was synthesised by activating kaolinite with an

19

alkaline solution at 100-150oC, to make use of its inorganic nature for fire-

resistant coatings in construction panels. Later, different types of, and synthetic

methods for producing, geopolymers have been developed. The resultant

geopolymer materials display advanced properties in terms of mechanics,

thermodynamics and surface chemistry which can be utilised in multiple

applications, such as adhesives, ceramics, foams, and toxic and radioactive

waste encapsulations (Davidovits, 2008).

The real breakthrough for making a geopolymer attractive was its application as

an alternative cement material to OPC. This was realised by generating fresh

geopolymer gels during concrete mixing to firstly bond the aggregates and then

harden the fresh geopolymer to form a final geopolymer concrete, which

displayed a similar cementitious property to that of OPC, as shown in Figure 1.6

(Kong et al., 2007, Rangan, 2007, Bakharev, 2005a).

Relevant research on geopolymer concrete was launched in the 1990s to

realise its considerable environmental benefits, such as extremely low CO2

emissions compared to OPC (Davidovits, 2002, CIA, 2011). Also, geopolymer

concrete possesses several superior properties, such as a higher early strength,

lower shrinkage, better thermal and chemical resistance, and more efficient

consumption of industrial by-products (such as fly ash wastes), if it has a

suitable mix and processing design (Davidovits, 2008, Duxson et al., 2007a,

Palomo et al., 1999, Rangan et al., 2005). Geopolymer concrete without any

other cement has already been created and proven by experiments to have

suitable strength and durability for use in construction (Rangan, 2007, Hardjito

and Rangan, 2005).


Figure 1.6: Schematic of principle of geopolymer binder in concrete mix (drew by the author based on the literature Kong, 2007, Rangan, 2007 and Bakharev,

2005a)

According to the most recent definition provided by the Geopolymer Institute, a

geopolymer is a material with a molecular structure comprised of covalent-

bonded chains or networks of T-O-T-O units (T is a geological element, such as

Si, Al, Fe and P) and is X-ray amorphous in ambient conditions but crystalline

at >500oC (Geopolymer Institute, 2012). This expands the initial concept of a

geopolymer which defined it as a polymerised material composed of amorphous

aluminosilicate backbones formed in alkaline conditions (Davidovits, 1989).

However, as the mainstream of current geopolymer technology is still focussed

on the conventional aluminosilicate-based geopolymer because of its relatively

mature techniques and controllable properties, this material is the basis of this

thesis.

An aluminosilicate-based geopolymer is composed mainly of SiO4 and AlO4

tetrahedra linked by covalent bonds. The schematic of its structure presented in

Figure 1.7 (Davidovits, 1976) shows that the Si and Al atoms are 4-coordinated

21

and the adjacent Si/Al tetrahedra linked by sharing the O atom in the corner

position. Also, although metal cations, such as Na+, K+, Li+ and Ca2+, and water

can be present for charge balance and structural stabilisation, they are more

likely to lie around the tetrahedra backbone instead of part of it (Barbosa et al.,

2000, Davidovits, 2008). In a geopolymer, the Si and Al tetrahedra units bond

with each other to form large network backbones, as shown in Figure 1.8

(Rowles et al., 2007, Barbosa et al., 2000). The chemical composition of an

aluminosilicate-based geopolymer can be expressed as:

Mn(-(SiO2)z-AlO2)n, wH2O

where M is the alkaline element, z is 1,2 or 3 to describe the Si/Al ratios in

different units, n is the degree of polycondensation and w the degree of

hydration (Davidovits, 2008, Davidovits, 1989).


Figure 1.7: 4-coordinated configuration of Si and Al tetrahedra in geopolymer (Davidovits, 1976)

The synthesis of an aluminosilicate-based geopolymer is normally based on

solid raw materials rich in alumina and silicate components, such as

metallurgical slags, fly ash and metakaolin (Duxson and Provis, 2008). A

geopolymer created from Class F fly ash has been well developed in recent

decades and is investigated in this research. A geopolymer material is

synthesised by activating raw fly ash using the alkaline activators’ sodium


hydroxide and sodium silicate under a suitable curing regime, as depicted in

Figure 1.9 (Kong et al., 2007, Rangan et al., 2005, Rangan, 2007).


Figure 1.8: Three-dimensional network of geopolymer structure (adapted from models proposed by Rowles et al., 2007 and Barbosa et al., 2000)

Figure 1.9: Scheme for synthesis of fly ash-based geopolymer

In principle, the entire geopolymerisation process used is divided into three

major stages. Firstly, the reactive contents in the raw fly ash, mainly the reactive

23

alumina and silicate, are dissolved under the activation of highly concentrated

alkalis. Then, the dissolved Al and Si contents, forced by electric and chemical

effects, orient and accumulate to form intermediate oligomers which are low-

degree polymerised units, some of which were identified by Swaddle and his

team (North and Swaddle, 2000, Davidovits, 2008). These oligomers further

polymerise into higher-degree polymers and later condense into a hardened

structure (Palomo et al., 1999, Xu and Van Deventer, 2000, Davidovits, 1988).

The ideal chemical reaction model proposed by Davidovits (1989) which

describes the geopolymerisation process using aluminosilicate-rich raw

materials represented by (Si2O5, Al2O2)n are shown below. Two series are

presented due to the different Si/Al ratios in the mixtures.

Series 1:

Series 2:

There has been further research on the mechanism of the geopolymer reaction.

Provis et al. (2005) investigated and proposed the reaction mechanism of a

geopolymer made from metakaolin presented in the schematic diagram in

Figure 1.10 which basically indicates the intermediate products during the

(Si2O5, Al2O2)n + nSiO2 + 4nH2ONaOH

KOH��⎯⎯⎯⎯⎯⎯⎯� n(OH)3Si − O − Al−(OH)2 − O − Si(OH)3

n(OH)3Si− O − Al−(OH)2 − O − Si(OH)3NaOH

KOH��⎯⎯⎯⎯⎯⎯⎯� (−SiO − O − Al−O − O − SiO− O −)n

+ 4nH2O

(Si2O5, Al2O2)n + 3nH2ONaOH

KOH��⎯⎯⎯⎯⎯⎯⎯� n(OH)3Si − O − Al−(OH)3

n(OH)3Si− O − Al−(OH)3NaOH

KOH��⎯⎯⎯⎯⎯⎯⎯� (−SiO− O − Al−O− O −)n + 3nH2O


different stages of geopolymerisation. More importantly, the end-products

include not only the amorphous phase but also the nano-crystalline phase that

is also possibly present in the final geopolymer. A similar reaction model for

more generalised geopolymerisation, which provides a more specific illustration

of the secondary reactions that occur during each stage, proposed by Duxson

et al. (2007a) is also presented in Figure 1.10.


Figure 1.10: Conceptual models for geopolymerisation proposed by Provis (2005) (left) and Duxson (2007) (right)

1.2.2 Advantages of Geopolymer Technology for FA-LWA Manufacture

Different from mainstream applications of geopolymers in cements, ceramics,

coatings, etc., this research makes use of geopolymer technology for the

manufacture of a FA-LWA. Its aim is to deal with the problems in the current

25

FA-LWA manufacturing method and realise the properties required for FA-LWA

through using a geopolymer material. Therefore, consistent with the above two

aims, the suitability of geopolymer technology is evaluated by comparing it with

a traditional manufacturing method. The traditional method selected is the most

common one that requires pelletisation and elevated-temperature sintering for

agglomeration and hardening, as previously discussed in subsection 1.1.5.

Details of both methods are presented in Table 1.3.

Table 1.3: Comparison of traditional method and geopolymer technology for FA-LWA manufacture

Stages in aggregate manufacture Traditional method Geopolymer technology

Mixing Water (silicate, cement, lime)

Alkaline hydroxide and silicate

Agglomeration Granulation and compaction Chemical bonding (optional vibration)

Hardening Sintering at elevated temperature >1000oC

Heating at mild temperature of 40-80oC

Table 1.3 lists the main stages of mixing, agglomeration and hardening in the

manufacture of FA-LWA. During mixing, the traditional method aims to stick the

loose fly ash particles together for subsequent granulation. The bonding agent,

such as water or other materials with cementitious properties, is used to

facilitate adherence of the fly ash particles. On the other hand, the new

geopolymer technology aims to gather the fly ash particles together and also to

activate fly ash for geopolymer reactions. Under the attack of alkali hydroxide

and silicate, fly ash particles develop to become fresh geopolymer which is

sufficiently viscous to make them connect with each other and also bond the


unreacted fly ash. The mixture after geopolymerisation ends in a uniform gel-

like mixture, which looks similar to traditional cement paste although its

manufacture is based upon different principles. Furthermore, fly ash will

undergo a complex chemical reaction that renders it into a solidified geopolymer

material which is suitable to be used as aggregate for concrete production.

The different mixing conditions of the above two methods, i.e. the traditional

palletisation and sintering method versus the geopolymerisation method, also

affect their agglomeration stages. In the traditional method, a granulation

process, or even compaction process, is needed to press the bonded fly ash

particles into green pellets (Bijen, 1986, Kayali, 2008), otherwise these particles

will be likely poorly bonded. However, when using geopolymer technology, the

mixture can be tightly bonded by fresh geopolymer, and the mixing force is

sufficient to complete this bonding, with no extra mechanical means required.

The traditional granulation process can thus be saved. Sometimes, a certain

vibration process might be needed if it is planned to mould the fresh

geopolymer mixture into a suitable shape for further processing, but that would

consume much less effort than pelletisation.

Another significant difference is the hardening stage during manufacture. As

previously discussed, a major drawback of the traditional method is the high

amount of power consumed to obtain the elevated temperature of greater than

1000oC required to harden the fly ash into strong aggregate products which

could result in a high cost and significant harm to the resources and

environment. Geopolymer technology, which is capable of transferring loose,

weak fly ash particles into compact, hard objects at mild temperatures (40-

80oC), could possibly address this drawback by significantly reducing the

27

operational temperature required for hardening. Despite the hydroxide and

silicate agents needed in the geopolymer-assisted aggregate industry, their

costs would still be much less than that of the power consumed in the traditional

method.

Apart from its simple, low-cost, eco-friendly manufacture, geopolymer

technology also has several advanced features for producing a desirable FA-

LWA product. A geopolymer material usually has good mechanical properties

with a low density range (Kong and Sanjayan, 2010, Swanepoel and Strydom,

2002, Xie and Kayali, 2014) which could be suitable for a FA-LWA demanding a

high strength/weight ratio. Geopolymer normally has advanced thermal, sound

and chemical insulation that could result in better durability for future

construction use (Duxson et al., 2007a, Bakharev, 2005h) and hence it is

expected that FA-LWA will further contribute to these beneficial properties. In

addition, it has been reported that the structure of a geopolymer is incredibly

resilient in the cases of exposure to high temperatures (Barbosa, 2003). These

features appear to be able to realise the properties expected of an artificial FA-

LWA, as discussed in subsection 1.1.3. Therefore, it is estimated that the new

geopolymer technology could replace the traditional method without seriously

weakening aggregates’ properties.

1.2.3 Previous Trials using Geopolymer Technology for LWA

Manufacture

Several initial studies have been conducted to explore the possibility of using

geopolymer technology in the manufacture of LWA products. However, most of

their products are not likely to be regarded as FA-LWAs as they are not made


solely from fly ash or fly ash derivatives, with those made from only fly ash

being weak in strength. Moreover, nearly all these research works are still in an

initial stage in terms of concrete application. Nevertheless, the experiences

gained from these applications of geopolymer technology are valuable guides

for this research.

Jo et al. (2007) proposed a kind of LWA called ‘ALFA’ made from fly ash,

cement, NaOH, Na2SiO3 and MnO2. This research aimed to initiate a

geopolymer reaction and a cement hydration and use the cementitious

materials formed later to bind all ingredients into hardened aggregate particles.

The produced ALFA aggregates had several good properties similar to normal

FA-LWAs when MnO2 was added, but very low strength when MnO2 was not

added (Jo et al., 2007). It is considered that MnO2 is necessary to produce

ALFA so as to act as the core that binds the geopolymer and hydrated cement.

In Jo’s research however, the formed geopolymer functions more as a bonding

agent which replaces water in its role in fresh mixture workability and does

away with sintering necessary in traditional FA-LWA manufacture.

Fansuri et al. (2012) investigated a method for converting a pure, fresh fly ash-

based geopolymer into pellets. This is like the pelletisation process in

conventional FA-LWA manufacture and solves the problem of granulating the

aggregates from the geopolymer material. The formed geopolymer pellets can

be later hardened by the normal geopolymer heating procedure, that is, mildly

heated for days (Fansuri et al., 2012). However, this method’s problems include

that more research is needed before it can be promoted for industrial

manufacture and the strengths of the pellets produced must be greatly

improved for structural use.

29

Bui et al. (2012) aimed to enhance the strength by adding ground-granulated

blast furnace slag (GGBFS) to a fly ash-based geopolymer mixture to produce a

LWA product that was sufficiently strong for concrete manufacture. An extra

benefit of using GGBFS was that its manufacture could be conducted at room

temperature (Bui et al., 2012). Their study agreed well with previous research

on room temperature-cured geopolymer made from a mixture of fly ash and

GGBFS (Davidovits, 2013). However, Bui et al. only researched the

manufacture of a coarse LWA product whereas, to complete a concrete mix,

natural sand is still needed and a high cement proportion (over 400 kg/m3) must

be used (Bui et al., 2012).

A study by Silva et al. which conducted research on making aggregates using

geopolymer technology was for the waste-water treatment industry not

construction (Silva et al., 2010). However, it describes a method as to produce

aggregate particles from raw materials using geopolymerisation. As expected,

the aggregate product in Silva’s research was weak in strength but, as it had

many penetrable micron pores to absorb the waste elements in water, it could

be seen as a successful example of building a micron-porosity system in a LWA

by geopolymer technology. So this method of manufacture is also consulted in

this research.

1.3 Scope of Thesis

This thesis studies a new type of artificial FA-LWA called a ‘geopolymer

aggregate’ made by geopolymer technology which is a low-cost, eco-friendly

process as it does not involve the high energy consumption of conventional FA-

LWA manufacturing methods. The geopolymer aggregate produced has


advanced characteristics that can function as both coarse and fine aggregate

portions in concrete manufacture to produce a high-performance lightweight

concrete.

This research work consists of the principles and effects of raw materials,

activator designs and synthetic conditions for geopolymerisation; the

mechanism, manufacture and properties of geopolymer aggregates; and the

utilisation of both coarse and fine geopolymer aggregates, in either the OPC

concretes or the geopolymer binder concretes. Also, mortar material that only

uses fine geopolymer aggregates is examined.

1.4 Techniques used in research

The techniques used in this research for manufacturing, property testing and

characterisation are briefly introduced here and will be further detailed in

following chapters.

The construction materials to be manufactured include fly ash-based

geopolymer aggregate mixes, pastes, and concrete and mortar samples using

different aggregates and cementitious materials. The techniques used include

different grades of mechanical mixers, moulding facilities, vibrating table,

electric oven and controlled environment room. Also, a mechanical jaw crusher

is used to create aggregate particles in appropriate size distributions.

For property testing, techniques are applied to evaluate the physical,

mechanical, structural and compositional properties of the materials produced.

The normal physical properties, such as density and porosity, are tested on

relevant fundamental facilities, and the particle sizes and surfaces using both

31

standard sieves and a Malvern Mastersizer 2000 laser diffraction particle size

analyser.

The mechanical tests of the strength properties of relevant samples are

conducted using a TECNOTEST compression testing machine (3000 kN

capacity). On the other hand, the aggregates’ crushing value is measured using

a SHIMADZU universal testing machine (1000 kN capacity). Moreover, the

rebound index of aggregates is measured using a Schmidt hammer.

Morphology, microstructure and porosity observations are made using a

combination of scanning electron microscopy (SEM) on a HITACHI TM 3000

facility and optical microscopy (OM) on a Zeiss AxioCam MRC digital camera.

The chemical compositions of fly ash and geopolymer materials are measured

using a combination of X-ray fluorescence (XRF), which identifies the

quantitative chemical composition, and X-ray diffraction (XRD), which identifies

the crystalline phases present in the tested material, and quantifies them and

the amorphous phases in combination with Rietveld refinement method. Also,

infrared (IR) spectroscopy is applied to identify variations in the chemical

functional groups, such as Si-O-Si and Si-O-Al, both before and after

geopolymerisation using a SHIMADZU spectrophotometer equipped with an

attenuated total reflectance (ATR) accessory.

1.5 Structure of Thesis

This thesis consists of nine chapters. The first is the introduction and literature

review, the last is the conclusion chapter while the main research work is

discussed in Chapters 2 to 8. A schematic of the relationship of each chapter to

the main research work is presented in Figure 1.11.


Chapter 1 depicts a wholistic picture of the development of the research work in

this thesis. It firstly provides the background to FA-LWA which is known as the

kind of advanced construction material highly needed in the current concrete

industry but has the shortcoming of consuming a high amount of energy during

in its manufacture. This leads to discussing the idea of making use of

geopolymer technology for low-cost, eco-friendly FA-LWA manufacture. It is

also expected that the geopolymer aggregate produced could be enhanced by

obtaining properties from advanced geopolymer materials based on previous

experiences of their use in construction. This aspect will also be explored.

This chapter then evaluates the principle of geopolymer technology, including

its history, raw materials, activating agents and conditions, reaction

mechanisms and models, and characteristics of its end-products. Based on this,

the suitability of geopolymer technology for FA-LWA manufacture is analysed

and the advantages of geopolymer materials for high-quality FA-LWA products

are highlighted.

Figure 1.11: Schematic of relationships among thesis chapters


The objectives of this research are later discussed in this chapter. The main one

is to investigate the manufacture of FA-LWA by geopolymerisation which

creates geopolymer aggregates of two types, coarse like ‘artificial stone’ and

fine like ‘artificial sand’. Another objective is to examine the suitability of the

geopolymer aggregate produced for concrete manufacture through using it in

OPC-based concretes and geopolymer binder-based concretes.

Chapter 2 launches a research work on the selection of suitable fly ash sample

for desired geopolymer synthesis. It investigates the basic characteristics of fly

ash properties, such as particle size distribution, specific surface area, loss on

ignition, chemical composition and crystalline/amorphous contents, and

examines the sensitivity of geopolymer synthesis to the minor differences

present in characteristics among different fly ash batches collected from the

same coal power station source. This was found to be more obvious in fresh

properties like workability and consistency, which is most likely to be due to the

different reaction rates of the fly ash samples containing different amorphous

SiO2, Al2O3 and CaO proportions. Thus, the author explains that to achieve

consistent manufacturing, one specific fly ash batch, which seems to be the

most suitable for geopolymer aggregates based on the testing results in

Chapter 2, is selected to create geopolymer materials for the work of this

research.

Chapter 3 concentrates on the mix design of geopolymer synthesis. A

systematic geopolymer mix design procedure is devised which uses the criteria

of SiO2/Na2O, H2O/Na2O and ‘water-to-geopolymer solids ratio’ (W/G ratio) to

distinguish between each geopolymer mix and calculate the final mix design, in

order to better reflect the relationships among the chemical components

35

involved in geopolymerisation. This mix design procedure is applied for all

geopolymer syntheses included in this thesis, including the ones for geopolymer

aggregates and the ones for the geopolymer binders in concrete and mortar

manufacture.

Chapter 4 concentrates on the curing regime of geopolymer synthesis, as a

valid factor that determines the performance of produced geopolymer materials.

The curing regime is researched by curing the fresh geopolymer made with the

selected mix design under various temperature, moisture and time conditions.

From the experimental results, it is seen that mild temperature heating is

beneficial for fresh geopolymer, that a higher temperature and a longer heating

time can facilitate growth of the geopolymer. Thus, such heating regime is first

applied for the manufacture of geopolymer aggregates and geopolymer binders

as presented and discussed in Chapters 5 to 7. On the other hand, it is found

that low-moisture curing and prolonged curing period can have a positive effect

on geopolymerisation, but these seem to be only significant for a room

temperature-cured geopolymer. Nevertheless, it is possible to make a room

temperature-cured geopolymer achieve a similar strength level as the heated

one by a well control of curing moisture and curing period. This finding is utilised

to make room temperature-cured geopolymer aggregate as detailed in Chapter

8.

Chapter 5 introduces the procedure for geopolymer aggregate manufacture.

The geopolymer material determined as suitable is synthesised based on the

research presented in Chapters 2, 3 and 4. Then, further mechanical

processing is applied to generate both coarse and fine aggregates with

appropriate particle sizes. A major feature of the manufacture of FA-LWA based


on the method of geopolymerisation is that the traditional elevated-temperature

sintering and high-power mechanical pressing/pelletising are done away with.

The relevant properties of the geopolymer aggregate produced, such as

appearance, grading, density, water absorption, porosity, crushing value and

rebound index, are measured. The chemical structure of geopolymer aggregate

is then analysed by IR spectroscopy. It is found that the geopolymer aggregate

generated is a type of FA-LWA composed of geopolymer structure with a low

density, high porosity and high water absorption capability but possessing good

mechanical behaviour. Geopolymer aggregate therefore, can have properties

which satisfy the regulations in relevant LWA standards, and are similar or even

better than other regular FA-LWA products.

Chapters 6 and 7 are concerned with issues of utilising the geopolymer

aggregates made as explained in Chapter 5, in concrete and mortar

manufacture in relation to the type of cementitious material used. In Chapter 6,

OPC which is the most widely used cementitious material in the world, is the

sole cement material investigated in this chapter. The research in this chapter

aims to provide an evaluation of the compatibility of geopolymer aggregates

against the current mainstream cementitious system.

In Chapter 7, a fly ash-based geopolymer binder, which has been considered a

potential alternative to OPC in recent decades, is investigated. In both Chapters

6 and 7, geopolymer aggregates serve as the sole aggregate materials for

coarse and fine aggregate portions. The mix design, sample preparation,

workability, density, and increases in strength and structure of concrete mixes

using the new geopolymer aggregate are investigated. The results indicate that

this geopolymer aggregate could qualify as a good substitute for natural

37

aggregates in concrete manufacture and a concrete made of it is capable of

achieving a high-performance lightweight concrete mixture with high strength

and low self-weight. Geopolymer aggregates could also be suitable for mortar

manufacture, when treated as a special type of concrete without a coarse

aggregate portion, to produce a high-performance mortar with a similar strength

but much lower self-weight than a normal one. The mortar research also

highlights the advantage of fine geopolymer aggregate made here as new

‘artificial sand’ for structural proposes.

Chapter 8 explores the manufacture and application of a special type of

geopolymer aggregate cured at room temperature without any heat treatment,

called a room temperature-cured geopolymer aggregate (RTGA), which is

different from those discussed in Chapters 5, 6 and 7. The reason for removing

the heat treatment is to further reduce the power consumed during aggregate

manufacture, and to save on the manufacturing step of heating. The properties

of RTGA, and subsequent concrete manufacturing using either OPC or

geopolymer binder, are researched in the same way as explained in previous

chapters. The results indicate that, although RTGA is weak in strength and

more porous than the other geopolymer aggregates that experience heat

treatment, and therefore not as good, it still has certain properties which qualify

it for use in construction. Such properties make RTGA a potential

environmentally friendly construction material in the future which certainly needs

further research.

38 2. Fly ash and its influence on fresh properties of geoploymer

Chapter 2

Fly Ash and its Influence on Fresh Properties of

Geopolymer

2.1 Introduction

As introduced in Chapter 1, for the geopolymer materials discussed in this

thesis, fly ash is the major aluminosilicate source required to provide reactants

for geopolymerisation. Hence, this chapter launches a research work on the

effects of fly ash characteristics for geopolymerisation, in order to find a suitable

fly ash sample for desired geopolymer synthesis. Meanwhile, the fresh

properties, which dominate geopolymer’s performance in a mechanical

processing of geopolymer aggregates manufacturing procedure, are also

studied.

The fly ash samples from different sources are greatly varied in characteristics,

as introduced in Chapter 1, subsection 1.1.4, which were found to result in

significant differences in the properties of final geopolymer products (Diaz et al.,

2010, Rickard et al., 2011). Meanwhile, since fly ash is just a by-product rather

than an end-product, controlling uniformity and consistency to suit particular

production purposes has not been strictly applied. In real practice, a common

way to guarantee the consistency of fly ash is to select a single source of fly ash,

mostly a particular coal power station, and only use the fly ash from this source

for the entire work of geopolymer synthesis (Rickard et al., 2011).

The way of selecting fly ash from a particular coal source seems to work when

using fly ash as a supplementary cementitious material, which is relatively less

39

affected by the fly ash variability. However, through recent research, the author

finds that the fly ash samples from one particular coal source with minor

different characteristics can result in significant differences in the properties of

produced geopolymers. This is rather obvious for fresh properties like

workability and consistency, which can be significant enough to result in

inconsistent final geopolymer products.

Therefore, the author considers it necessary to strictly control the source of fly

ash, so as to assure the consistency for fresh geopolymer mix and subsequent

geopolymer aggregate products. This has been researched and is reported and

discussed in this chapter.

Three fly ash samples from different batches of one particular coal power

station are selected and investigated. It is found that all selected fly ash can

produce a fresh geopolymer mix workable for future mechanical processing.

However, the fresh properties of different mixes are quite different. This

difference is considered to be mainly due to the differences in amorphous SiO2,

Al2O3 and CaO contents in the selected fly ash samples. An evaluation is later

made on the performance of each fresh geopolymer mix against the mechanical

processing, based on which one specific batch of fly ash is preferentially

recommended to be used in later geopolymer aggregate research.

Also, the current study discovers that the hardened properties of geopolymers,

such as density and strength, are also varied for different fly ash samples. This

drives a further topic as to whether an appropriate fly ash selection may

enhance the mechanical behaviour of geopolymer aggregates. This question is

briefly discussed in this chapter. However, a further detailed discussion relating


to the hardened properties of geopolymer will be presented in Chapter 5, in

combination with the results of relevant testings directly related to the produced

geopolymer aggregates.

2.2 Selection and Characterisation of Fly Ash

2.2.1 Selection of Fly Ash Samples

The fly ash samples selected and discussed in this chapter were sourced from

three different production batches during the years 2012-2014 from the Eraring

thermal power station in New South Wales, Australia. The production dates are

2012-Dec-13, 2013-Jul-25 and 2014-Jan-31, which are at approximately 6

month interval. These samples are referred to as ‘FA1’, ‘FA2’ and ‘FA3’

respectively, throughout this chapter. The relevant characteristics of the

selected fly ash samples are researched and discussed in following subsections,

to distinguish each fly ash sample and later relate to the performance of the

fresh geopolymer mixes.

2.2.2 Density, Particle Size and Specific Surface Area

The relative densities of the three fly ash samples FA1, FA2 and FA3 are

measured based on Australian standard AS3583 (AS, 1991) and shown in

Table 2.1. It is seen that the selected fly ashes are not consistent in density,

very likely due to the different combustion and cooling procedures they have

been through (Kutchko and Kim, 2006).

On the other hand, particle size distribution and specific surface area of fly ash

particles were measured using a Malvern Mastersizer 2000 laser diffraction

particle size analyser (Figure 2.1), to check if the selected three fly ash samples

41

are varied in above physical properties as usually found in the fly ash from

different sources (Blissett and Rowson, 2012). The results were automatically

recorded every minute of the one hour test duration. The final results are the

average values of the last 10 records. This is to make the results accurately

reflect the situation of a well dispersed fly ash sample. The particle size and

surface information of FA1, FA2 and FA3 are shown in Table 2.2 and Figure 2.2.

Table 2.1: Relative densities of fly ash

Fly ash Density (kg/m3)

FA1 2210

FA2 2130

FA3 2060

Figure 2.1: Malvern Mastersizer 2000 laser diffraction particle size analyser

In Table 2.2, the parameter of D(x%) is used to describe the general trend in

particle size distribution, meaning the diameter where a certain percentage of

the volume distribution is below it. For example, FA1 has a D(10%)=2.93 µm,

meaning that there are 10% in volume of the total FA1 particles that are finer


than 2.93 µm. The results show that FA1 and FA2 have similar size values for

D(10%) and D(50%) while FA3 has smaller particle size values for these

proportions. Otherwise, FA1, FA2 and FA3 share the similar D(90%) values.

These results indicate that FA3 has a finer particle size distribution than FA1 or

FA2, which can also be seen from the distribution curves in Figure 2.2. This is

also consistent with the higher specific surface area value found for FA3 sample.

So it is seen that the selected fly ash samples have different particle size and

surface conditions, despite that the differences are not significant. As these fly

ash samples are from a same coal source, their different physical properties

might be caused by the different combustion history of them (Blissett and

Rowson, 2012). Nevertheless, all three fly ash samples have a proportion

of >75% particles passing 45µm sieve, indicating that they belong to the same

‘fine’ grade of fly ash as prescribed in Australian standard AS 3582 (AS, 1998f).

Table 2.2: Results of particle size distribution of the chosen fly ash batches

Code Specific surface (m2/g)

D(10%) (µm)

D(50%) (µm)

D(90%) (µm)

Passing 45µm sieve (%)

FA1 1.70 2.93 19.22 70.60 79.44

FA2 1.67 2.57 20.30 73.90 77.77

FA3 2.69 1.43 17.88 71.98 79.42

43

Figure 2.2: Particle size distribution curves of fly ash samples

2.2.3 Loss on Ignition (LOI)

Table 2.3: LOI values of different fly ash samples

Code Production date of fly ash batch

Temperature of combustion (oC)

Duration of combustion (hour) LOI (%)

FA1 2012-Dec-13 750 1 0.97%

FA2 2013-Jul-25 750 1 1.71%

FA3 2014-Jan-31 750 1 0.91%

A standard loss on ignition (LOI) test regulated in Australian standard AS 3583

(AS, 1991) was done to measure residual carbon content in fly ash. All fly ash

samples were firstly oven-dried at 105oC for 24 hours to evaporate the free

water content. After this, the oven-dried fly ash was combusted at 750oC for 1

hour in a Muffle furnace. The mass difference before and after the combustion

was measured to calculate the LOI value. For each fly ash sample, the final LOI

0

1

2

3

4

5

6

0.01 0.1 1 10 100 1000 10000

Perc

enta

ge in

tota

l vol

ume

(%)

Particle size (µm)

FA-1

FA-2

FA-3


value is represented by the average mass loss of three samples prepared and

combusted separately, as shown in Table 2.3. FA2 shows a slightly higher LOI

value than the other two fly ash samples, despite that all three fly ashes have a

low LOI value less than 2%, implying that they are from an almost fully coal

combustion process.

2.2.4 Chemical Oxide Composition (based on XRF)

An X-ray fluorescence (XRF) test was conducted to identify the proportion of

chemical oxides present in fly ash. The chemical composition results obtained

from XRF are shown in Table 2.4, where it is seen that the compositions of the

three fly ash samples are basically similar. This similarity must be due to the

shared coal source and combustion history. SiO2 and Al2O3 possess the

dominating proportions of the selected fly ash samples. The summation of their

proportions exceeds 75% by mass. These two components are the main

reactants participating in the geopolymer reactions (Davidovits, 2008).

Otherwise, there are also several other minerals like Fe2O3, CaO, K2O, TiO2,

etc. commonly found in fly ash, and their proportions in the tested fly ash

batches in this research are far lower than those of SiO2 and Al2O3.

2.2.5 Crystalline and Amorphous Contents (based on XRD)

X-ray diffraction (XRD) was applied to identify the crystalline phases present in

selected fly ash samples, using a PANalytical Empyrean X-ray diffractometer

(Figure 2.3) with Cu Kα radiation at room temperature, and a scan range of 10-

80o 2θ values. Moreover, the proportions of crystalline and amorphous phases

were quantified by X-ray diffraction measurements, where Rietveld refinements

were carried out based on a FullProf suite software (Rodríguez-Carvajal, 2001).

45

A crystalline corundum (α-Al2O3) was added in a mass ratio of 20% of the total

scanned sample (fly ash + corundum) to serve as a spike phase to complete

the quantification (Diaz et al., 2010, Ward and French, 2006). The patterns and

related Rietveld refinements are presented in Figure 2.4.

Table 2.4: Chemical composition of each fly ash sample tested by XRF

Oxides FA1 (wt.%) FA2 (wt.%) FA3 (wt.%)

SiO2 58.60 58.00 57.36

Al2O3 20.20 22.21 22.11

Fe2O3 9.25 7.55 8.13

CaO 4.67 6.09 4.70

K2O 3.02 1.40 3.09

TiO2 2.34 2.79 2.45

SO3 1.04 0.84 1.10

SrO 0.34 0.55 0.49

ZrO2 0.30 0.29 0.26

MnO 0.16 0.14 0.19

Rb2O 0.04 - 0.04

Y2O3 0.04 0.04 0.04

V2O5 - 0.12 -

ZnO - - 0.05

SiO2 + Al2O3 78.80 80.21 79.47

SiO2 / Al2O3 2.90 2.61 2.60

Si/Al (molar) 2.47 2.22 2.21


Figure 2.3: PANalytical Empyrean X-ray diffractometer

In the X-ray diffraction patterns shown in Figure 2.4, the dots denote the test

results. The uppermost solid line is the plot of the theoretical calculations. The

differences between the test and the theoretical results are indicated by the

difference curve shown in the lowermost portion of the diagrams. Agreement

between the experimental and the theoretical results is generally good apart

from slight mismatch near 26.5o which is the vicinity of a number of overlapping

peaks from the different phases. The vertical short bars indicate the Bragg

positions used for refinement according to the Rietveld method, where the three

rows of these bars from top to bottom are corundum, quartz and mullite,

respectively.

47

Figure 2.4: X-ray diffraction patterns and corresponding Rietveld refinement results for fly ash samples FA1, FA2 and FA3


In Figure 2.4, apart from the intentionally added corundum, distinctive amounts

of quartz and mullite are detected. A trace amount of magnetite seems to be

present but the amount is too small to be accurately quantified. The results of

the proportions of crystals and amorphous content in each fly ash sample by

Rietveld quantification are shown in Table 2.5. A complete example of the

calculations applied to the data and used to determine the crystalline and

amorphous contents, is demonstrated in the Appendix A. Fly ash FA1 has the

most crystals as 9.9% of quartz and 19.0% of mullite compared to the other two

fly ashes FA2 and FA3. The total crystalline proportions in FA2 and FA3 are

basically quite similar. Nevertheless, FA2 has a slightly higher mullite while FA3

has a slightly higher quartz.

Table 2.5: Quantification on crystals and amorphous content in fly ash

Items FA1 (wt%) FA2 (wt%) FA3 (wt%)

Quartz 9.9 6.0 6.5

Mullite 19.0 16.6 15.0

Amorphous 71.1 77.4 78.5

Total SiO2 by XRF 58.6 58.0 57.4

Total Al2O3 by XRF 20.2 22.2 22.1

Total SiO2 + Al2O3 by XRF 78.8 80.2 79.5

Amorphous SiO2 43.4 47.3 46.6

Amorphous Al2O3 6.5 10.3 11.3

Amorphous SiO2 + Al2O3 49.9 57.6 58.0

Amorphous SiO2/Al2O3 6.6 4.6 4.1

49

Moreover, considering the previous XRF composition results in combination

with the results obtained from XRD, one can also approximately calculate the

proportion of amorphous SiO2 and Al2O3 present in each fly ash sample. This is

done by subtracting the crystalline SiO2 and Al2O3 from the total amounts of

SiO2 and Al2O3 obtained from XRF results. The calculated amorphous SiO2 and

Al2O3 proportions are thus presented in Table 2.5. FA1 has the least amorphous

SiO2 and Al2O3 at around 49.9%, while the respective quantities for FA2 and

FA3 are quite similar with a value that is nearly 8% higher than FA1. Otherwise,

FA1 has the highest ratio of amorphous SiO2 to amorphous Al2O3 of about 6.6,

which is much higher than that of FA2 or FA3, which is only around 4.1 - 4.6.

2.3 Geopolymer Mixes

A blended alkali solution composed of NaOH and Na2SiO3 was used to activate

the selected fly ash samples for geopolymer synthesis. This alkali solution was

prepared by mixing the ingredients of NaOH flakes (>99% purity), Grade D

Na2SiO3 solution (29.4% SiO2, 14.7% Na2O) and deionised water in specified

proportions as designed in Table 2.6.

In Table 2.6, the mix design for all the mixes has been done such that the fly

ash quantity is maintained at a constant value of 1500 grams, and the same

activator is applied for the mixes made with the three fly ash samples FA1, FA2,

and FA3. The relevant geopolymer mixes are referred to as GEO1, GEO2 and

GEO3 correspondingly. The design of these mixes therefore, allows the

properties of the produced geopolymers to be predominantly influenced by the

unique characteristics of the used fly ash.


The activator was prepared through a two-step method. At first, a 14.1M NaOH

solution as designed was made by dissolving NaOH flakes into deionised water.

This was done manually, using appropriate necessary handling protective

measures. The procedure of NaOH dissolution can release large amount of

heat, hence the prepared NaOH solution was stored for one day prior to mixing

so as to cool down. Meanwhile, the NaOH solution was sealed to prevent any

contact with H2O and CO2 from the atmosphere. The second step was to

combine the solutions of NaOH and Na2SiO3 as designed with a Na2SiO3/NaOH

ratio of 3.2 just before geopolymer synthesis.

Table 2.6: Mixes of fly ash-based geopolymer with different fly ashes

Paste Fly ash Fly ash/ Activator

Mix quantities (grams)

Fly ash NaOH flakes

Na2SiO3 solution

Deionised water

GEO1 FA1 2.05 1500 68.93 556.19 105.27

GEO2 FA2 2.05 1500 68.93 556.19 105.27

GEO3 FA3 2.05 1500 68.93 556.19 105.27

Note: the NaOH flakes and deionised water present in this table are to be combined to prepare a 14.1M NaOH solution, which is used along with Na2SiO3 solution for activation purpose.

The steps of geopolymer sample preparation are displayed in Figure 2.5. The

geopolymer mixes GEO1, GEO2 and GEO3 were made by adding the prepared

activator onto the fly ash samples FA1, FA2 and FA3 respectively. A Hobart

mixer was used to mix the fly ash and activator until a uniform mixture was

achieved. A portion from the obtained fresh geopolymer mixture was tested for

fresh mix properties, and the remaining portion was used to cast samples for

density determination and strength testing. Cubic 50mm×50mm×50mm steel

51

moulds, with polyethylene sheets stuck onto the inner surface to facilitate

demoulding, were used to cast the needed samples. A vibrating table was used

to compact the fresh mixture, and the vibration continued until the surface of

fresh mixture was level and no bubbles appeared. The moulds were then

sealed with a lid and placed in a controlled environment room (ER), at 23±1oC

and 50% relative humidity (RH), for 24 hours before applying a heating curing

regime at 80oC for 72 hours in an electrical oven. After heating, the

geopolymers were demoulded once they cooled down to room temperature,

and then placed in the ER until further density and strength testing.

2.4 Fly Ash’s Influence on Fresh Properties

It is found that the variation in geopolymer mixes caused by different fly ash

batches mainly appears in fresh properties. Two aspects of fresh geopolymer

mixes, namely workability and consistency, are thus focussed on, in this section.

They are normally applied for describing fresh Portland cement, and in current

research, used to reflect the fresh properties of different geopolymer mixes.

Although geopolymer is a different mixture, it exhibits a similar cementitious

property as fresh cement paste mix (Kong and Sanjayan, 2010) and therefore is

thought suitable to resemble fresh Portland cement paste regarding the

performance of workability and consistency. The performance of each fresh mix

is evaluated for a selection of suitable fly ash for geopolymer aggregates.

Moreover, possible reasons causing the observed varied fresh properties are

analysed, to provide information needed for fly ash selection from a broader fly

ash source in future practice.


Figure 2.5: Preparation steps (1) Hobart mixer, (2) well-mixed fresh geopolymer, (3) steel moulds with polyethylene cover, (4) well-compact geopolymer, (5)

geopolymer after demoulding and (6) geopolymer after heating

2.4.1 Slump and Mini-flow Method

A combined slump and mini-flow method was used to evaluate the workability of

fresh geopolymer. This method is carried based on a mini-sized slump cone

(Figure 2.6) whose dimensions are: 19 mm (top diameter), 38 mm (bottom

(1) (2)

(3) (4)

(5) (6)

53

diameter) and 57 mm (height). This cone can be treated as a small-sized

version of the regular slump cone for fresh concrete regulated in Australian

standard AS 1012 (AS, 1993).

Slump value is a common factor for workability evaluation. A slump test was

measured on fresh geopolymers following the same procedure of conventional

slump testing. The result is referred to as ‘initial slump’.

On the other hand, the mini-flow method, which is usually devised for assessing

the effect of air entraining and plasticizing admixtures on Portland cement paste

(Zhor and Bremner, 1998), is to assess the flow of fresh geopolymer under

vibration. Mechanical vibration is quite widely used for geopolymer compaction

in practice (Hardjito and Rangan, 2005, Kong and Sanjayan, 2010) and will be

applied in a geopolymer aggregate manufacturing procedure (to be stated in

Chapter 5). Assessing flow under vibration is therefore needed in order to

investigate whether there is a variation of flow behaviour caused by different fly

ash samples. Apart from this, the author aims to use mini-flow result to manifest

the variations in workability of different mixes, which are relatively less obvious

in the slump test, possibly due to the high viscosity of geopolymers (Criado et

al., 2009).

For mini-flow test, the spread of the fresh geopolymer just after lifting the cone

was first measured and described as ‘initial spread’. The spread measurements

were taken along four different halves of the spread area (Figure 2.7). The

estimated spread value was calculated as the average of the four

measurements. The area of initial spread was then approximately represented

by a circular area whose diameter is the calculated average spread value.


Figure 2.6: Schematic of mini-sized slump cone for slump and mini-flow testings

Figure 2.7: Scheme of spread measurement using the mini-flow method

55

The mini-flow test was also performed to measure the spread of fresh

geopolymer flow upon vibration. To deal with the especially stiff geopolymer

material, a vibrating table (220-240V AC, 50HZ) was used instead of the

conventional flow table. The vibration was applied for 10 seconds. During this

period, the fresh geopolymer, immediately after slump test, was left to flow

freely on the vibrating table. The expansion of spread was measured at three

time points as (1) once the vibration stopped, (2) three minutes after the

vibration stopped, and (3) ten minutes after the vibration stopped. The spread

areas were measured in the same way as explained earlier.

The workability results of the fresh geopolymer mixes made with fly ashes FA1,

FA2 and FA3 are shown in Table 2.7, with images showing the performance of

different geopolymer mixes in Figure 2.8.

Table 2.7: Workability of produced fresh geopolymer mixes

Code

Before vibration After 10-second vibration

Initial slump (mm)

Initial spread (cm2)

Spread at once (cm2)

Spread 3 mins (cm2)

Spread 10 mins

(cm2)

GEO1 0 11.34* 12.34 13.15 13.42

GEO2 20 18.33 39.43 39.80 40.55

GEO3 10 13.50 27.74 28.19 29.19

*11.34cm2 is the bottom area of the mini-sized cone which indicates that there is no spread in this case.


Figure 2.8: Images of geopolymer flows before (left column) and once after (right column) vibration in mini-flow test

In Table 2.7, the geopolymer mixes GEO1, GEO2 and GEO3 display very

different slump behaviours. GEO2 has a relatively high initial slump value of 20

mm. It seems that such mix could be easily compacted with only little externally

applied compaction effort. Differently, GEO1 has almost no slump at all, and

GEO3 has a slump of 10 mm. The initial spread results show a similar trend to

GEO1 GEO1

GEO2 GEO2

GEO3 GEO3

57

that of the slump results. GEO2 has the largest spread area indicating the best

workability, followed by GEO3 which has a lower spread value than GEO2 but

still obvious initial spread. However, GEO1 results in a very stiff mixture without

detectable spread. It may therefore be concluded that geopolymer workability

could be quite different for different batches even if the fly ash used is produced

in the same power station.

Such differences in workability are further manifested after the vibration

treatment in the mini-flow test. The curves describing the increase of

geopolymer spread under vibration are shown in Figure 2.9. From this Figure, it

is seen that GEO2 shows a rapid increase in spread area from 18.33 cm2 to

40.55 cm2 in 10 minutes after vibration. Likewise, GEO3 also has an increase in

spread area from 13.50 cm2 to 29.19 cm2. However, it is found that GEO1 has a

reluctant flow after vibration, with only a slight spread increase from 11.34 cm2

to 13.42 cm2. Thus, it seems that the geopolymers made from fly ashes FA2

and FA3 can be more easily compacted by mechanical vibration.

Nevertheless, with the assistance of mechanical vibration, the geopolymer

GEO1 can also flow with an increasing spread. Although this increase is not

significant after a 10-second mechanical vibration, it is considered that a

prolonged vibration time more than 10 seconds should be useful to enlarge the

spread, and possible to make the mix of GEO1 properly compacted. For this

case, the fresh mix made with fly ash FA1 is expected to be qualified for

undertaking the required mechanical processing in geopolymer aggregate

manufacture. Indeed, this is realised as seen in Chapter 5, section 5.3 where a

prolonged vibration can work to obtain a well-cast geopolymer sample.


Figure 2.9: Spread area increase by mechanical vibration of geopolymer paste

On the other hand, GEO2 presents a very high fluidity, as seen in Table 2.7,

which can be easily compacted even with no need of mechanical vibration.

However, the high fluidity of GEO2 is not primary pursued in a manufacture of

geopolymer aggregates, primarily because this high fluidity may lead to a

negative effect, such as a weak end-product with loose geopolymer structure

(Rangan, 2007). Since all these fly ashes are found to be practical for

geopolymer aggregates, the fly ash FA2 is not selected for its potential risks.

Also, GEO3 seems good with a moderate workability neither too stiff nor too

fluid before vibration, but has a high spread increase after vibration, as seen in

Figure 2.9. The 10-second vibration applied seems to be too long for GEO3,

because it generates a large spread which is more than twice in area than its

original. This is not needed in a geopolymer aggregate manufacture, and a

shortened vibration time may be more appropriate.

0

5

10

15

20

25

30

35

40

45

initial spread spread at once spread in 3mins

spread in 10mins

Spre

ad a

rea

(cm

2 )

GEO1

GEO2

GEO3

59

Among the selected fly ash samples, FA1 and FA3 are therefore considered

more suitable for the further geopolymer aggregate manufacture. Nevertheless,

the author suspected that the geopolymer made with FA3 may deform too fast

under vibration, and is hardly controlled from a perspective of laboratory

research work. In contrast, the fresh geopolymer made with FA1 is not

vulnerable to mechanical force, and leaves enough time for specific operations

and observations. It may be stiff at the beginning, but can be processed with

assistance of a prolonged mechanical vibration. Therefore, according to the

author’s opinion, FA1 is preferably selected for future geopolymer aggregate’s

making, even though both FA1 and FA3 display a workable performance based

on the result of this subsection.

2.4.2 Vicat Plunger Penetration

A standard consistency test was also made on the fresh geopolymers, in order

to provide a further way other than workability test for evaluating the fresh

properties of different geopolymer mixes. Determination of standard consistency

was performed following the method prescribed in Australian standard AS 2350

(AS, 2006), same as for normal Portland cement mix. A Vicat plunger

penetration method was used, and the consistency of fresh geopolymer was

reflected by the distance of plunger penetration, as illustrated in Figure 2.10 and

Table 2.8.


Figure 2.10: Standard Vicat plunger for consistency testing

Table 2.8: Penetration distance of Vicat plunger of fresh geopolymer mixes

Code Penetration distance (mm)

GEO1 2

GEO2 8

GEO3 4

For all geopolymer mixes, the plunger was released at the same height of 42

mm before releasing the plunger, so that the penetration distance shown in

Table 2.8 can be used to assess the consistency of each fresh geopolymer mix.

Otherwise, it is noticed that the penetration value for geopolymer paste is

generally far less than the penetration value for Portland cement paste of

normal consistency, possibly due to the unique high stiffness of fresh

geopolymer. The obtained penetration results shown in Table 2.8 are only used

for comparison between the tested geopolymers.

GEO1 GEO2

GEO3

61

It can be seen that the fresh geopolymers made with different fly ash samples

result in different penetration distances, where the descending order of

resistance to plunger penetration is GEO1>GEO3>GEO2. The GEO1 made

with fly ash FA1 has the highest resistance, indicating the highest stiffness,

which is in accordance with the former workability test. Besides, it is found that

the geopolymer with a higher workability shows a lower resistance to plunger

penetration. This result, on one side, proves again that the fresh properties of

geopolymer can be different when using different fly ash samples. Also, all three

fly ash samples would lead to stiff geopolymer mixes, although their effects are

slightly varied. Mechanical assistance seems to be necessary for further

processing.

2.4.3 Evaluation on Fly Ash’s Characteristics for Fresh Properties

From the above reported results, it can be concluded that the fly ash samples

sourced from different production batches of the same power station would still

result in quite different performance of the produced geopolymers. Such

differences are quite considerable for the fresh properties like workability and

consistency. Knowing that the mix design and curing condition remained the

same for all the mixes, it is reasonable to infer that such differences in

geopolymers are mainly caused by the differences in certain characteristics of

the selected fly ashes, which are researched in this subsection. The outcomes

are expected to update the knowledge of fly ash-based geopolymer, and to

provide the needed information for advising fly ash selection in future

geopolymer technology.


Particle Size and Specific Surface Area

Table 2.2 and Figure 2.2 indicate FA1 and FA2 have similar particle size and

surface conditions. It has been argued that the increase in specific surface area

of the fly ash may result in increased water demand in order to maintain a

certain workability level (Kohno and Komatsu, 1986). Nevertheless, it has also

been argued that due to the spherical nature of fly ash particles, workability

would in fact be enhanced (Khatri et al., 1995, Kwan and Li, 2013). Further

enhancement to the workability would occur due to efficient particle packing,

which may result in void filling leading to making water more available for

lubricating the mixture and hence improving both workability and strength

(Brouwers and Radix, 2005, Isaia et al., 2003). Therefore, based on the

detected physical fly ash characteristics, the geopolymer pastes made from FA1

and FA2 are expected to share similar workability. Such expectation is also

supported by the recent finding that the fly ash’s reactivity for geopolymerisation

is relevant to its particle size and surface information (Kumar and Kumar, 2010).

This however, has not been the case. The geopolymer mixes using FA1 and

FA2 turn out to have significant differences in workability and consistency, as

shown in the results of Table 2.7.

Examination of Table 2.2 and Figure 2.2 also shows that the fly ash FA3 clearly

has a finer particle size distribution and higher specific surface area compared

to FA1 and FA2. This is expected to provide an improved particle reactivity for

FA3 due to its broader surface contact with the activator (Kumar and Kumar,

2010). Table 2.7 shows that the geopolymer GEO3 has a better workability than

GEO1. Such better workability, at least partially, could be owing to the

enhanced particle characteristics of FA3 as discussed above. However,

63

contrary to similar expectation, the paste GEO2 which is made from fly ash FA2,

results in a better workability than GEO3, and indeed far better than GEO1

whose fly ash FA1 is very similar in its physical characteristics to FA2, as seen

from Table 2.2, Figure 2.2 and Table 2.7. This is not expected if workability is

considered to be depending solely on the particle size distribution and specific

surface area values.

The above discussion implies that there should be something else affecting the

development of a geopolymer. Thus it is necessary to also pay attention in this

regard to the role played by the chemical composition and the

crystalline/amorphous proportion in the fly ash.

SiO2, Al2O3 and CaO Contents

The differences in the chemical composition of fly ash samples seem to be quite

influential. It has been previously concluded that the whole geopolymer

synthesis is started by the dissolution from fly ash of SiO2 and Al2O3 as initiated

by alkali activation. Afterwards, the dissolved SiO2 and Al2O3 compounds act as

precursors to trigger subsequent synthetic reactions (Davidovits, 1989,

Davidovits, 2008). Because of this, a higher proportion of active SiO2 and Al2O3

is very likely to lead to a higher degree of early dissolution, which may result in

making this effect dominant factor in shaping the early stage properties (Duxson

and Provis, 2008, Palomo et al., 2004).

With the materials discussed in this investigation, the dissolution of SiO2 and

Al2O3 is most likely to be determined by the unique composition of SiO2 and

Al2O3 contents in the different fly ash samples, as the other conditions of the

synthesis have been intentionally kept constant. A higher amount of SiO2 and


Al2O3 contents would be better for enhancing the geopolymer reactions.

Furthermore, a higher proportion of the amorphous phase would also be

preferred because the amorphous phase has been found to be more reactive

than the crystalline phase (van Jaarsveld et al., 2003, Chen-Tan et al., 2009,

Williams and van Riessen, 2010).

The extreme low workability of GEO1 seems to be associated with the higher

contents of quartz and mullite but lower content in amorphous SiO2 and Al2O3 of

FA1, as seen in Table 2.5. In contrast, the mixes GEO2 and GEO3, which are

from the fly ash samples containing more amorphous SiO2 and Al2O3, result in

much better workability. Whether this characteristic is in any way responsible for

the difference in workability is not certain at this time and should be further

investigated. Nevertheless, workability, which is usually decided by the

characteristics of fluidity, cohesion and viscosity of the fresh mixture, can be

influenced by the solid/liquid proportion of the mixture. A lower solid proportion

is likely to lead to an increased fluidity, and subsequent improved workability.

For the geopolymer mixes here, the initial solid/liquid proportion of the mixture

was intentionally kept the same by using the same mix design for GEO1, GEO2

and GEO3 mixes. Therefore, the workability is expected to be identical until fly

ash particles start to dissolve. When the reactive part of fly ash starts to

dissolve as a result of the activators action, the solids amount in the fresh

mixture reduces in favour of an increase in the amount of solution. The fly ash

containing more reactive contents, for example more amorphous SiO2 and

Al2O3, is therefore expected to dissolve faster and result in the mixture with an

even lower solid proportion. Such dissolution could be fast enough as to affect

the property of early stage mixture, like workability. This argument is supported

65

by the finding that the dissolution of SiO2 and Al2O3 happens almost

immediately after adding the activator (Chen-Tan et al., 2009, Li et al., 2011).

Thus, it may be reasonable to conclude that the mixture using the fly ash

containing more amorphous SiO2 and Al2O3 could quickly gain a lower

solid/liquid proportion immediately after mixing and thus impart higher

workability to the fresh mixture.

Additionally, CaO is another component in fly ash apart from SiO2 and Al2O3

that can influence the properties of fresh geopolymer mixes, like workability and

setting times (van Jaarsveld et al., 2003). Using the leaching test, it has been

found that the dissolution rate of CaO can be very fast, and sometimes even

faster than the dissolution rate of Al2O3 (Lee and Van Deventer, 2002). This

indicates that CaO may also dissolve at the early stage of mixing, increase the

proportion of liquid, and further enhancing the workability of fresh geopolymer in

a similar way as that of amorphous SiO2 and Al2O3 discussed in the previous

paragraph. It may also be supported by the relevant research on the effect of

CaO content for early cement properties (Lea, 1970), despite that more

research is clearly needed in a geopolymer system. From Table 2.4, it is seen

that FA2 has a slightly higher CaO content than the other two fly ash samples.

This may explain the better workability of GEO2 compared to GEO3, despite

the fact that FA2 and FA3 share a similar content of amorphous SiO2 and Al2O3.

2.5 Fly Ash’s Effect on Hardened Properties

Although this chapter mainly focuses on fly ash and its influence on fresh

properties, a finding on the different densities and strengths present in produced

geopolymers draws a further notice on fly ash’s effect on hardened properties.


For the purposes of a supplementary illustration on fly ash variability and a

secondary suggestion for fly ash selection, this finding is reported and briefly

discussed in this section. More detailed discussions on hardened geopolymers

will be made in relation to geopolymer aggregate materials in following chapters.

The density of air-dried hardened geopolymer was measured by dividing its

mass by its volume just before performing compressive strength testing, which

was done using a TECNOTEST compression testing machine (3000 kN

capacity) (Figure 2.11). The loading rate was set constant at 0.33 MPa/sec,

based on the recommended loading rate range in US standard ASTM C109

(ASTM, 2012c). The strength value for every geopolymer mix was represented

by the average value for three samples.

The density and compressive strength values are shown in Table 2.9. GEO2

and GEO3 present similar air dry density values, while GEO1 yields a higher air

dry density compared to the former two mixes. On the other hand, GEO1 has

the highest 7-day compressive strength, of nearly 53 MPa, while GEO2 and

GEO3 share the lower 7-day strength values of around 40 MPa. It is observed

that the hardened geopolymer products originated from different fly ash

samples are different in density and strength, which is likely to be caused by the

varied fly ash characteristics, since the applied mix designs and curing regimes

are same for all mixes.

67

Figure 2.11: Compressive strength testing on a 3000 KN TECNOTEST machine

Table 2.9: Air dry density and compressive strength of produced geopolymers

Code Fly ash Air dry density (kg/m3)

Compressive strength (7d) (MPa)

Standard deviation

GEO1 FA1 1690 52.94 1.59

GEO2 FA2 1630 39.22 0.41

GEO3 FA3 1630 41.05 1.27

It has been expected that a mix using a more reactive fly ash should receive an

improved formation of geopolymer structure, and therefore a more enhanced

strength. Yet the results of strength of the mixes made from FA2 and FA3 are

actually lower than that of FA1, as seen from Table 2.9. Hence, it is estimated

that some other factors may also influence the strength.


As seen in Table 2.5, the amorphous SiO2/Al2O3 ratios are 6.6, 4.6 and 4.1 for

FA1, FA2 and FA3 respectively. At the same time, it is noticed that the strength

values for the three ash mixes are approximately 53 MPa, 39 MPa and 41 MPa

respectively. While the difference between the strength relating to FA2 and FA3

cannot be said to be significant, it can be said quite comfortably that the

strength difference relating to FA1 when compared with that of FA2 or FA3 is

significant. The author attributes the reason for this strength superiority of FA1

to the relatively high amorphous SiO2/Al2O3 ratio. Indeed, other researchers

have found this ratio to affect the strength of geopolymers (De Silva et al., 2007,

Palomo et al., 2004).

The low strength of GEO2 may also be related in part to the high carbon

content in the original fly ash FA2, which is higher than FA1 and FA3 as seen in

Table 2.3. The unburned carbon is much more porous than the other minerals

present in fly ash (Blissett and Rowson, 2012). Too high carbon content in fly

ash is likely to absorb much of the added alkali activator and thus inhibit the

progress of geopolymerisation, which is likely to reduce the strength of

produced geopolymer (Diaz et al., 2010).

On the other hand, even though it has been once reported that CaO content

can react to form the calcium-containing hydrates and thus enhance the

strength of geopolymers (van Jaarsveld et al., 2003), the effect of CaO is not

significant in the mixes of this research, possibly due to a minor variation in

CaO proportions among the selected fly ash samples (Table 2.4).

Moreover, for the geopolymer mixes discussed here, it seems that the

proportions of amorphous and crystalline phases in fly ash may less affect the

69

hardened properties, despite that they are thought to be the major reasons for

different fresh properties in previous sections. It has been reported that less

amorphous content in fly ash would lead to relatively insufficient reactants

dissolved for geopolymerisation, and thus result in lower strength. The crystals,

which are not reactive, have been believed to almost not contribute to

geopolymer formation (Bakharev, 2006, Rickard et al., 2011). The findings in

previous sections prove the low reactivity of crystals in the fly ash. However, it is

questionable that, after a long curing period, whether these crystals do not show

evidence of having reacted efficiently enough to provide significant increase in

strength and whether the notion that crystalline formations remain more or less

inert even if the geopolymer is subjected to a long curing regime is correct.

Table 2.10 shows the difference that occurred in the crystalline phase between

its original content in the fly ash and the final content in the geopolymer, by

conducting X-ray diffraction supplemented by Rietveld quantification process.

This is illustrated by comparing the different crystalline quantities between the

pre-mixed ingredients (including fly ash and activator solids) and the final

geopolymer, as seen in Table 2.10.

The summation of crystals in the pre-mixed ingredients before

geopolymerisation is 24.1%, 18.8% and 17.9%, for GEO1, GEO2 and GEO3

respectively, which are greatly varied. Despite this, the crystalline proportions of

the final geopolymers are reduced to be 15.6%, 14.5% and 14.2%, for GEO1,

GEO2 and GEO3 respectively. These indicate that the crystals could also be

activated, which agrees a previous finding of the author that the mullite in fly

ash can be reacted if given suitable conditions (Xie and Kayali, 2014), despite

the fact that they are inert in most cases (Fernández-Jiménez and Palomo,


2003). The activated crystals, which are similarly composed of Si and Al

compounds, must have been transformed into geopolymer products in a

process similar to that of the amorphous phase, albeit in a relatively slow

reaction rate. This is expected to further contribute to geopolymer formation.

Table 2.10: Quantification results on crystals and amorphous content before and after geopolymerisation

Original fly ash Phase

Fly ash + activator solids

wt.%

Geopolymer wt.%

Difference wt.%

FA1

Amorphous 75.9 84.3 +8.5

Quartz 8.3 6.1

Mullite 15.8 9.5

Sum of crystals 24.1 15.6 -8.5

FA2

Amorphous 81.2 85.5 +4.3

Quartz 5.0 4.0

Mullite 13.8 10.5


FA3

Amorphous 82.1 85.8 +3.7

Quartz 5.4 5.1

Mullite 12.5 9.1


Besides, the degrees of crystal activation occurring in the different mixes are

different, as shown in Table 2.10 where the quantity of activated crystals is

8.5%, 4.3% and 3.7% for GEO1, GEO2 and GEO3 respectively. The fly ash

with a higher crystalline proportion results in a higher quantity of activated

crystals. This indicates that the fly ash with less amorphous content would not

71

necessarily lead to insufficient geopolymerisation, but may undergo a rather

higher degree of crystal activation that further contributes to geopolymer

formation. This would most likely be influenced by the activators’ strength and

nature as well as the curing conditions.

This research on density and strength performance of geopolymer mixes

provides advisory information for fly ash selection. As introduced in Chapter 1, a

geopolymer aggregate is designed with a low density and good strength

behaviour. In this research, the hardened geopolymers have an air dry density

of 1600-1700 kg/m3, which is in a low density range if it is used as an aggregate

(Clarke, 1993). Meanwhile, this geopolymer material can result in a

compressive strength of around 40-50 MPa, which is considered acceptable for

a load bearing material in constructions. These facts indicate that the

geopolymer materials proposed here should be suitable for producing

geopolymer aggregates.

Among the three fly ash samples, the fly ash FA1 is likely to be recommended

because of the superior strength of its geopolymer product. Nevertheless, the

geopolymers made with FA2 and FA3 are not significantly different in

mechanical properties. However, the practicality of fly ash FA1 for geopolymer

aggregates still needs a further discussion, and this will be made in Chapter 5 to

specifically examine the physical and mechanical development of the

geopolymer aggregate samples made from FA1.

2.6 Conclusions

This chapter presents an important component of the research reported in this

thesis. This component is meant to develop the current understanding of the


role of fly ash in geopolymerisation. It leads to the conclusion that the fresh

properties of geopolymer, such as workability and consistency, can be very

sensitive to the characteristics of fly ash batch applied, even if the ash source is

not changed. This conclusion has not been previously reported in literature

dealing with geopolymer manufacturing issues. This is however, as indicated by

the experimental results of this chapter, must be an issue of concern to be

considered before conducting the subsequent geopolymer aggregate research.

Based on the research and discussion presented in this chapter, one specific fly

ash batch is selected, for the consistency of fly ash properties, and for the

performance of its subsequent fresh geopolymer product which is considered

suitable for the further geopolymer aggregate manufacture. This fly ash is the

one produced on Dec 13th, 2012 from Eraring thermal power station. It will be

used as the fly ash precursor for geopolymerisation involved in this research

and reported and discussed in the following chapters.

73

Chapter 3

Geopolymer Mix Design

3.1 Introduction

This chapter researches a systematic mix design procedure which confirms the

proportions of fly ash, NaOH flakes, Na2SiO3 solution and water needed for a fly

ash-based geopolymer mix.

In this mix design procedure, the above mentioned ingredients are separated

into two categories as fly ash and activator. A chemical formula

Na2O·xSiO2·yH2O is used to express the activator during the mix design

procedure. Meanwhile, another parameter which is water-to-geopolymer solids

ratio (W/G ratio) is used to inter-relate the use of fly ash with that of the activator.

For this case, three criteria, namely; SiO2/Na2O, H2O/Na2O and W/G, are

proposed to distinguish each mix and to calculate the final mix design, which is

a feature of the mix design procedure presented in this chapter. Otherwise, the

factors conventionally related with a geopolymer mix, such as the molarity of

NaOH solution and the mass ratio of Na2SiO3/NaOH, can be obtained from the

outcome of this mix design procedure. This mix design procedure is able to fulfil

the need of current research and will be used for the manufacture of

geopolymer materials discussed in the remaining chapters of this thesis.

3.2 Materials and Mix

The applied ingredients for geopolymer mix include fly ash, NaOH flakes,

Na2SiO3 solution and water. Among them, fly ash is from the 2012-Dec-13 fly

ash batch obtained from the Eraring thermal power station, whose

74 3. Geopolymer mix design

characteristics are detailed in Chapter 2; ‘NaOH flakes’ is a commercial product

with a purity of >99%; Na2SiO3 is sourced from a Grade D silicate solution (29.4%

SiO2, 14.7% Na2O); water is deionised water generated from a laboratory water

purification machine. These ingredients comply with the normal synthetic

method for a fly ash-based geopolymer (Davidovits, 2008).

The geopolymer mix follows a general three-step mixing procedure (Hardjito

and Rangan, 2005, Rangan, 2007) which is shown in Figure 3.1: 1) a NaOH

solution with a certain molarity is first prepared by dissolving NaOH flakes in the

correct amount of water; 2) this NaOH solution is then combined with a Grade D

Na2SiO3 solution to form the activator; and 3) this activator is later poured on

the fly ash to activate it for geopolymerisation to take place.

Also, the following aspects need to be noticed when processing above mixing

procedure. First, the NaOH solution made from a dissolution of NaOH flakes is

not immediately used, but has to be stored in sealed condition for a certain

period before the mix is made (Arioz et al., 2012). This is because the

dissolution of NaOH flakes can release large amount of heat, and therefore

requires a certain period to cool down. On the other hand, this NaOH solution is

not pre-mixed with the Grade D Na2SiO3 solution. These two solutions are only

combined just before the start of the activation of fly ash for geopolymerisation,

i.e. the third step of above mixing procedure. This is in order to avoid a possible

transformation in the ratio of SiO2/Na2O in the used Na2SiO3 solution if earlier

pre-mixing it with NaOH solution was done (Iler, 1979, Nauman and Debye,

1951, Engelhardt et al., 1975).

75

Figure 3.1: Mixing procedure for fly ash-based geopolymer

Moreover, it needs to be stated that, the ingredients and mixing procedure

discussed in this section are for the real practical work of geopolymer mix, and

not relevant to the mix design procedure. In the mix design procedure that will

be introduced later, some other criteria are designed and used for calculations,

but they would not affect the process of practical work. After the completion of

mix design procedure, the ingredients shown in the final mix design are still

processed following the steps in Figure 3.1.

3.3 Terminology

This section introduces several terms used in a systematic geopolymer mix

design procedure for differentiating among geopolymer mixes.

3.3.1 Fly Ash/Activator Mass Ratio (F/A)

The F/A parameter is used to define the relationship between the fly ash, which

refers to the mass of fly ash particles in use, and the activator which refers to

the mass of the activator composed of NaOH and Na2SiO3 solutions. It is

provided mainly to facilitate the production of a convenient and practical mix,

and to make it easy to repeat mixes for research purposes.


3.3.2 Activator Composition Na2O·xSiO2·yH2O

On the other hand, the composition of the alkali activator (NaOH and Na2SiO3

solutions) is expressed in a chemistry-based way, as suggested by Provis et al.

(2009), that is, expressed in the form of mole ratios such as Na2O·xSiO2·yH2O,

where x and y stand for the mole ratios to be designed. The values of

SiO2/Na2O and H2O/Na2O can be treated as criteria which are referred by users

so as to create new geopolymer designs as required.

In the formula Na2O·xSiO2·yH2O, retaining Na2O as one mole results in x and y

being the values of the mole ratios of SiO2/Na2O and H2O/Na2O respectively.

By providing fixed values of x and y when specifying the mix design, the

composition of the activator is able to be determined while the amounts of

NaOH, Na2SiO3 and water required can be calculated later.

3.3.3 Water-to-geopolymer Solids (W/G) Ratio

A further criterion, which is the W/G ratio proposed by Rangan (2007), is used

to build another connection between the activator and fly ash apart from the F/A

to truly reflect the function of the internal components in the activator.

In the W/G ratio, water (W) refers to the mass of H2O present in the activator,

as in Na2O·xSiO2·yH2O, while geopolymer solids (G) refer to the total mass of

the materials of the geopolymer that are in the solid phase (which include the fly

ash) and the solid parts in the liquids (the solutes), that is: the solid masses of

fly ash, Na2O and SiO2.

From these definitions, it is clear that the W/G value highlights the role of H2O in

the activator Na2O·xSiO2·yH2O while the other components, Na2O and SiO2,

77

are reflected by known x and y values. The W/G ratio is used to correlate these

components with the amount of fly ash to be mixed to confirm the amounts of

ingredients required for a given mass of fly ash or given volume of the total

mixture and complete the final mix design.

For users who apply this ratio in their design, the W/G ratio needs to be

specified at the beginning, which is as significant in terms of workability and

strength as is the water to cement ratio for a regular Portland cement mix. An

appropriate W/G ratio is able to be obtained from the previous work of Hardjito

and Rangan (Rangan, 2007, Hardjito and Rangan, 2005) but needs to be

further checked in trial tests of specific raw materials.

3.4 Mix Design Procedure

A flowchart of the mix design procedure for geopolymer mix is presented in

Figure 3.2. In this figure, the mix design procedure is separated into two stages

with required steps linked by solid lines. Besides, the squares indicating the

known information by users are linked to these steps by dashed lines. In stage

1, the activator is prepared based on the designed criteria SiO2/Na2O (x value)

and H2O/Na2O (y value) for the activator Na2O·xSiO2·yH2O. Then, in stage 2,

the W/G ratio is specified, the amounts of NaOH, Na2SiO3 and water required

for the target mass of the fly ash to be activated are calculated and the value of

F/A is obtained to guide the practical work.

Figure 3.2: Flowchart of procedure for geopolymer mix designs


To better illustrate this procedure for creating a geopolymer mix design, the

following examples are provided.

Stage 1 - suppose the values of x and y are specified as x=1 and y=13, and

hence the chemical formula of the activator is expressed as Na2O·SiO2·13H2O.

From knowledge of chemistry, one mole of Na2O·SiO2·13H2O is composed of

one mole of Na2O, one mole of SiO2 and thirteen moles of H2O.

The molar mass values of the components Na2O, SiO2 and H2O can be

calculated based on the known molar mass values of their constituent elements

which can be checked from the periodic table of elements. It is thus calculated

that their molar mass values are 62 grams/mole, 60 grams/mole and 18

grams/mole respectively. Therefore, the total molar mass value of

Na2O·SiO2·13H2O is:

62 ∗ 1 + 60 ∗ 1 + 18 ∗ 13 = 356 grams/mole

This means that, for 1 mole of Na2O·SiO2·13H2O, the real mass is:

356 gramsmole� ∗ 1 mole = 356 grams

As, of the 356 grams of the activator Na2O·SiO2·13H2O, there are 62 grams of

Na2O, the mass proportion of Na2O to the total Na2O·SiO2·13H2O can be

obtained as:

Na2O:

62356

= 0.1742

Similarly, the mass proportions of SiO2 and H2O are calculated as:

81

SiO2:

60356

= 0.1685

H2O:

13 ∗ 18356

= 0.6573

The above mass proportions are corresponding to one mole of the activator

Na2O·SiO2·13H2O as 356 grams in total. Such mass proportions are not

changed once the activator’s composition is confirmed, that is, once the x and y

values are decided, even if the activator’s total amount is different. Therefore,

they can be used to calculate the required amount of each ingredient for any

specific amount of activator.

Therefore, if we need to prepare 100 grams of the activator Na2O·SiO2·13H2O,

the masses of the components Na2O, SiO2 and H2O are:

Na2O:

100 ∗ 0.1742 = 17.42 grams

SiO2:

100 ∗ 0.1685 = 16.85 grams

H2O:

100 ∗ 0.6573 = 65.73 grams

The three components Na2O, SiO2 and H2O come from the three ingredients

NaOH flakes, Na2SiO3 solution and deionised water. Among these three


ingredients, the Na2SiO3 solution is the only source of SiO2. Thus, the 16.85

grams of SiO2 must come solely from it. As introduced earlier, the applied

Grade D Na2SiO3 solution is composed of 29.4% SiO2, 14.7% Na2O and 55.9%

water in mass. Therefore, the amount of Na2SiO3 solution required for 100

grams of the activator is:

16.8529.4%

= 57.31 grams

There are two sources for the component Na2O; the Na2SiO3 solution and

NaOH flakes. As previously mentioned, the applied Na2SiO3 solution has a

proportion of 14.7% Na2O. Therefore, the amount of Na2O coming from it can

be calculated as:

57.31 ∗ 14.7% = 8.42 grams

Then, the remaining Na2O should come from the NaOH flakes. As 17.42 grams

of Na2O in total are required, the amount provided from the NaOH flakes is:

17.42 − 8.42 = 9 grams

The amount of NaOH flakes required to provide these 9 grams of Na2O

depends on the Na2O concentration of the specific NaOH flake product. As the

product used in this research can be treated as pure NaOH, the Na2O

concentration found above (that is the 9 grams in this example) is equivalent to

the Na2O proportion specified in the chemical formula NaOH. The chemical

formula NaOH can be rewritten as Na2O·H2O to clearly indicate the proportion

of Na2O which can be calculated based on the known molar mass values of

Na2O (62 grams/mole) and H2O (18 grams/mole) as:

83

6262 + 18

= 0.775

Then the amount of NaOH flakes required is:

90.775

= 11.61 grams

Finally, the amount of deionised water can be calculated by subtracting the

above two ingredients from 100 grams of the total activator amount as:

100 − 57.31 − 11.61 = 31.08 g

This amount of deionised water is the water that needs to be added to the 11.61

grams of NaOH flakes so that the resulting NaOH solution plus the 57.31 grams

of Na2SiO3 solution will give us the required initial 100 grams of activator whose

specified x and y were 1 and 13 respectively.

From the above we can see that, in order to prepare 100 grams of the

activator Na2O·SiO2·13H2O, the amounts of the ingredients should be

11.61 grams of NaOH flakes, 57.31 grams of Grade D silicates solution and

31.08 grams of deionised water.

The above calculations could also be used as a standard procedure for

preparing the activator for a geopolymer mix. The only variables are the values

of x and y in the activator, and the compositions of the applied ingredients which

can all be designed or acquired by users.


Stage 2 - suppose that a geopolymer mix with a higher workability is required.

Based on relevant knowledge 1, the W/G value is set to be 0.26 while the

activator Na2O·SiO2·13H2O introduced in the previous example is still used.

Suppose that the prepared activator aims to activate 1000 grams of fly ash for

geopolymer manufacture. From the definition of the W/G value, we can see that:

(WG� ) =

mass (H2O)mass (fly ash) + mass(Na2O) + mass(SiO2) = 0.26

(where the ‘mass (H2O)’ is the mass of the H2O in the formula

Na2O·SiO2·13H2O which is referred to here as ‘the activator’)

If we simultaneously divide the numerator and denominators in the middle

portion of this equation by ‘mass(Na2O·SiO2·13H2O)’, which is indeed the mass

of the alkali activator, this equation becomes:

�WG� �

=

mass (H2O)mass(Na2OˑSiO2ˑ13H2O)

mass (fly ash)mass(Na2OˑSiO2ˑ13H2O) + mass(Na2O)

mass(Na2OˑSiO2ˑ13H2O) + mass(SiO2)mass(Na2OˑSiO2ˑ13H2O)

= 0.26

From the definition of F/A, we can see that:

mass (fly ash)mass(Na2OˑSiO2ˑ13H2O) = (F

A� )

Therefore, one of the denominators in the equation can be simplified and the

whole equation written as:

1 Gained from author’s experience and previous report of Rangan (2007)

85

�WG� � =

mass (H2O)mass(Na2OˑSiO2ˑ13H2O)

(FA� ) + mass(Na2O)

mass(Na2OˑSiO2ˑ13H2O) + mass(SiO2) mass(Na2OˑSiO2ˑ13H2O)

= 0.26

where the other numerators and denominators are actually the mass

proportions of Na2O, SiO2 and H2O in the activator. As discussed in the

previous paragraphs of this example, the values of these proportions are equal

to the molar mass value proportions of Na2O, SiO2 and H2O in the chemical

formula Na2O·SiO2·13H2O as:

Na2O:

mass (Na2O)mass(Na2OˑSiO2ˑ13H2O) =

molar mass (Na2O)molar mass (Na2OˑSiO2ˑ13H2O) =

62356

= 0.1742

SiO2:

mass (SiO2)mass(Na2OˑSiO2ˑ13H2O) =

molar mass (SiO2)molar mass (Na2OˑSiO2ˑ13H2O) =

60356

= 0.1685

H2O:

mass (H2O)mass(Na2OˑSiO2ˑ13H2O) =

molar mass (H2O)molar mass (Na2OˑSiO2ˑ13H2O) =

13 ∗ 18356

= 0.6573

Therefore, the previous equation for the W/G can be simplified as:

�WG� � =

0.6573�F

A� � + 0.1742 + 0.1685= 0.26

Then,

�FA� � =

0.65730.26

− 0.1742 − 0.1685 = 2.1854


From this, we can obtain the amount of activator needed for 1000 grams of fly

ash as:

10002.1854

= 457.58 grams

From the previous paragraphs in this example, it is known that 11.61 grams of

NaOH flakes, 57.31 grams of Grade D silicates and 31.08 grams of deionised

water are required for 100 grams of the activator Na2O·SiO2·13H2O. Therefore,

the amounts needed for 457.58 grams of such an activator are:

NaOH flakes:

457.58 ∗11.61100

= 53.13 grams

Na2SiO3 solution:

457.58 ∗57.31100

= 262.24 grams

Deionised water:

457.58 ∗31.08100

= 142.22 grams

If the values of x and y are changed, we can still obtain the required results after

following the calculation steps indicated in the previous paragraphs of this

example to obtain the amounts of the ingredients needed for 100 grams of the

activator. Thereafter we can follow the steps detailed above.

We have therefore established that , if we want to activate 1000 grams of fly

ash with a W/G value of 0.26 and use the activator expressed as

Na2O·SiO2·13H2O, the amounts of the other three ingredients required are

87

53.13 grams of NaOH flakes, 262.24 grams of Grade D silicates and 142.22

grams of deionised water. Meanwhile, for this case, the F/A ratio is nearly

2.19.

The above calculations are then expressed in units of kg/m3 in the final mix

design which indicates the mass required per 1m3 of the mix. This is more

suitable for practical use which requires a mix design to be simple and direct.

Based on the known density values, each ingredient’s volumetric proportion can

be calculated and its mass per 1m3 volume of the mix obtained.

From known information, the densities of applied fly ash, NaOH flakes, Grade D

Na2SiO3 solution and deionised water are set to be 2210 kg/m3, 2130 kg/m3,

1520 kg/m3 and 1000 kg/m3 respectively.

The volume of 1000 grams of fly ash is:

10002210 ∗ 1000

= 4.52 ∗ 10−4 m3

The volume of 53.13 grams of NaOH flakes is:

53.132130 ∗ 1000

= 2.50 ∗ 10−5 m3

The volume of 262.24 grams of Na2SiO3 solution is:

262.241520 ∗ 1000

= 1.73 ∗ 10−4 m3

The volume of 142.22 grams deionised water is:

142.221000 ∗ 1000

= 1.42 ∗ 10−4 m3


Therefore, the volumetric proportion of fly ash is:

4.52 ∗ 10−4

(4.52 ∗ 10−4 + 2.50 ∗ 10−5 + 1.73 ∗ 10−4 + 1.42 ∗ 10−4)= 0.571

Similarly, the volumetric proportions of other three ingredients are:

NaOH flakes

2.50 ∗ 10−5

(4.52 ∗ 10−4 + 2.50 ∗ 10−5 + 1.73 ∗ 10−4 + 1.42 ∗ 10−4)= 0.032

Na2SiO3 solution

1.73 ∗ 10−4

(4.52 ∗ 10−4 + 2.50 ∗ 10−5 + 1.73 ∗ 10−4 + 1.42 ∗ 10−4)= 0.218

Deionised water

1.42 ∗ 10−4

(4.52 ∗ 10−4 + 2.50 ∗ 10−5 + 1.73 ∗ 10−4 + 1.42 ∗ 10−4)= 0.179

The ingredients’ volumetric proportions shown above are also their volumetric

proportions in a 1 m3 geopolymer mix. Therefore, their mass quantities in the

mix can be calculated as:

Fly ash

0.571 ∗ 2210 = 1261.91 kg/m3

NaOH flakes

0.032 ∗ 2130 = 68.16 kg/m3

Na2SiO3 solution

89

0.218 ∗ 1520 = 331.36 kg/m3

Deionised water

0.179 ∗ 1000 = 179.00 kg/m3

Hence, we can obtain a complete mix design for a geopolymer mix using

an activator Na2O·SiO2·13H2O with a W/G value of 0.26. This mix design is

shown in Table 3.1 and requires 1261.91 kg/m3 of fly ash, 68.16 kg/m3 of

NaOH flakes, 331.36 kg/m3 of Grade D Na2SiO3 solution and 179.00 kg/m3

of deionised water. The calculation steps discussed here also work for other

geopolymer mixes with various W/G values and/or activator compositions.

Therefore a systematic geopolymer mix design procedure can be established.

Table 3.1: Final mix design of the geopolymer mix example (kg/m3)

W/G value

SiO2/Na2O (x value)

H2O/Na2O (y value)

F/A value

Mix design (kg/m3)

Fly ash NaOH flakes

Na2SiO3 solution

Deionised water

0.26 1.00 13 2.19 1261.91 68.16 331.36 179.00

3.5 Molarity of NaOH and Mass Ratio Na2SiO3/NaOH

In the mix design procedure presented in the last section, the criteria used to

distinguish each geopolymer mix, and therefore required to be designed by

users, are SiO2/Na2O, H2O/Na2O and W/G values. However, in a conventional

geopolymer mix design, the factors of the molarity of NaOH solution and the

mass ratio of Na2SiO3/NaOH are often important to be defined (Rangan, 2007,

Kong and Sanjayan, 2010, Chindaprasirt et al., 2007) as they more directly

describe the solutions used in a practical mix as shown in Figure 3.1. Therefore,


the information of these two factors under the previous mix design procedure is

discussed in this section.

The molarity of NaOH solution and the mass ratio of Na2SiO3/NaOH can be

calculated from the final mix design made from the previous mix design

procedure, as illustrated by following example:

As introduced, a complete mix design is obtained and shown in Table 3.1 for

the example proposed in section 3.4 which, uses an activator Na2O·SiO2·13H2O

with a W/G value of 0.26. The ingredients in this mix design are 1261.91 kg/m3

of fly ash, 68.16 kg/m3 of NaOH flakes, 331.36 kg/m3 of Grade D Na2SiO3

solution and 179.00 kg/m3 of deionised water.

As displayed in Figure 3.1, the amount of NaOH solution practically used in a

geopolymer mix is a sum of the amounts of NaOH flakes and deionised water,

which is:

68.16 + 179.00 = 247.16 kg/𝑚𝑚3

Therefore, the mass proportion of NaOH solids in this NaOH solution is:

68.16247.16

∗ 100% = 27.58%

Using a chemistry manual it is able to check the molarity of a NaOH solution

which has a mass proportion of 27.58% is 8.7M (specified at a condition of 20oC

and 101.3 kPa atmospheric pressure) (ZXXK, 2012).

On the other hand, the mass ratio of Na2SiO3/NaOH can be calculated based

on the masses of Na2SiO3 (331.36 kg/m3) and NaOH (247.16 kg/m3):

91

331.36247.16

= 1.34

From the above, it is now shown that for a geopolymer mix designed with

SiO2/Na2O=1, H2O/Na2O=13 and W/G=0.26, the molarity of NaOH solution is

8.7M and the mass ratio of Na2SiO3/NaOH solutions is 1.34. These results

can describe the true nature of the used NaOH and Na2SiO3 solutions in a

practical geopolymer mix. Moreover, they can be used to compare with other

literature for scientific analysis as well as practical purposes.

Also, it is found out that the values of above factors, such as the molarity of

NaOH solution and the mass ratio of Na2SiO3/NaOH, are different for different

geopolymer mixes, as seen from some examples presented in Table 3.2 made

based on the mix design procedure in section 3.4.

In the first 5 mixes of Table 3.2, the values of SiO2/Na2O (x value) and W/G are

set constant, but the value of H2O/Na2O (y value) is varied from 11 to 15. It is

observed that a higher y value results in a higher free water usage, and a lower

molarity of NaOH solution. The mix with y=11 obtains the highest molarity as

12.5M while the mix with y=15 obtains the least molarity as 6.8M. On the other

hand, a higher y value also leads to a lower Na2SiO3/NaOH ratio, going down

from 1.76 to 1.06 for y=11 and y=15, respectively. It is thus concluded that the

molarity of NaOH solution and Na2SiO3/NaOH ratio are affected by the criterion

of H2O/Na2O used in the mix design procedure.

Moreover, for the mixes No.2, No.6 and No.7, the values of y and W/G are the

same while the value of x is varied. It is seen that a higher x value results in an


obvious increase in the mass ratio of Na2SiO3/NaOH, which is reasonable since

x value determines the ratio of SiO2/Na2O in the activator.

Otherwise, for the mixes No.3, No.8 and No.9, the values of x and y are kept

the same while the value of W/G is changed from 0.26 to 0.22. These three

mixes are found to have the same molarity of NaOH solution as 8.7M, and the

same mass ratio of Na2SiO3/NaOH as 1.34, despite their different W/G ratios.

This seems due to the fact that, the activator’s composition is the same

because of the same x and y values were fixed for these three mixes. So the

NaOH and Na2SiO3 solutions used to compose this activator will also be the

same for these mixes, and not affected by the variation in W/G ratio.

The above finding provides a further knowledge that the geopolymer mixes

designed with different W/G ratios could still be able to end up with the same

molarity of NaOH solution only if they use the same x and y values. For this

case, the variation in W/G ratio will only reflect on the relationship between the

fly ash and activator. Moreover, W/G can be treated as an independent factor

when studying the development of a geopolymer, where any effect on its

properties that may be observed from a change of W/G ratio, is not caused by

any change in NaOH molarity.

Table 3.2: Molarity of NaOH solution and mass ratio of Na2SiO3/NaOH for different geopolymer mix designs

No. SiO2/Na2O (x value)

H2O/Na2O (y value) W/G Fly ash

(kg/m3) NaOH flakes

(kg/m3) Na2SiO3 (kg/m3)

Water (kg/m3)

NaOH solution molarity (M)

Na2SiO3/NaOH mass ratio

1 1.0 11 0.26 1235.44 79.74 394.27 144.16 12.5 1.76

2 1.0 12 0.26 1250.08 72.85 360.00 163.34 10.4 1.52

3 1.0 13 0.26 1261.91 68.16 331.36 179.00 8.7 1.34

4 1.0 14 0.26 1272.81 62.03 306.61 193.23 7.5 1.20

5 1.0 15 0.26 1281.81 57.73 285.43 205.11 6.8 1.06

6 1.25 12 0.26 1232.49 56.19 453.21 117.77 10.8 2.61

7 1.5 12 0.26 1214.76 39.25 547.88 71.46 12.1 4.95

8 1.0 13 0.24 1311.75 63.51 313.86 170.15 8.7 1.34

9 1.0 13 0.22 1363.93 59.82 295.62 160.27 8.7 1.34


3.6 An Alternative Mix Design Method – using NaOH Solution

instead of NaOH Flakes

A workable mix design procedure is discussed in previous sections, which can

successfully create geopolymer mixes for research purposes. The created mix

design consists of the mass proportions of fly ash, NaOH flakes, Na2SiO3

solution and water for 1 m3 fresh geopolymer mix.

In this section, a modified mix design is proposed, which uses a NaOH solution

instead of NaOH flakes to be the source of NaOH in the final mix design, for

safety and efficiency concerns. As seen from previous sections, the practical

mix requires an activator composed of NaOH and Na2SiO3 solutions (Figure

3.1). Meanwhile, the ingredients used in the previous mix design are fly ash,

NaOH flakes, Na2SiO3 solution and water (Table 3.1). For this case, a step of

dissolving NaOH flakes by water is needed to prepare a required NaOH solution,

which is a very hazardous step, and always has to be repeated to create

different molarities for scientific or other purposes. This step of the dissolution of

NaOH flakes can even release a large amount of heat which requires a strict

protection and a long period to cool down (Xie and Kayali, 2014). Because of

the above concerns, the author proposes a modified mix design here so as to

minimise repeating this hazardous step every time a mix is made.

To clearly illustrate the above idea, a modified mixing procedure is created and

shown in Figure 3.3. This Figure is different from the old one shown in Figure

3.1, in that a NaOH solution which has a sufficiently high molarity is firstly

created from NaOH flakes by water dissolution, and acts as the source (stock)

of NaOH. Then, this high molarity NaOH solution is used along with water and

95

Na2SiO3 solution to serve as the activator for a real geopolymer mix. The

remaining parts of the general procedure are the same and unchanged from the

mixing procedure shown in Figure 3.1.

Figure 3.3: A modified mixing procedure for fly ash-based geopolymer

A 16M (mol/L) NaOH solution is selected to be the high molarity NaOH solution

in Figure 3.3 (this 16M solution is specified at a condition of 20oC and 101.3

kPa atmospheric pressure), because its molarity is high enough for all

geopolymer mixes needed in this thesis. An abundant stock of this 16M NaOH

solution can be prepared in advance for different mixes. This way, ideally, it only

needs one time of the dissolution of NaOH flakes, so as to prepare this 16M

NaOH solution2.

2 The user may choose a molarity other than 16M if that is more suitable. This does not change the method or procedures adopted here.

Figure 3.4: Flowchart of procedure for an alternative geopolymer mix design method


Correspondingly, the mix design procedure is also modified to suit the new

mixing procedure of Figure 3.3, as displayed in Figure 3.4. In this new mix

design procedure, the steps from 1.1 to 2.6 are exactly the same as those in the

old one in Figure 3.2. After these steps, the user is able to obtain the needed

masses of NaOH flakes, Na2SiO3 solution and water for activating a specified

mass of fly ash (1000 grams in the example proposed here). Based on these, it

can calculate the proportions of ingredients for the new mix design that uses a

16M NaOH solution as the source of NaOH.

This is better illustrated based on the following example:

To start with, from the example discussed in section 3.4, to activate 1000 grams

of fly ash with an activator Na2O·SiO2·13H2O and a W/G ratio of 0.26, we

required 53.13 grams of NaOH flakes, 262.47 grams of Na2SiO3 solution and

142.22 grams of deionised water.

The 262.47 grams of Na2SiO3 solution will not be changed since the required

amount of SiO2 is constant in both mix design procedures. Therefore, in the

new mix design which uses a 16M NaOH solution, all the 53.13 grams of NaOH

flakes are provided by the 16M NaOH solution.

It can be looked up that a 16M NaOH solution at 20oC and 101.3 kPa

atmospheric pressure conditions has a NaOH solid mass proportion of 43.68%

and a density of 1470 kg/m3 (ZXXK, 2012).

So, the required mass of the 16M NaOH solution is:

53.1343.68%

= 121.63 grams

99

Apart from the NaOH solids, the water that will be present in the 16M NaOH

solution is therefore:

121.63 ∗ (1 − 43.68%) = 68.50 grams

Now, the amount of total water needed in the initial design so that the formula

Na2O·SiO2·13H2O is fulfilled, has been established as equal to 142.22 grams

(needed for the activation of 1000 grams of fly ash). However, as can be seen

from the above result we notice that some of the water that is needed (in the

activation of 1000 grams fly ash which is 68.50 grams of water), is already there

in the 16M NaOH solution that we have prepared. This means that we need to

add free deionised water to make up to the quantity of water of 142.22 grams so

as to fulfil the initial requirement of the formula Na2O·SiO2·13H2O.

Therefore, the amount of deionised water to be added in the mix design is:

142.22 − 68.50 = 73.72 grams

Hence, we can see that to activate 1000 grams of fly ash, the other

ingredients are 121.63 grams of 16M NaOH solution, 262.47 grams of

Na2SiO3 solution and 73.72 grams of deionised water.

Later, a final mix design expressed in units of kg/m3 is created and shown in

Table 3.3, using the method that is exactly similar to what has been done in the

final part of the stage 2 of the procedure in section 3.4.

That is: for 1000 grams of fly ash and 262.47 grams of Na2SiO3 solution, the

volumes are not changed, which are 4.52 × 10-4 m3 and 1.73 × 10-4 m3.

The volume of 121.63 grams of 16M NaOH solution is:


121.631470 ∗ 1000

= 8.27 ∗ 10−5 m3

The volume of 73.72 grams of water is:

73.721000 ∗ 1000

= 7.37 ∗ 10−5 m3

Then, the volumetric proportions of the ingredients are:

Fly ash

4.52 ∗ 10−4

(4.52 ∗ 10−4 + 8.27 ∗ 10−5 + 1.73 ∗ 10−4 + 7.37 ∗ 10−5)= 0.579

16M NaOH solution

8.27 ∗ 10−5

(4.52 ∗ 10−4 + 8.27 ∗ 10−5 + 1.73 ∗ 10−4 + 7.37 ∗ 10−5)= 0.106

Na2SiO3 solution

1.73 ∗ 10−4

(4.52 ∗ 10−4 + 8.27 ∗ 10−5 + 1.73 ∗ 10−4 + 7.37 ∗ 10−5)= 0.221

Deionised water

7.37 ∗ 10−5

(4.52 ∗ 10−4 + 8.27 ∗ 10−5 + 1.73 ∗ 10−4 + 7.37 ∗ 10−5)= 0.094

Then, the mass quantity of each ingredient for 1 m3 mix is:

Fly ash

0.579 ∗ 2210 = 1279.59 kg/m3

16M NaOH solution

101

0.106 ∗ 1470 = 155.82 kg/m3

Na2SiO3 solution

0.221 ∗ 1520 = 335.92 kg/m3

Deionised water

0.094 ∗ 1000 = 94.00 kg/m3

That is:

For the geopolymer mix using an activator Na2O·SiO2·13H2O with a W/G

value of 0.26, it requires 1279.59 kg/m3 fly ash, 155.82 kg/m3 16M NaOH

solution, 335.92 kg/m3 Grade D Na2SiO3 solution and 94.00 kg/m3

deionised water.

Table 3.3: Final mix design using a 16M NaOH ingredient of the geopolymer mix example (kg/m3)

W/G value

SiO2/Na2O (x value)

H2O/Na2O (y value)

F/A value

Mix design (kg/m3)

Fly ash 16M NaOH

Na2SiO3 solution

Deionised water

0.26 1.00 13 2.19 1279.59 155.82 335.92 94.00

Note: despite that the NaOH solution presented here is 16M, the true molarity of the NaOH solution as it ended up in the mixture is in fact 8.7M as shown in Table 3.4.

It needs to be noticed that, this 16M NaOH solution is only used for a safe and

efficient mix, and does not mean that the true molarity of NaOH solution in the

mixture is as high as 16M. As seen from Table 3.4, the mix design shown in

Table 3.3 ends up in a NaOH solution with a molarity of 8.7M, following the

calculation steps introduced in section 3.5. This molarity is the same as the


molarity of NaOH solution in the mix design of Table 3.1, meaning that the two

mix design procedures proposed here and in section 3.4 would not change the

molarity of NaOH solution.

Table 3.4: Comparison of the molarity of NaOH solution between different mix designs

Mix design

using NaOH flakes

Mix design using NaOH

solution

W/G ratio 0.26 0.26

SiO2/Na2O (x value) 1 1

H2O/Na2O (y value) 13 13

F/A ratio 2.19 2.19

Fly ash (kg/m3) 1261.91 1279.59

NaOH flakes (kg/m3) 68.16 -

16M NaOH solution (kg/m3) - 155.82

Na2SiO3 solution (kg/m3) 331.36 335.92

Deionised water (kg/m3) 179.00 94.00

Molarity of NaOH solution (M) 8.7 8.7

Otherwise, the figures in the mix design of Table 3.3 are slightly different from

those of Table 3.1, despite that these two tables are made with the same values

of x, y and W/G. Nevertheless, the author considers it reasonable since the

above two mix designs are theoretically calculated based on two different

mixing procedures, where one uses NaOH flakes and the other uses a 16M

NaOH solution. The calculated figures in above mix designs are relevant to the

volumetric proportions and densities of the ingredients. However, in the case of

103

using the NaOH as flakes, when the volumes were calculated, the NaOH

volume was based on the density of the flakes. On the other hand, when the

NaOH volume was calculated in the modified method, the density of NaOH 16M

solution which was used was that sourced from the standard, hence an

inevitable discrepancy will occur, albeit slight. Moreover, in the case of

cementitious materials mix design, the actual mix design is usually obtained

after a further modification based on trial tests from the theoretically calculated

one (Neville, 1996). Therefore, it seems that a similar modification process is

required for dealing with the discrepancy in the above geopolymer mix designs.

Furthermore, the mass proportions of the chemical components involved in

geopolymerisation, such as fly ash, NaOH, Na2SiO3 and water, are not

influenced because they are only relevant to the criteria of SiO2/Na2O,

H2O/Na2O and W/G that are set to be the same in above two mix design

procedures, as introduced in sections 3.4 and 3.6.

3.7 Mix Designs used in this Thesis

The mix design researched in this chapter aims to create the mix designs for

geopolymer mixes needed in the rest of the research and discussed in following

chapters, including mixes for the geopolymer pastes, the geopolymer

aggregates, and the geopolymer binders for concrete and mortar productions.

The mix designs used in these investigations, which have been done following

the mix design procedure illustrated in this chapter, are presented in Table 3.5.

For safety and efficiency reasons, as preferred by the author, the mix designs in

Table 3.5 use a 16M NaOH solution as the stock solution according to the

principles explained in the previous paragraphs. Thus, the 16M NaOH solution


appears in Table 3.5 as an ingredient of the activator. The mix designs for the

binder in concrete and mortar mixes as presented in the Table 3.5, are made for

1 m3 purely geopolymer mix, but can be modulated in quantities according to

the real proportion of geopolymer binder in any specific mix.

Table 3.5: Mix designs for geopolymers used in this thesis

x y W/G F/A Fly ash (kg/m3)

16M NaOH (kg/m3)

Na2SiO3 (kg/m3)

Water (kg/m3)

Purely Geopolymer, paste mixes

1.25 11 0.23 2.16 1310.65 131.04 461.90 13.62

1.25 11 0.24 2.05 1283.42 135.00 475.88 14.03

1.25 11 0.25 1.96 1256.87 138.87 489.52 14.44

1.25 11 0.26 1.86 1230.98 142.65 502.82 14.83

1.25 11 0.22 2.28 1338.98 127.01 447.68 13.20

1.25 12 0.24 2.16 1297.98 123.12 433.97 43.42

1.25 13 0.26 2.06 1259.41 119.46 421.07 71.84

Purely Geopolymer, aggregate mixes

1.25 11 0.24 2.05 1283.42 135.00 475.88 14.03

1.25 12 0.24 2.16 1297.98 123.12 433.97 43.42

1.25 11 0.26 1.86 1230.98 142.65 502.82 14.83

Purely Geopolymer, concrete mixes

1.25 11 0.24 2.05 1283.42 135.00 475.88 14.03

1.25 11 0.26 1.86 1230.98 142.65 502.82 14.83

Purely Geopolymer, mortar mixes

1.25 11 0.26 1.86 1230.98 142.65 502.82 14.83

3.8 Conclusions

This chapter presents a mix design procedure for a fly ash-based geopolymer

made from fly ash, NaOH flakes, Na2SiO3 solution and water. This mix design

procedure distinguishes each geopolymer mix by the criteria of SiO2/Na2O,

105

H2O/Na2O and W/G, with which a final mix design can be created to satisfy the

manufacture of desired geopolymer materials. This procedure offers a

consistent and systematic method of creating the geopolymer mix designs used

in this research and presented in the following chapters of this thesis.

106 4. Curing regime for geopolymer synthesis

Chapter 4

Curing Regime for Geopolymer Synthesis

4.1 Introduction

This chapter focuses on the effect of curing regime for fly ash-based

geopolymer synthesis. Curing regime generally regulates the atmospheric

conditions applied to facilitate the development of a fresh geopolymer to obtain

the desired properties. In this chapter, the effect of curing temperature is firstly

researched. This involves a mild-temperature heat curing method (including

different heating temperature and heating time) and a room-temperature curing

method. Apart from this, the effect of moisture condition during the curing of

geopolymer is also researched, in order to enhance the properties of end-

product through a better control of the moisture condition. Moreover, the effect

of curing period for room temperature-cured (ambient-cured) geopolymer is

discussed. The outcomes of this chapter aim to provide knowledge needed for

the decision of curing regime in future geopolymer aggregate manufacture.

4.2 Previous Research on Curing of Geopolymer Synthesis

Over recent decades, there has been a great deal of research on developing

the curing regime for fly ash-based geopolymer synthesis.

A heat treatment is always preferred to attain a good and fast gain in strength of

a geopolymer, which is a thermoset inorganic polymer, and is particularly critical

for a geopolymer created from fly ash which needs relatively higher activation

energy than other raw materials (Bakharev, 2005a, Jiang and Roy, 1990). Such

heating usually proceeds at mild temperatures of 40-80oC for days after the

107

casting of a fresh geopolymer (Provis et al., 2009, Rangan, 2007, Palomo et al.,

1999, Kong et al., 2007).

Although a high curing temperature is better for the development of a fresh

geopolymer (Sindhunata et al., 2006), if it is too high, it can have a negative

effect on the end product (van Jaarsveld et al., 2002). On the other hand, a

longer heating time can improve the properties of geopolymers, but the rate of

improvement is only significant in the first 24 hours and then it becomes much

slower (Fernández-Jiménez et al., 2006). It has been suggested that,

considering the balance between outcomes and costs, an optimum curing

regime for fly ash-based geopolymer concretes is 60oC for 24 hours (Hardjito

and Rangan, 2005, Rangan, 2007). Moreover, several hours of resting before

heating is found to be beneficial for the strength gain in a geopolymer

(Bakharev, 2005a).

The idea of ambient curing held at room temperature with no heat treatment has

also been investigated (Rovnaník, 2010). Although the properties of a

geopolymer are weakened without heat treatment, to compensate for this,

several methods have been tried, such as using a prolonged curing period (Vijai

et al., 2010), reducing the fly ash’s particle size by mechanical milling (Kumar

and Kumar, 2010, Temuujin et al., 2009) or grinding (Somna et al., 2011),

adding reactive components such as GGBFS (Nath and Sarker, 2012,

Davidovits, 2013), and employing a relatively lower curing moisture (Perera et

al., 2007, Xie and Kayali, 2014). Along with heated geopolymers, a room

temperature-cured geopolymer and the relevant curing conditions that affect its

development are also researched in this chapter.


4.3 Curing Temperature

The effect of curing temperature for geopolymer manufacture is firstly

investigated. For a fly ash-based geopolymer, a mild-temperature heating from

40-80oC is always preferred in research and in practical field, where the heating

temperature and heating time are influential for final properties (Provis et al.,

2009, Rangan, 2007, Palomo et al., 1999, Kong et al., 2007). Despite this, an

ambient curing conducted at room temperature (20-23oC) can also be workable

for geopolymer synthesis (Rovnaník, 2010). Although the above literature on

the effects of heating and ambient-curing is already quite detailed, the author

still considers it necessary to examine these effects for the geopolymer mixes

included in this thesis, because these geopolymers are created from a unique

fly ash sample as seen from Chapter 2.

Applying a certain heat treatment to a fresh fly ash-based geopolymer is a

traditional method for facilitating the geopolymer’s development, and normally

consists of the following steps (Provis et al., 2009, Rangan, 2007):

a) rest the fresh geopolymer samples in sealed moulds at room

temperature for a certain period of 6-48 hours called the ‘resting time’;

b) place the geopolymer samples in an oven for a certain period (24-72

hours) and heat at a mild temperature (40-80oC);

c) remove the geopolymer samples from the oven and rest them at room

temperature for a certain period (8-24 hours) to cool down; and

d) demould the geopolymer samples and continue to rest them at a

controlled room-temperature environment for a certain period before use

for tests or applications.

109

In contrast, under the ambient-curing regime, which does not include any heat

treatment, fresh geopolymer samples are normally placed in a specified room-

temperature curing condition for a certain period. They are usually sealed in

moulds to prevent moisture loss but can also be demoulded for different

ambient conditions after they are initially hardened. Ambient-cured geopolymer

may not develop as fast as the heated one, but is also able to achieve a high

quality in end-product (Kumar and Kumar, 2010), and therefore is considered in

this section.

4.3.1 Geopolymer Mix

Table 4.1: Mix design of geopolymers for curing regime research

Fly ash batch 2012-12-13

W/G value 0.24

SiO2/Na2O (x value) 1.25

H2O/Na2O (y value) 11

Fly ash/Activator (F/A) 2.05

Fly ash (kg/m3) 1283.42

16M NaOH solution (kg/m3) 135.00

Na2SiO3 solution (kg/m3) 475.88

Water (kg/m3) 14.03

The mix design shown in Table 4.1 is applied for the geopolymer mixes used in

this section. In this mix design, SiO2/Na2O and H2O/Na2O ratios are specified as

1.25 and 11 respectively, based on the previous research of Provis et al. (2009).

Besides, the W/G ratio is set to be 0.24, based on Rangan’s research (Rangan,


2007) and the author’s trial mixes, which is suitable for making a geopolymer

mix with sufficient workability and strength. This mix design is created following

the mix design procedure proposed in Chapter 3. Cubic 50mm×50mm×50mm

geopolymer samples are manufactured following the same procedure

previously illustrated in Chapter 2, section 2.3.

4.3.2 Curing Regime

Different curing temperature and time are designed to be evaluated as

presented in Table 4.2. A heat treatment at 40-80oC for 1 or 3 days is

investigated, as suggested in previous literature (Provis et al., 2009, Rangan,

2007, Palomo et al., 1999, Kong et al., 2007). Also, an ambient curing regime

held in the ER at 20oC and 50% RH is researched. All samples go through a

specified 7-day curing regime after casting and then undergo the required

testing.

Table 4.2: Curing regimes investigated for optimum geopolymer growth

Code Resting

time (hours)

Heat (oven)

Heating time (hours)

Temperature (oC)

Testing age (days)

20ER 6 - - 20 7

H40-1d 6 Yes 24 40 7

H40-3d 6 Yes 72 40 7

H60-1d 6 Yes 24 60 7

H60-3d 6 Yes 72 60 7

H80-1d 6 Yes 24 80 7

H80-3d 6 Yes 72 80 7

111

4.3.3 Strength

The strength of hardened geopolymers cured at different conditions has been

given a special attention in this investigation. This strength performance is

represented by the compressive strength value of cubic geopolymer sample,

which has been measured using a TECNOTEST compression testing machine

(3000 kN capacity). The loading rate was set constant at 0.33 MPa/sec, based

on the recommended loading rate range in US standard ASTM C109 (ASTM,

2012c). The compressive strength result shown in Figure 4.1 is related to each

specific curing regime regulated as shown in Table 4.2, where each result is the

average of six samples from the same mixing batch.

Figure 4.1 basically demonstrates that a higher temperature and longer heating

time results in a higher strength gain, which agrees with previous finding

(Sindhunata et al., 2006). The cubic geopolymer sample which has been

through a heat curing at 80oC for 3 days achieved the highest strength of 52.94

MPa. On the other hand, the two curing regimes of 80oC for 1 day and 60oC for

3 days resulted in a comparable compressive strength. From the above, a

curing regime of 80oC for 3 days performs best in strength, and therefore is

preferred for the manufacture of geopolymer aggregates introduced in Chapter

5.

The ambient-cured geopolymer sample has the lowest compressive strength,

which is far behind those of the heated ones. This proves the general

understanding of the advantage of heating for fly ash-based geopolymer

(Sindhunata et al., 2006, Rangan, 2007). Meanwhile, it shows a significant gap

in strength between the ambient-cured and heat-cured geopolymer samples.


For a short-term curing procedure, such as 7 days, it seems that an ambient-

cured geopolymer cannot provide the needed strength for further structural use,

unless a modification on the curing regime is made.

Figure 4.1: 7-day compressive strengths of geopolymers under different curing regimes (corresponding to the mix in Table 4.1)

4.4 Curing Moisture Condition

Curing regime for geopolymers generally concentrates on curing temperature

and period, but rarely on the moisture condition. Nevertheless, an appropriate

curing moisture condition is likely to affect the developments in a fresh

geopolymer, based on the current findings (Perera et al., 2007, Xie and Kayali,

2014). Hence, in this section, research on the effect of curing moisture on the

developments in geopolymer, is presented.

2.18

12.99

28.68

39.46

24.45

38.65

52.94

0

10

20

30

40

50

60

20 40 60 80

Com

pres

sive

str

engt

h (M

Pa)

Curing temperature (oC)

ER

heat 1d

heat 3d

113

This ‘curing moisture’ research concentrates on the ambient-cured geopolymer

instead of the heated one. This is because a heated geopolymer has already

been much developed in its microstructure by heat treatment (Bakharev, 2005a,

Jiang and Roy, 1990), and therefore is considered less affected by the moisture

condition applied afterwards.


The geopolymer mixes designed here were devoted for the research into the

effects of different curing moisture conditions. These mixes were designed with

different initial water amounts so as to observe the possible effects (if any) of

the curing moisture conditions in relation to differences in the original water

content.

To fulfil this, the geopolymer mixes are intentionally designed so that they all

contain equal masses of the fly ash and activator solutes while their water

contents are different. Following the mix design procedure in Chapter 3, these

mix designs involve making the ratio of fly ash to Na2O+SiO2 constant and

varying the ratio of H2O/Na2O (the y value in Na2O·xSiO2·yH2O). The final mix

designs are presented in Table 4.3 in which all these designs are based on fly

ash content of 1000 grams in order to highlight the variation in water content.

Here, as the usages of NaOH and Na2SiO3 solutions are constant, the water

amount from these two solutions must be the same for all mixes. So, the

variation in water content among the mixes is rather realised by the quantity of

free water, as seen in Table 4.3. For 1000 grams of fly ash, 94.89 grams of 16M

NaOH and 334.34 grams of Na2SiO3 solutions remain constant, with the free


water varying from 9.82 grams to 57.05 grams. The mixing procedure for

geopolymer samples is the same as described in the last section.

Table 4.3: Mix designs for geopolymer mixes with different initial water contents

SiO2/Na2O (x value)

H2O/Na2O (y value) W/G F/A

Mix designs (grams)

Fly ash 16M NaOH

Na2SiO3 solution Water

1.25 11 0.22 2.28 1000 94.89 334.34 9.82

1.25 12 0.24 2.16 1000 94.89 334.34 33.43

1.25 13 0.26 2.06 1000 94.89 334.34 57.05

4.4.2 Curing Regime

During the first 7 days, the curing regime was the same for all samples, namely:

keeping the sample in a sealed mould and placing it inside the ER at 20oC. The

curing regimes are designed to vary the curing moisture at a certain time (7

days) after the casting of the fresh geopolymer. In this way, curing moisture

condition can be varied during the polycondensation stage which is a stage

more vulnerable to moisture change (Zuhua et al., 2009, Davidovits, 1989).

After mixing, fresh geopolymer samples were sealed in the moulds, rested for 6

hours and placed in the ER at 20oC for the first 7 days. After this 7-day ambient

curing, the geopolymer samples were demoulded, and later cured in the ER

under three different moisture conditions displayed in Table 4.4: (1) unsealed

(A); (2) sealed (AS); or (3) dried inside a desiccator over the anhydrous silica

gel (AD). The sealed curing aims to retain water during 7 to 14 days after

casting and the unsealed and desiccated aimed to release water in a semi-dry

condition with 50% RH and a dry condition respectively. A vacuum facility was

115

applied to accelerate the drying process of desiccated curing by reducing the

internal pressure in the desiccator during the first 24 hours of the drying process.

These three different regimes continued for the next 7 days. The samples were

then tested at the final age of 14 days.

Table 4.4: Specified curing moisture conditions for produced geopolymers

Code Unsealed (A)

Sealed (AS)

Desiccated (AD)

Testing age (days)

A-14d Yes - - 14

AS-14d - Yes - 14

AD-14d - - Yes 14

4.4.3 Strength

The compressive strength results for the group A, AS and AD geopolymer

mixes after 14 days are presented in Figure 4.2. For the same curing condition,

the mix with a lower W/G ratio, which means a lower initial water content,

results in a higher strength. On the other hand, the order of the compressive

strengths of the samples mixed with the same W/G ratio is AD>A>AS which is

the same for all the mixes involved in this experiment and indicates that, while

the desiccation method can enhance strength development, the surface-sealed

one decreases it. It seems that the retention of water in geopolymers during 7 to

14 days (AS group) can result in lower strengths than when water is allowed to

escape (A and AD groups), which is possibly due to the role of water in

influencing the reaction equilibrium of geopolymerisation. It has been reported

that a lower amount of water is inclined to facilitate a higher extent of

polycondensation, which is the dominant reaction during 7-14 days, due to the


shift of reaction equilibrium according to the water content (Zuhua et al., 2009).

It is considered that the curing at a low-moisture condition could accelerate the

release of water in the mix, and therefore positively shift the polycondensation

reaction and generate more geopolymer products. This may explain the better

strength outcomes of A and AD samples as observed in this research.

Figure 4.2: Compressive strengths of 14-day ambient-cured geopolymers coded as A-14d, AS-14d and AD-14d with different W/G values

So it is suggested that, for better structural development, the water in the

mixture seems to be better released after a certain curing period. This could be

realised by exposing the geopolymer to a low moisture condition (like the 50%

RH room), or by using a desiccator. Nevertheless, further research is still

required in this area to find out if strength enhancement obtained by partial or

enhanced drying should be adopted as a general principle for all geopolymer

mixes. Besides, it is necessary to start a long-term investigation so as to

observe whether the strength enhancement gained by a low-moisture condition

16.16 14.46 17.45

14.32 13.66

16.31

10.81 10.6

13.54

02468

101214161820

A-14d AS-14d AD-14d

Com

pres

sive

str

engt

h (M

Pa)

Curing conditions

W/G=0.22

W/G=0.24

W/G=0.26

117

can be maintained for such ambient-cured geopolymers as the ones reported

above.

This section also suggests that, an early demoulding of a geopolymer sample to

make it exposed to a low-moisture condition may facilitate its strength

development. If such geopolymer is not concerned about achieving certain

shape, texture or dimensions, such as producing aggregates, then such early

demoulding may be done even slightly before the full set of the fresh

geopolymer. However, it is considered by the author that a too early demoulding

should not be suitable, because an abundant amount of water in the mix is

highly needed in an early stage (first 2-3 days of curing) for dissolution and

hydrolysis reactions (Zuhua et al., 2009). A research on an optimum

demoulding time is needed in the future.

Moreover, combined with the strength result shown in Figure 4.1, (even though

the mixes represented in Figure 4.1 are not the same as those in Figure 4.2) yet

it can be concluded when studying the two Figures that there is a remarkable

increase in strength from 7 days to 14 days of the tested ambient-cured

geopolymer samples. This phenomenon implies a possible benefit from a longer

curing time on an ambient-cured geopolymer. This phenomenon will be

discussed in the following sections that focus on the effect of curing period.

4.5 Curing Period (Short-term)

This section deals with the research on the effect of curing period for

geopolymer synthesis. Both heat curing and ambient curing regimes are

included. It is found that a heated geopolymer is stronger than an ambient-

cured geopolymer if both of them are cured for the same period. But an


ambient-cured geopolymer is likely to obtain a more obvious strength gain after

a prolonged curing period, and therefore would eventually achieve a

comparable strength with the heated one that is made with the same mix design

but heated for a short curing period. Therefore, it seems that a prolonged

ambient curing period would be workable for improving the performance of an

ambient-cured geopolymer material.


The geopolymer mixes manufactured from the same mix designs shown in the

last section (Table 4.3) are made and used for experiments regarding the curing

period. These include the mixes with different initial water contents in order to

serve the purpose of doing a satisfactorily more complete investigation.

4.5.2 Curing Regime

Two curing series are used, one is a mild-temperature heat treatment from 40-

80oC, and the other is an ambient-cured regime with samples held in the ER at

20oC and 50% relative humidity (RH). Details of the curing regimes used

throughout this section are presented in Table 4.5 and, under both, the freshly

mixed geopolymers are first sealed and rested for 6 hours in the ER, for an

enhanced geopolymer development (Bakharev, 2005a). Then, those to be

heated are placed in an electric oven at 60oC for 4 or 24 hours, and later

removed and demoulded, and stored in the ER for further testing. Also, a

controlled ambient-curing procedure is undertaken to seal fresh geopolymer

samples in moulds and keep them in the ER at 20oC. They are sealed to

achieve sufficiently uniform reactions in the first 7 days, then some are directly

119

tested at the age of 7 days while the rest are demoulded and exposed to the

atmosphere of the ER for a further 7 days.

All three geopolymer mixes detailed in Table 4.3 were manufactured and cured

at the four different curing regimes included in Table 4.5. For each specific mix

and each specific curing regime, the result is the average of three geopolymer

samples made from the same batch.

Table 4.5: Specified curing procedures for heat-cured and ambient-cured geopolymers

Code Resting time (hours)

Temperature (oC)

Heating time (hours)

Testing age (days)

H-24h

6

60 24 7

H-4h 60 4 7

A-7d 20 - 7

A-14d 20 - 14

4.5.3 Density and Strength

The density of a cubic geopolymer sample was measured by dividing its mass

by its volume, with the latter specifically measured using a water displacement

method. Also, a normal compressive strength test was conducted to represent

the mechanical property of each geopolymer produced.

The density and strength outcomes for the heated geopolymers are presented

in Figure 4.3. For all three mixes, the H-24h samples yield lower densities than

the H-4h ones which implies that heating may help to release more water into

the semi-dry atmosphere of 50% RH after demoulding and, therefore, the longer

the heating period, the lower the density.


Figure 4.3: Density and compressive strength results for heated geopolymer samples

The H-24h samples achieved higher compressive strength values than the H-4h

ones. Moreover, when the curing period is the same, a sample with a lower

1.67

1.65

1.68

1.67

1.70

1.69

1.62

1.63

1.64

1.65

1.66

1.67

1.68

1.69

1.70

1.71

H-4h H-24h

Den

sity

(g/c

m3 )

Heating period

W/G=0.22

W/G=0.24

W/G=0.26

15.71

42.07

11.05

36.25

10.18

30.32

0

5

10

15

20

25

30

35

40

45

0 4 8 12 16 20 24 28

Com

pres

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h (M

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Heating period (hour)

W/G=0.22

W/G=0.24

W/G=0.26

121

W/G ratio obtains a higher compressive strength which agrees with previous

Rangan’s work on geopolymer concretes (Rangan, 2007).

Figure 4.4: Density and compressive strength results of ambient-cured geopolymer samples

1.73

1.72

1.74

1.73

1.77

1.75

1.69

1.70

1.71

1.72

1.73

1.74

1.75

1.76

1.77

1.78

A-7d A-14d

Den

sity

(g/c

m3 )

Curing period

W/G=0.22

W/G=0.24

W/G=0.26

3.54

16.16

3.52

14.32

3.18

10.81

0

2

4

6

8

10

12

14

16

18

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Com

pres

sive

str

engt

h (M

Pa)

Curing period (day)

W/G=0.22

W/G=0.24

W/G=0.26


Similarly, the density and compressive strength of an ambient-cured

geopolymer are also measured and presented in Figure 4.4. The mix with a

lower W/G ratio has a lower density. Besides, for the mix with the same W/G

ratio, A-7d sample has a higher density than the A-14d one which, it is

concluded, is the result of a mass reduction in water due to exposure to the 50%

RH atmosphere in the ER from 7 to 14 days.

It can be seen that an ambient-cured geopolymer after 7 days obtains a much

lower strength than that of a heated geopolymer cured within the same period.

This is consistent with the previous finding reported in section 4.3 of this chapter.

However, the same ambient-cured mix could end in a much higher strength

after 7 more days of ambient curing held in a 50% RH atmosphere, as seen in

Figure 4.4. Apart from a benefit from a reduced moisture condition, as

discussed in last section, a prolonged curing time also seems effective for a

strength gain in an ambient-cured geopolymer. In contrast, it has been reported

that a longer curing time at a room temperature could not significantly enhance

the strength of a heated geopolymer (Fernández-Jiménez et al., 2006). A 14-

day ambient-cured sample is observed to achieve a comparable strength with a

7-day 4-hour heated sample made from the same mix design, which is same for

all tested mixes with different W/G ratios. Therefore, it is considered that a

prolonged curing may function like a short-term heating for geopolymer

development.

Also, from this research, it is observed that a prolonged room-temperature

drying at a low humidity condition (50% RH) is likely to be an ideal curing

regime for geopolymer in terms of strength development. This is however,

different with the case of OPC, which is preferred to be cured at a room-

123

temperature and a high humidity condition (100% RH). A high water content is

required for an advanced cement hydration process in a OPC system (Lea,

1970). However, this seems not to be the case in a geopolymer one, where a

low-moisture condition after the first 7 days of curing is likely to result in a strong

end-product.

Moreover, same as the heated samples, the ambient-cured sample with a lower

W/G ratio results in a higher strength performance. From this, it is estimated

that a low initial water content may enhance the strength gain, and should be

incorporated with a prolonged curing period in geopolymer synthesis.

4.5.4 Scanning Electron Microscopy

A scanning electron microscopy (SEM) testing is used to observe the pace of

micro-structural development in the networks of produced geopolymers, and

was conducted on a HITACHI TM 3000 tabletop SEM facility at 15 kV. It was

applied on the geopolymer pieces remaining after strength tests, with four

different parts observed for each unique geopolymer mix and the dominant type

of structure seen in the SEM images chosen to be the representative image.

The images for the 7-day and 14-day ambient-cured samples with W/G=0.22

and 0.26 are presented in Figure 4.5(a) to (d). For comparison, an image of the

7-day 4-hour heat-cured sample with W/G=0.22 is presented in Figure 4.5(e).


Figure 4.5: SEM images of geopolymers with W/G=0.22: (a) A-7d, (b) A-14d and (e) H-4h; and with W/G=0.26: (c) A-7d and (d) A-14d

The ambient-cured geopolymer with W/G=0.22 displays a clear densification

process in structure from 7 to 14 days. In Figure 4.5(a), it can be seen that the

125

fly ash particles are partially dissolved and start to form small, loosely

connected blocks after 7 days of ambient curing, Then, after 14 days, they

become denser and more closely connected with a reduction in the intermediate

pores, as seen in Figure 4.5(b) tracking the structural formation pace proves

that this geopolymer can develop with a prolonged ambient curing period, which

could explain its strength gain during this period as can be seen in Figure 4.4.

The ambient-cured sample with W/G=0.26 shows a similar densification

process, but less significant than that of the sample with W/G=0.22. A higher

proportion of unreacted fly ash particles with a loose particle connections are

found as shown in Figure 4.5(c) and (d). This may be due to the lower alkali

strength caused by relatively higher water content in the W/G=0.26 sample.

Moreover, it can be noticed that there is similarity between the structure in

Figure 4.5(b), which depicts the development of a 14-day ambient-cured

geopolymer with W/G=0.22, and that in Figure 4.5(e), which represents the

same mix but heated at 60oC for 4 hours. The above two samples that have

been through different curing regimes display similar structures, and this is

consistent with their similar strength outcomes as shown in Figure 4.3 and

Figure 4.4. These similarities in micro-structural development and strength

values may indicate that the effect of heat curing (for 4 hours in this case) may

be achieved by a prolonged curing at room temperature in a relatively dry

environment.

The result of the research presented in this section, in combination with that of

section 4.4, demonstrate a possibility of applying a prolonged curing period with

a relatively dry atmosphere to enhance the strength of an ambient-cured


geopolymer. However, the research presented in this section only focuses on a

short-term period, and therefore the strengths of produced ambient-cured

geopolymers are still not very high. To deal with this, a long-term curing period

is researched and presented in the next section.

4.6 Curing Period (Long-term)

In section 4.5, it has discovered that an ambient-cured geopolymer can achieve

a strength and structural performance comparable with a heated geopolymer

when given a longer curing time. However, as this finding is arrived at during

only a very short period, the outcome of strength benefit is not very significant

yet. In this section, a long-term experiment is carried out to evaluate the

possibility of making a high-strength ambient-cured geopolymer qualified for

real structural use.

It is considered that, one shortcoming of an ambient-cured geopolymer should

be its extremely low geopolymer reaction rate, due to the absence of heat

treatment (Bakharev, 2005a, Jiang and Roy, 1990). This may explain its

extremely low-strength outcome after a short-term curing as shown in Figure

4.1. Otherwise, as a fresh ambient-cured geopolymer usually lasts longer in a

fresh stage without an early set observed, it is assumed that the extent of

geopolymer reaction can be enhanced in a longer curing period, which is the

motive of the research work in this section. This prolonged ambient-cured

geopolymer manufacture is considered appropriate for an application that does

not need a fast strength gain, such as the geopolymer aggregates. The

knowledge gained from this section will be further applied for the manufacture of

room temperature-cured geopolymer aggregates as explored in Chapter 8.

127


A mix design with a moderate water content of W/G=0.24 is selected, which is

the same as the one in Table 4.1. This mix contains a proper initial water

amount which is neither too dry nor too wet, which is considered more suitable

to represent the normal geopolymer in practical use. The geopolymer samples

are made following the same procedure presented in the last section. Three

geopolymer samples from the same batch are prepared and tested for each

specific curing regime.

4.6.2 Curing Regime

The geopolymer samples produced are placed in a container in the ER at 20oC

and 50% relative humidity for the different periods of 7, 14, 21, 28, 56 and 84

days. These samples are only sealed in the first 7 days, and later unsealed for

the rest of the curing periods. They are immediately tested once they reach their

specified curing terms.


The specific gravities of the cubic geopolymer samples versus their different

curing periods are measured and presented in Figure 4.6. It can be seen that

the specific gravity of a geopolymer reduces in the first 14 days, slightly

increases from 14 to 28 days and reduces again from 28 to 84 days. Basically,

a longer curing period leads to a lower specific gravity.


Figure 4.6: Variations in specific density versus long-term curing period at room temperature of fly ash-based geopolymers produced

The compressive strength values of the geopolymer samples cured for different

periods are measured and presented in Figure 4.7 in which it can be seen that

longer curing periods cause higher strength gains. Despite a low strength of

less than 10 MPa in the initial days, the geopolymers tested display much

higher strengths which reach 43.96 MPa after 84 days, which is a high enough

strength as far as structural elements are concerned.

Based on the slope of the strength curve in Figure 4.7, it can be seen that the

rate of strength gain is much faster in the first 28 days, achieving almost 10

MPa increments every 7 days. Then, this rate begins to slow down, and there is

little further strength gain during 56 to 84 days, which implies that the strength

of this kind of geopolymer mix is close to the limit after 84 days of ambient

curing.

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

0 14 28 42 56 70 84 98

Spec

ific

grav

ity

Curing period at room temperature (days)

129

Figure 4.7: Compressive strength versus long-term curing period at room temperature of fly ash-based geopolymer whose mix design is in Table 4.1

Moreover, by comparing the geopolymer samples presented in this section with

the heated ones presented in section 4.3 using the same mix design (the

W/G=0.24 one in Table 4.1), as shown in Figure 4.8, it is found that a prolonged

curing period results in an ambient-cured geopolymer with a comparable to, or

even higher strength than those of heated geopolymers. From Figure 4.8, the

84-day ambient-cured geopolymer has a compressive strength of 43.96 MPa,

lower than only that of the geopolymer H80-3d (which undergoes heating at

80oC for 3 days) but similar to those of geopolymers H80-1d and H60-3d, and

much higher than the other heated samples.

Nevertheless, ambient curing still has a limit in terms of strength gain as it

cannot cause a fresh geopolymer to attain the same strength level that can be

realised by heat treatment. Combining Figure 4.7 and Figure 4.8, the highest

05

101520253035404550

0 14 28 42 56 70 84 98

Com

pres

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str

engt

h (M

Pa)

Curing period at room temperature (day)


strength achieved under ambient curing is less than 45 MPa whereas a

geopolymer heated at 80oC for 3 days can reach more than 50 MPa. In practice,

if the aim is to produce a high-level strength of more than 50 MPa within a

relatively short time, a heat treatment must be selected. This is also the reason

that a geopolymer aggregate made under heat treatment is primarily

researched in this thesis.

Figure 4.8: Comparison of compressive strengths of geopolymers under different curing regimes (where all samples are made from the mix design

shown in Table 4.1)

The above findings in this section have specifically focussed on the geopolymer

mix with x=1.25, y=11 and W/G=0.24, which was previously introduced. These

findings indicate the positive effect of a long-term room-temperature curing for

strength development of this selected geopolymer mix, which is helpful for a

deeper understanding on the principles of curing period. A prolonged curing

regime is likely to be a potential way for enhancing the strength of an ambient-

0

10

20

30

40

50

60

84d ER H80-3d H80-1d H60-3d H60-1d H40-3d H40-1d

Com

pres

sive

str

engt

h (M

Pa)

Curing regime

131

cured geopolymer. This observation needs further investigation which needs to

be based on a wider selection of geopolymer mixes.

4.7 Conclusions

This chapter reports and discusses the research that deals with the effect of

curing regime on the development of fly ash-based geopolymer. The factors

studied here include curing temperature, moisture regime and curing period.

Based on the results, a heat curing method is still the better one for a fast

development in strength and structure of geopolymer mix. However, the

development of an ambient-cured geopolymer is found to be enhanced with a

reduced moisture condition and a prolonged curing period. Through this, an

ambient-cured geopolymer with a comparable strength as the heated one can

be realised, provided the time is not a constraint.

The research work reported in this chapter helps to find an optimum curing

regime used in future geopolymer aggregate manufacture. A heat curing regime

of 80oC for 3 days is selected for its advanced capability to achieve the high

strength of a fresh geopolymer, and will be applied for the manufacture of

geopolymer aggregates discussed in Chapter 5. This curing regime will also be

used for a synthesis of geopolymer binder required in concrete and mortar

mixes as presented in Chapter 7. In addition, another ambient curing regime

operating at 20oC for 84 days with a low-moisture curing condition is used for

the room temperature-cured geopolymer aggregate manufacture and will be

presented in Chapter 8.

132 5. Geopolymer aggregate (GA)

Chapter 5

Geopolymer Aggregate (GA)

5.1 Introduction

This chapter presents research on geopolymer aggregate (GA) as a new type of

FA-LWA for concrete application. It is based on the research presented in

Chapters 2, 3 and 4, which determined the fly ash, mix design procedure and

curing regime required for fly ash-based geopolymer synthesis.

Compared to traditional FA-LWA products, the manufacture of this geopolymer

aggregate is designed to be more advanced as it uses chemical bonding

instead of mechanical pressing for agglomeration, a more flexible gel-like

geopolymer for easy processing and does not involve elevated-temperature

sintering. Therefore, it aims to create a low-cost, simple and eco-friendly

artificial FA-LWA manufacturing process.

This geopolymer aggregate is composed of geopolymer materials and, based

on relevant experiments, is found to have a low specific gravity, high porosity

and high water absorption capacity but good mechanical behaviour. It is of

adequately good quality to satisfy relevant engineering standards and has

comparable properties to those of other commonly used FA-LWAs. Therefore,

this geopolymer aggregate is anticipated for the manufacture of a high-

performance lightweight concrete (LWC) with desired properties like low self-

weight and high strength/mass ratio, like the conventional FA-LWAs. And this

geopolymer aggregate may further promote this LWC industry due to its low-

cost and eco-friendly features.

133

5.2 Mix Design used for Geopolymer Aggregate

It is considered that the properties of geopolymer aggregates would be greatly

affected by the selection of geopolymer mix design. However, a complete

experimental research on the effects of different mix designs would be a huge

work requiring a long-term investigation. Instead, in this section, a desired mix

design for geopolymer aggregate manufacture is selected based on the

analysis of previous results in the literature and trial mixes by the author.

As discussed in Chapter 3, the geopolymer mix design is determined by the

criteria SiO2/Na2O, H2O/Na2O and W/G. Among them, SiO2/Na2O and

H2O/Na2O reflect the composition of the activator which is made of NaOH and

Na2SiO3 solutions. From the research of Provis et al. (2009), it was found that

usually there is an optimum range of the SiO2/Na2O ratio, which is 1.0 to 1.5, for

achieving an advanced mechanical property. Also, a H2O/Na2O ratio of 11 was

recommended in the same research (Provis et al., 2009). In addition, in the

research presented in Chapter 4, section 4.4, the mixes with a SiO2/Na2O ratio

of 1.25 and a H2O/Na2O ratio of 11-13 have been proven to be workable for

making strong geopolymer products. For these considerations, the values for

SiO2/Na2O and H2O/Na2O are specified to be respectively, 1.25 and 11 for the

geopolymer aggregate research presented in this chapter.

On the other hand, Hardjito and Rangan have studied a range of W/G ratio from

0.16 to 0.24, discovering that a lower W/G ratio in this range can result in a

higher strength performance (Hardjito and Rangan, 2005, Rangan, 2007). This

proposed range of W/G, however, is found to be not perfectly suitable for the

geopolymer mixes included in this research. Some mixes with a W/G ratio


below 0.22, which should be workable according to Hardjito and Rangan’s

reports, are actually too dry to be uniformly compacted. Therefore, trial mixes

are made and tested for strength so as to find a proper W/G ratio.

The geopolymer mixes are designed with the same SiO2/Na2O (1.25) and

H2O/Na2O (11) but with different W/G ratios ranging from 0.23 to 0.26. The

resulting mix designs, which are created based on the proposed mix design

procedure in Chapter 3, are presented in Table 5.1. Cubic 50mm×50mm×50mm

geopolymer samples are manufactured following the same procedure

previously illustrated in Chapter 2, section 2.3. A heat curing regime at 80oC for

3 days is used to harden the fresh samples.

Table 5.1: Geopolymer mix designs with different W/G values

W/G value

SiO2/Na2O (x value)

H2O/Na2O (y value)

F/A value

Mix designs (kg/m3)

Fly ash 16M NaOH

Na2SiO3 solution

Deionised water

0.23 1.25 11 2.16 1310.65 131.04 461.90 13.62

0.24 1.25 11 2.05 1283.42 135.00 475.88 14.03

0.25 1.25 11 1.96 1256.87 138.87 489.52 14.44

0.26 1.25 11 1.86 1230.98 142.65 502.82 14.83

The compressive strength of the hardened geopolymer samples on the 7th day

after mixing was measured, using a TECNOTEST compression testing machine

(3000 kN capacity). The final compressive strength value for each unique

geopolymer mix is the average of six cubic samples, as displayed in Figure 5.1.

As is clear in Figure 5.1, the geopolymers made in this research can achieve

good strengths higher than 45 MPa within 7 days. Also, the geopolymer with

135

W/G=0.24 has the highest of all the mixes tested, 52.94 MPa, while that with

W/G=0.26 has the lowest of 47.16 MPa.

The difference in W/G ratio is not significant for the tested hardened samples.

This might be due to a more advanced structural growth under a high

temperature curing as 80oC, as reported in Chapter 4, section 4.3. Meanwhile, it

is found that, a lower W/G ratio does not certainly result in a higher strength, as

used to be proposed in Hardjito and Rangan’s literature (Hardjito and Rangan,

2005, Rangan, 2007). The author thinks this could be due to the unique

characteristics of the selected fly ash batch in this research. After all, the

geopolymer mix with W/G=0.24 is found to have the best strength among the

tested mixes, and therefore recommended for the geopolymer aggregate

research.

Figure 5.1: 7-day compressive strengths of geopolymers with different W/G values

49.85 52.94

49.33 47.16

0

10

20

30

40

50

60

0.23 0.24 0.25 0.26

Com

pres

sive

str

engt

h (M

Pa)

W/G value


5.3 Manufacturing Procedure

This section introduces a manufacturing procedure for geopolymer aggregates

in combination with the author’s research. In Chapter 1, subsection 1.2.2, the

author illustrated the desired advantages of introducing geopolymer technology

in cost and environmental issues for creating geopolymer aggregate as a new

FA-LWA product. These advantages are expected to be realised in a real

manufacture of geopolymer aggregates presented in this section.

In principle, a desired geopolymer material is firstly created, using a normal fly

ash-based geopolymer mix with suggested fly ash, mix design and curing

regime as discussed in the previous parts of this thesis. This is to make a

hardened material with suitable performance for an aggregate product through

a low power-oriented process. Then, a mechanical processing is applied to

produce the aggregate particles from the created hard geopolymer material, to

save the mechanical pelletisation process as usually required in conventional

FA-LWA manufacture. A specific flowchart describing the manufacturing

procedure is presented in Figure 5.2 with specific steps discussed in the

following subsections.

137

Figure 5.2: Flowchart of manufacturing procedure for geopolymer aggregate

5.3.1 Mixing and Casting

In this step, a suitable geopolymer material is created from the 2012-Dec-13 fly

ash batch, as proposed in Chapter 2. The mix design specified with W/G=0.24,

SiO2/Na2O=1.25 and H2O/Na2O=11, which is recommended in this research as

of section 5.2, is now presented in Table 5.2 and used to create the needed

geopolymer material.

A normal geopolymer mixing procedure is carried out. Firstly, fly ash and an

alkali activator are mixed for 15 to 20 minutes using a 10-litre volume

mechanical mixer. Then, the uniform fresh geopolymer is cast into a flat


stainless steel container, which is later fixed on a vibrating table. A vibration for

1 to 2 minutes shapes the geopolymer mixture to the internal dimensions of the

container. A steel container is used because it is sufficiently strong to be held

on a vibrating table and the sample does not stick to it in a short time. Also, it is

flat to ensure that the thickness of the produced geopolymer is less than the

size of the opening in the jaw crusher. Later, considering that the fresh

geopolymer mixture can stick to the steel surfaces especially while being

heated, the cast geopolymer is transferred to another same-sized plastic

container, and cut into the shape of chunks (Figure 5.3) for facilitating a

convenient curing and washing procedure as required and will be introduced in

next subsections. The chunks were created by cutting the geopolymer pastes

that were cast in the trays, using a large spatula when they were still fresh.

Finally, the plastic container with geopolymer chunks inside is sealed with a lid

before heating.

Table 5.2: Geopolymer mix design for geopolymer aggregate manufacture


W/G value 0.24




Fly ash (kg/m3) 1283.42



Water (kg/m3) 14.03

139

Figure 5.3: Fresh geopolymer chunks in plastic container

5.3.2 Curing

A conventional heat-curing regime of 80oC for 3 days is applied to harden the

manufactured fresh geopolymer chunks because, based on the research

presented in Chapter 4, section 4.3, heat still seems to be the most powerful

method for enhancing the strength of a fresh geopolymer.

The fresh geopolymer chunks are sealed in a plastic container and rested in the

ER (20-23oC) for 24 hours to allow sufficient time for the initial stage

geopolymer reactions. Then, it is oven-heated at 80oC for 3 days in an electric

oven to maximise development of the geopolymer. After heating, it is returned

to the ER and de-moulded after cooling down to room temperature. The curing

moisture in the ER is set to 50% relative humidity (RH), a low-moisture

condition adapted from the research outlined in Chapter 4, section 4.4. This

post-heating period might be theoretically treated as a part of the entire curing

regime, however, it usually does not lead to obvious changes in hardened

properties of geopolymer (Rangan, 2007, Hardjito and Rangan, 2005). For


consistency, in this research, it is specified that all geopolymer chunks

experience a same curing regime of 28 days in total after mixing, including the

days of heating and the days of storage in the ER, and are then subjected to

further processing.

5.3.3 Washing and Air Drying

After curing, washing is required to remove any residual caustic alkaline

activator that might be present in the pores of the hardened geopolymer

samples due to incomplete geopolymerisation process. This is for a more user-

friendly aggregate product, which is done by soaking the geopolymer samples

in a sealed water tank and changing the water every day. For thorough washing,

the soaked geopolymer samples were in a shape of small-sized chunk, as

shown in Figure 5.4. The washing continues until the pH value of water in the

tank is neutral range (pH=5-9).

Figure 5.4: Soaking of hardened geopolymer chunks in water tank

Here, the duration of washing is decided by the pH variation of the solution in

the water tank where geopolymer aggregates are soaked in. For a consistent

pH value result, which is determined by the amount of alkalis and the volume of

141

solution, it has been decided that 5 kg of geopolymer chunks are soaked in 20

litres of water in the tested water tank. This way can basically assure the

consistency of pH test and is simple to be completed in the practical work. As

seen from Figure 5.5, it takes 4-5 days of washing to reduce the pH value to a

neutral range for the geopolymer chunks produced here. Afterwards, it is

considered that most alkali content has been removed, and the final

geopolymer aggregate product is much less hazardous for users.

Figure 5.5: pH value after every day’s washing

After washing, the geopolymer chunks are dried in air. For the manufacturing

procedure proposed in this research, no additional heat is applied for drying,

because it is still unsure if a heat treatment applied on a geopolymer that has

already been hardened by heating, could lead to any further reaction and

modify its structure. Although this is a very interesting aspect that needs further

exploration, the author considers it beyond the scope of this research.

0

2

4

6

8

10

12

1 2 3 4 5

pH v

alue

Washing period (day)


5.3.4 Mechanical Crushing, Sieving and Storage

Later, a mechanical crushing process is applied to reduce the dimensions of the

hardened geopolymer chunks into the proper range of aggregate particles. This

step produces aggregates after the hardening of fresh mixtures instead of prior

to it, and is likely to save a pelletisation process in usual FA-LWA manufacture

(Clarke, 1993, Swamy and Lambert, 1981). On the other hand, the aggregate

particles made this way is expected to be in an angular or irregular shape rather

than a spherical shape, which may influence the workability (Shilstone, 1999,

Kockal and Ozturan, 2010) but will be likely to enhance the strength with an

improved interlocking effect (Kayali, 2008, Zhang and Gjørv, 1990, Lo et al.,

2004, Wasserman and Bentur, 1996) of their concrete mixes.

A jaw crusher (Figure 5.6) is applied to complete this step of mechanical

crushing, which functions to squeeze particles to make them broken based on a

compressive force generated from the relative movement of two jaws (normally,

one jaw is fixed and the other jaw reciprocates). Jaw crusher is a convenient

way of breaking particles into small sizes, which has been previously used in

conventional LWA manufacture (Hu et al., 2009, Kayali, 2008), and hence is

applied here for geopolymer aggregate manufacture.

This jaw crusher can generate particles with a wide distribution of sizes which is

controlled by two factors, the gear of the jaw crusher and the cycling time of

crushing. These two factors are combined and referred to as the crushing

program. The gear number selected determines the opening size at the lower

end of the crushing chamber, which is varied between 1 mm and 30 mm with

ten different opening widths set up. Normally, one crushing program results in

143

only one particle size distribution of the produced aggregates. Therefore, an

assessment combined with trial tests is required to determine the proper

crushing program for the desired particle size.

Figure 5.6: Jaw crusher used for manufacture of aggregate particles

As, in the following chapters, a geopolymer aggregate sample with a nominal

size of 10 mm is required for concrete manufacture, the target maximum size of

jaw crushing is 10 mm. To achieve this, trial crushing processes are carried out,

with the resultant different particle size distributions tracked and shown in

Figure 5.7. It can be seen that the particle size of a sample can mostly be

reduced to less than 10 mm after two crushing cycles at gear-7 (opening size is

around 22 mm) and one at gear-5 (opening size is around 15 mm), this program

is selected for making an aggregate sample with a nominal size of 10 mm. The

obtained sample is later sieved using a 4.75 mm standard sieve, with the

portion resided on the sieve collected to be the coarse aggregates.

On the other hand, the portion of the sample passing the 4.75 mm sieve is

further crushed to make fine aggregates. Considering the usual expectation of

there being finer particles in a fine aggregate portion, a gear-1 (opening size is


around 1 mm) crushing is used. However, it seems that the particle size of the

sample is hardly finer after three crushing cycles at gear-1, as seen in Figure

5.7, so the program for producing fine aggregates is set to be three times

crushing at gear-1.

Figure 5.7: Grading curves of geopolymer aggregate samples after each jaw-crushing cycle

From the above, the final crushing program for making both coarse and fine

aggregate portions is confirmed as:

• using gear-7 for 2 cycles;

• using gear-5 for 1 cycle;

• sieving and removing the coarse particles (>4.75mm); and

• using gear-1 for 3 cycles.

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

0.01 0.1 1 10 100

Cum

ulat

ive

perc

enta

ge p

assi

ng (%

)

Sieve size (mm)

Gear 7-1st time

Gear 7-2nd time

Gear 5-1st time

Gear 1-1st time

Gear 1-2nd time

Gear 1-3rd time

145

The prepared coarse (4.75-10 mm) and fine (<4.75 mm) geopolymer aggregate

samples are maintained in an air dry condition and then stored in sealed

containers.

5.4 Appearance

The air-dried coarse and fine geopolymer aggregates produced by the

procedure in section 5.3 are displayed in Figure 5.8. The geopolymer

aggregates display a unique light-grey appearance, with angular particle shapes

formed through the step of jaw crushing. Their surfaces look smooth but feel a

little rough and are not dusty after washing.

Figure 5.8: Coarse (>4.75mm) (left) and fine (<4.75mm) (right) geopolymer aggregates in air-dried condition

An enlarged image of a geopolymer aggregate captured using a Lynx stereo

inspection microscope (Figure 5.9) is shown in Figure 5.10. A geopolymer

aggregate presents a rough and uneven surface with porous craters which can

lead to it having a lightweight property and may also strengthen its interlocking

with the cement matrix in a concrete mix (Kayali, 2008, Zhang and Gjørv, 1990).

The properties affected by these shape and surface characteristics of a

geopolymer aggregate are investigated in the following sections.


Figure 5.9: Lynx stereo inspection microscope for observation of surface of geopolymer aggregate

Figure 5.10: Enlarged image (40 times) of particle surface of geopolymer aggregate (length approximately 3mm)

The pores detected in a geopolymer aggregate can be formed for several

reasons: 1) the space left by the residual activator solution; 2) the space

occupied by the air introduced during mechanical mixing and vibration; and 3)

the space caused by the curls of the polymerised structure of geopolymer, as

has been previously observed (Kriven et al., 2003, Sindhunata et al., 2006).

1 mm

147

Although these reasons might provide possible explanations for the porous

craters detected in a geopolymer aggregate (Figure 5.10), further research is

certainly needed.

5.5 Grading

Grading indicates the particle size distribution and is a significant factor for

describing the quality of a geopolymer aggregate sample. In a concrete mix,

well-graded aggregates consisting of particles in appropriate size distributions

are always preferred for densified compaction. In contrast, single-sized

aggregates, which are always not properly compacted with much space left

among the particles, may weaken the structure and require a high amount of

cement, and therefore are not preferred (Neville, 1996).

The grading of two geopolymer aggregate samples is discussed. One is a

sample collected directly from the jaw crusher and referred to as the ‘original

geopolymer aggregate’, the grading of which is determined by only the nature of

the jaw crusher and applied crushing program (subsection 5.5.1). The other is a

sample further refined after jaw crushing to meet the requirements of relevant

standards (subsection 5.5.2).

5.5.1 Original Geopolymer Aggregate

Grading of the original geopolymer aggregate sample is tested based on the

standard sieve analysis, with the results shown in Table 5.3 and subsequent

grading curves in Figure 5.11. Besides, the grading curves of the laboratory-

used crushed stone and river sand, which are two common types of natural

aggregates for concrete use, are also presented in Figure 5.11 for comparison.


Table 5.3: Typical sieving analysis of coarse and fine geopolymer aggregates

Sieve size Mass retained (g)

Percentage retained (%)

Cumulative percentage retained (%)

Cumulative percentage passing (%)

Coarse

13.20 mm 0.0 0.0 0.0 100.0

9.50 mm 2.6 0.3 0.3 99.7

6.70 mm 536.5 53.0 53.3 46.7

4.75 mm 457.0 45.2 98.4 1.6

<4.75 mm 16.0 1.6 100.0 0.0

Total 1012.1

Fine

4.75 mm 0.2 0.1 0.1 99.9

2.36 mm 21.9 9.1 9.1 90.9

1.18 mm 46.5 19.4 28.6 71.4

600 µm 81.7 34.1 62.7 37.3

425 µm 67.3 28.1 90.8 9.2

300 µm 14.9 6.2 97.0 3.0

150 µm 5.9 2.5 99.5 0.5

75 µm 0.9 0.4 99.9 0.1

<75 µm 0.3 0.1 100.0 0.0

Total 239.4 Fineness modulus 3.88

149

Figure 5.11: Grading curves of geopolymer aggregates and regularly used crushed stone and river sand

The coarse geopolymer aggregates display a good grading, with an even size

distribution in the range of 4.75 to 9.50 mm, which satisfies the regulations in

the ASTM C330 standard (ASTM, 2009) and is very close to that of the normal

weight crushed stone. Therefore, it is considered that these aggregates can be

directly used in concrete with no need for further refinement.

On the other hand, the fine geopolymer aggregates do not show a good size

distribution but a high value of fineness modulus which is also not as fine as

that of the river sand, as seen in Table 5.3 and Figure 5.11. Moreover, they fail

to meet relevant regulations in the standard ASTM C330 presented in Table 5.4,

with their percentages of particles passing 300 µm too small. As this is likely

due to the limitation of the jaw crusher previously discussed, further refinement

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

0.01 0.1 1 10 100

Cum

ulat

ive

perc

enta

ge p

assi

ng (%

)

Sieve size (mm)

Crushed stone

River sand

GA coarse

GA fine


of the fine geopolymer aggregates is needed before applying them to concrete

making.

Table 5.4: Grading requirements regulated in ASTM C330 for lightweight concretes (printed in ASTM, 2009)

(This table has been removed due to copyright restrictions)

5.5.2 Refined Geopolymer Aggregate

A refinement process for modulating the grading of fine geopolymer aggregates

to make them suitable for concrete is introduced. This is done by regrouping the

sample of fine geopolymer aggregates. Firstly, the desired mass proportion

value of each size fraction is designed, based on the range of each fraction

regulated in ASTM C330 (Table 5.4), and presented in Table 5.5. Then, the

amount of aggregates required from each fraction is calculated and included in

the final geopolymer aggregate sample.

For example, suppose that 1000 grams of well-graded fine geopolymer

aggregates are needed. From Table 5.5, the mass proportions for the size

fractions 1.18-4.75 mm, 300µm-1.18 mm, 150-300 µm and 75-150 µm are 20%,

60%, 15% and 5% respectively. Therefore, the amounts of aggregates required

from each of these fractions are 200 grams, 600 grams, 150 grams and 50

grams which are included in the final fine geopolymer aggregate sample.

151

In the grading curves shown in Figure 5.12, it can be seen that the new refined

sample of fine geopolymer aggregates has an increased proportion of particles

passing 300 µm compared with that in the old sample, and is much closer to

that of river sand. After refinement, according to ASTM C330, these fine

geopolymer aggregates are qualified for application in concrete manufacture.

Therefore, the refinement procedure discussed in this subsection is applied to

prepare the fine geopolymer aggregate portion in future concrete mixes that are

discussed in the following chapters.

Table 5.5: Refined values of fine geopolymer aggregates based on ASTM C330

Sieve size Percentage retained

Cumulative percentage

retained

Cumulative percentage

passing

4.75 mm 0% 0% 100%

1.18 mm 20% 20% 80%

300 µm 60% 80% 20%

150 µm 15% 95% 5%

75 µm 5% 100% 0%


Figure 5.12: Grading curves of river sand, and original and refined fine geopolymer aggregates

5.6 Density and Water Absorption

Density and water absorption capacity are two important factors that reflect the

physical quality of aggregates, especially for a LWA which normally has a low

density and high absorption capacity due to its highly porous structure (Neville,

1996). These two factors regarding the geopolymer aggregate produced in

section 5.3 are measured in reference with the Australian standard AS 1141

and shown in Table 5.6 (AS, 1999).

At this point, the author finds it necessary to describe the way to acquire the

oven-dry density, that is, the density of an aggregate sample with no water left

in its penetrable pores. Although, normally, a heat treatment at 100oC to 105oC

is used to reach this condition according to the regulations in standard AS 1141

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

0.01 0.1 1 10

Cum

ulat

ive

perc

enta

ge p

assi

ng (%

)

Sieve size (mm)

River sand

Orignial GA fine

Refined GA fine

153

(AS, 1999), the suitability of this for a geopolymer aggregate is arguable

because, based on current knowledge it is still not clear whether a heat

treatment on a heat-hardened geopolymer can lead to any further structural

change. Therefore, another way to realise the ‘oven-dry’ condition is applied in

this section, by removing the water under a low-pressure condition in a silica-gel

desiccator assisted by a vacuum pump.

Also, it is noted that, to make the tested geopolymer aggregate sample easier to

get dried, the sample is firstly placed in the desiccator used for this type of

drying immediately after being in air-dried condition right after the jaw crushing.

For this case, the ‘oven dry mass’ is measured first, and the other parameters

required (like ‘SSD mass’) are measured later. Compared to the standard order

of measurements suggested in AS 1141, the method used in this research need

further caution to prevent any material loss during the test.

Table 5.6: Density and water absorption capacity of geopolymer aggregate

Coarse (28-day)

Fine (28-day)

Specific gravity (SSD) 1.84 2.01

Specific gravity (OD) 1.66 1.77

Dry loose bulk density (kg/m3) 876.8 1117.2

Water absorption capacity (in 24 hours) 11.0% 13.7%

SSD - saturated surface dry condition; OD - oven dry condition

From Table 5.6, a coarse geopolymer aggregate has specific gravities of 1.84

and 1.66 under saturated surface dry and oven dry conditions respectively


which are lighter than those of a regular aggregate and belong to the area of

LWAs (Chandra and Berntsson, 2002). A fine geopolymer aggregate has a

similarly low specific gravity but of a slightly higher value, with specific gravities

of 2.01 and 1.77 under saturated surface dry and oven dry conditions

respectively. The dry loose bulk densities of coarse and fine geopolymer

aggregates are 876.8 kg/m3 and 1117.2 kg/m3 respectively which are also

sufficiently low to be considered LWAs (ASTM, 2009). Therefore, a geopolymer

aggregate is a type of LWA created from coal fly ash and referred to as a type

of FA-LWA product.

On the other hand, both coarse and fine geopolymer aggregates have high

water absorption capacities of above 10%, much higher than those of normal

weight aggregates (NWAs) (Neville, 1996), which is also of concern during a

concrete mix (ACI, 1998, ACI, 2003). Nevertheless, it has been seen that a

geopolymer product solely made from Class F fly ash can have a high capacity

of water absorption (Görhan and Kürklü, 2014). Several previous LWA samples

with high water absorptions are also found to end up in good quality concrete

products (Jo et al., 2007, Swamy and Lambert, 1981). So it is estimated that the

produced geopolymer aggregate may also lead to a good concrete mix if it can

be properly applied.

Furthermore, the author finds that the synthetic conditions, namely mix design

and curing regime, can greatly affect the density and water absorption capacity

of geopolymer aggregate. This is reasonable since geopolymer aggregates are

composed of geopolymer materials which can be affected by these conditions

(Duxson et al., 2007a). The variations in density and water absorption caused

by geopolymer synthetic conditions are illustrated in following paragraphs.

155

Geopolymer aggregate samples are made using the synthetic conditions

presented in Table 5.7. Here, the geopolymer mixes are expressed using the

proposed criteria in Chapter 3, to more clearly indicate the different composition

in each mix. The density and absorption characteristics of produced geopolymer

aggregates are measured and shown in Table 5.8. For aggregate samples No.

1, No. 2 and No. 3, it can be seen that a longer heating time results in a higher

density and lower water absorption. This may suggest a more densified

structure which is possibly due to a better development of geopolymer under a

long duration of heat treatment (Palomo et al., 2004, Rangan, 2007).

Table 5.7: Various mix designs and curing regimes for geopolymer aggregate manufacture

Code x value (SiO2/Na2O)

y value (H2O/Na2O) W/G F/A Curing

temp. (oC) Curing

days (d)

1 1.25 11 0.24 2.05 80 1

2 1.25 11 0.24 2.05 80 2

3 1.25 11 0.24 2.05 80 3

4 1.25 12 0.24 2.16 80 3

5 1.25 11 0.26 1.86 80 3

Note: the values of x and y are used to specify the composition of activator Na2O·xSiO2·yH2O; W/G means water-to-geopolymer solids ratio; F/A means fly ash to activator ratio. These are all detailed in Chapter 3.

Also, the results for samples No. 3 and No. 4 show that a higher water content

with a higher H2O/Na2O (y value) in the activator of the initial mix can lead to a

lower density and higher water absorption capacity of the end-product when the

same curing regime is applied. This may be due to the weak geopolymerisation


process caused by higher water content in the initial geopolymer mix (Xie and

Kayali, 2014).

Table 5.8: Density and water absorption values of geopolymer aggregate samples made from various mix designs and curing regimes

Code Specific gravity (SSD)

Specific gravity (OD)

Water absorption capacity

1 1.76 1.52 16.0%

2 1.79 1.58 13.3%

3 1.84 1.66 11.0%

4 1.80 1.55 15.7%

5 1.94 1.68 15.8%


Sample No. 5 is based on sample No. 3 but with a higher amount of activator

added, although the composition of the added activator is unchanged.

Compared with sample No. 3, sample No. 5 has a slightly higher density, and a

much higher water absorption capacity that requires a more strict control in

concrete mix (ACI, 1998). It is considered that the extra activator in sample No.

5 should generate a higher degree of geopolymerisation and, therefore, a more

densified structure, as previously observed (Duxson et al., 2005) and reflected

by its higher density. But this variation in density is found not very obvious for

these two tested samples. On the other hand, the fact that the extra activator

added in the mix of No.5 also introduces more water and leaves more space in

the hardened structure may explain the much higher water absorption capacity

(15.8%) of this mix, which makes it less preferred in real practice.

157

In summary, the mix of No. 3 can result in satisfactory specific gravity and water

absorption capacity values, for which reasons, the author considers it preferable

for the manufacture of geopolymer aggregates.

5.7 Internal Pores

This section investigates the internal pores in the structure of a geopolymer

aggregate which is considered the main reason for its low specific gravity. The

relevant pore information is analysed using an optical microscopic (OM) method.

A geopolymer aggregate sample is mounted and polished to expose its internal

cross-section, and then an image of which is captured by a Zeiss AxioCam

MRC digital camera.

Traditionally, the information on internal pores of aggregates is analysed by

mercury intrusion porosimetry (MIP) or gas adsorption equipment. MIP uses

mercury to penetrate into the pores of tested samples and record the pressure

responses during penetration, based on which information porosity can be

obtained. MIP can cover sizes ranging from 0.0055 µm to 360 µm. However,

several of its limitations make it unsuitable for this research; for example,

mercury usually does not penetrate all the pores in a tested sample which will

underestimate the volume of pores. Also, the high pressure of mercury can

easily break the walls of the inner pores, thereby overestimating the proportion

of large pores (Korat et al., 2013, Diamond, 2000).

On the other hand, the gas adsorption method, which may use several types of

gas such as nitrogen, argon and krypton, can also be used for research on

internal pores. The principle of gas adsorption is to use the gas to penetrate and

attach to the inner surfaces of the pores, and then record variations in the


volume and pressure to obtain porosity information. Although it is relatively

more accurate than MIP and has been applied in geopolymer materials (Duxson

et al., 2007e, Sindhunata et al., 2008), its maximum detectable size typically as

360 nm (Korat et al., 2013, Bouguerra et al., 1998) is considered by this author

to be not large enough for the analysis of geopolymer aggregates.

For above reasons, a microscopic method that can mostly cover micron-sized

pores is proposed in this research. The major target here is to observe the

pores present in the internal structure of geopolymer aggregates, and to

measure the size and area distributions of the pores in different sizes. Further

information on the pores of LWAs can also be conducted using a X-ray micro-

tomography (micro-CT) (Korat et al., 2013).

Geopolymer aggregate samples are selected randomly from stockpiles, washed

with water and then washed with ethanol and dried at 40oC. Afterwards, a flat

surface of the cross-section of a sample, as required by the OM method, is

created by the polishing procedures illustrated in Figure 5.13, that is: making a

mixture of epoxy resin and hardener with a mass proportion of

epoxy/hardener=25/3; soaking the aggregate sample in the fresh mixture in a

plastic mould to hold the sample; curing the sample at room temperature until

the fresh resin hardens; milling the resin with the sample inside using a grinder

on an abrasive paper with a grit size of P4000 until a flat surface appears;

polishing the sample to a 6 µm finish on the diamond plate of a polisher. Later,

the created surface is ready for microscopic observation.

159

Figure 5.13: Polishing procedures (1) mixing resin and hardener, (2) and (3) soaking sample in resin, (4) covering sample in hardened resin, (5) grinder and

(6) polisher

A bright field image of a geopolymer aggregate displaying its direct reflection of

the vertical incoming light is captured by a Zeiss AxioCam MRC digital camera,

and shown in Figure 5.14 in which the dark areas with no light reflected are the

pores and the bright areas are the surface of the cross-section. This image

(1) (2)

(3) (4)

(5) (6)


shows a large range of pores of different shapes and sizes inside the

geopolymer aggregate which indicates that it has a porous internal structure

similar to its surface structure. This explains the lightweight performance of

produced geopolymer aggregates with a similar porous internal structure as the

other FA-LWAs reported before (Kayali, 2008, Swamy and Lambert, 1981).

Figure 5.14: Bright field image of geopolymer aggregate sample

Information regarding the porosity, like that of the size, area and number of

pores, is measured based on the captured bright field image assisted by the

ImageJ image-processing software. At first, a specific zone of the cross-section

of geopolymer aggregate sample is captured (like the left image in Figure 5.15).

Then, ImageJ converts the captured image into a black-and-white one to better

distinguish the pore areas from the structure, with the threshold of grey scale

set to 154. Then, it automatically measures the size, area and number of

identified pores (black areas) in the image on the right in Figure 5.15. The

minimum size of the identified pores is 1.452 µm, with the smaller pores

distorted in the captured image due to the limitation of the applied facility

161

although this resolution is considered adequate for analysing an aggregate

sample. To better represent the porosity information of the geopolymer

aggregate produced in this chapter, 27 images at different positions in the

geopolymer aggregate are captured and measured following the same

procedure as above, with the results presented in Figure 5.16 and Figure 5.17.

Figure 5.15: Original captured image (left) and converted black-and-white image (right) of geopolymer aggregate sample

Figure 5.16: Size distribution of detected pores in geopolymer aggregates as obtained using an optical microscopy method

0%

2%

4%

6%

8%

10%

12%

14%

0.1 1 10 100 1000

Perc

enta

ge in

tota

l por

es (%

)

Pore size (µm)

200 µm


Figure 5.17: Distribution of areas occupied by detected pores in geopolymer aggregates

Figure 5.16 displays the size distribution of pores detected in the geopolymer

aggregate sample, which indicates that the detected pores are in a wide size

range of 1 µm to 250 µm, and the portion within 1 µm to 10 µm is dominant. On

the other hand, Figure 5.17 shows that the pores of 10 µm to 50 µm occupy

most areas, despite there being less of this fraction than of 1 µm to 10 µm pores.

From above results, it is concluded that the pores present in the structure of

geopolymer aggregates are micron-sized, and mostly concentrated in a size

range less than 50 µm. Meanwhile, only around 0.1% of the detected pores are

larger than 100 µm as seen from Figure 5.16. This means that the pores in the

geopolymer aggregate structure are concentrated in a relatively small size

range as far as this feature of LWAs is concerned (Korat et al., 2013). Therefore,

it is envisaged that geopolymer technology may help reduce the size of pores

0.0%5.0%

10.0%15.0%20.0%25.0%30.0%35.0%40.0%45.0%50.0%

1-10µm 10-50µm 50-100µm 100-500µm

Perc

enta

ge in

tota

l por

ous

area

(%

)

Pore size (µm)

163

formed in an aggregate sample, and mitigate the defects of large-sized internal

pores.

On the other hand, based on the microscopic images in Figure 5.14, the surface

of a geopolymer aggregate’s structure is not continuous but has a large amount

of micron-sized pores. This amount of pores seems sufficient to provide the

needed porous volume to realise the lightweight of a geopolymer aggregate, as

seen from the density results in section 5.6, despite that the volume of each

single pore would be small due to its small pore size. Moreover, this type of

microstructure containing more homogeneously distributed small-sized pores is

likely to be less fragile compared to the one containing large-sized pores (Masi

et al., 2014).

5.8 Crushing Value

For a FA-LWA, its mechanical property is significant because it determines its

capability in resisting external loads in further construction applications. This is

normally reflected by the mechanical performance of a concrete made with a

specific FA-LWA product (Kayali, 2008, Lo et al., 2004) which is considered a

more convincing examination method. On the other hand, several methods that

are directly conducted on aggregate samples are also proposed, to obtain initial

estimations of their mechanical properties (Bui et al., 2012), such as the

standard crushing value test which is regulated in Australian standard AS 1141

(AS, 1999) and applied in this section.

The crushing value test assesses the capability of an aggregate sample to bear

an external mechanical pressure. As required in AS 1141, a fraction of the 9.50

mm to 13.20 mm of geopolymer aggregate particles manufactured in section


5.3 is selected for the crushing value test, which is operated on a SHIMADZU

universal testing machine (1000 kN capacity) shown in Figure 5.18. The load is

made according to a certain procedure in AS 1141 for applying a force at a rate

of 40 kN per minute until 400 kN. After the test, the aggregate sample (Figure

5.19) is sieved through a 2.36 mm sieve to measure its mass proportion of

particles under a size of 2.36 mm which is its crushing value (AS, 1999). The

crushing value of the selected geopolymer aggregate sample is 23.43%, as

shown in Table 5.9.

Figure 5.18: Crushing value test of geopolymer aggregate sample

Generally, higher crushing values indicate weaker structures of the aggregates.

The crushed normal weight stone used in the locality has a crushing value of

16.90%, while usually, those of FA-LWAs are between 26% and 35% (Kayali,

2008). Therefore, the 23.43% crushing value is relatively lower, indicating a

better capability of the new geopolymer aggregate in resisting the external

pressure in a crushing value test. Based on this, geopolymer aggregate sample

165

seems to have a comparable or even slightly stronger structure than the usual

FA-LWAs, implying its potential to be undertaken for concrete manufacture.

Nevertheless, it is still weaker compared to the normal weight crushed stone.

Figure 5.19: Appearance of geopolymer aggregate sample after crushing value test

Table 5.9: Crushing value of geopolymer aggregate sample tested

Property Geopolymer aggregate

Crushing value (%) 23.43

5.9 Rebound Test by Schmidt Hammer

The mechanical performance of geopolymer aggregate is further evaluated by

rebound test based on a Schmidt hammer. Rebound test is a common way of

describing the surface hardness and penetration resistance of natural rocks

(Katz et al., 2000, Cerna and Engel, 2011). As the created geopolymer

aggregate can be treated as a new kind of ‘artificial rock’ to replace the natural

rock in a concrete mix, this rebound test is considered appropriate for providing

a supplementary knowledge on the property of geopolymer aggregate in

comparison with other natural rocks regularly used in concretes.


The rebound test is conducted according to the procedure regulated in ASTM

C805 (ASTM, 2014). The tested sample is a block of synthesized ‘geopolymer

rock’ with a flat and smooth surface in the dimensions of

130mm×130mm×100mm, as required in a rebound test. This ‘geopolymer rock’

was made by normal geopolymer synthetic method with the same mix design

and curing regime as those for geopolymer aggregates, and therefore has the

same material base as the geopolymer aggregate.

Table 5.10: Results of Schmidt hammer rebound test

Hammer’s angle: -90o, vertical-down

Operation temperature: 20-23oC

Distance between each impact point: 25 mm

Rebound index (kPa)

No.1 40 No.6 39

No.2 38 No.7 37

No.3 37 No.8 38

No.4 37 No.9 37

No.5 41 No.10 42

Mean rebound index (kPa): 38.6

Standard deviation (kPa): 1.8

The rebound result is displayed in Table 5.10 in which the final rebound index is

a mean value of ten separate impacts. It is seen that the rebound index of

geopolymer aggregates is 38.6, which is much higher than the chalk (23.9),

close to limestone and sandstone (41.5-50.8), but lower than syenite and

granite (65.0-73.4) (Katz et al., 2000). From this result, it can be concluded that

167

geopolymer aggregate is expected to have similar mechanical performance to

limestone and sandstone, which are two regular types of natural rock for

concrete use (Neville, 1996), and is capable of making the moderate-strength

concrete products like limestone and sandstone aggregates (Sengul et al.,

2002). This implies that the newly made geopolymer aggregate, as a type of

FA-LWA, may be able to serve the need in a structural concrete. Further

discussions will be made on this part in the following chapters, in combination

with the more direct mechanical outcomes from the concrete samples that are

made with geopolymer aggregate.

5.10 Infrared (IR) Spectroscopy

As infrared (IR) spectroscopy is a common method for identifying chemical

function groups, such as Si-O-Si, Si-O-Al and -OH, and has been applied in

geopolymer characterisations (Davidovits, 2008). It is used to further assess the

chemical structure of the geopolymer aggregates produced. This is conducted

using a SHIMADZU infrared spectrophotometer equipped with an attenuated

total reflectance (ATR) accessory, from 4000 cm-1 to 650 cm-1 with a 4 cm-1

resolution, single bounce and total of 32 scans.

The IR spectra of the fly ash and geopolymer aggregate are presented in Figure

5.20. For the fly ash, there is a strong band at 1056.99 cm-1 which indicates the

presence of an amorphous phase that normally lies in the range of 950 cm-1 to

1250 cm-1 (Lee and Van Deventer, 2003) and could be the Si-O-Si and Si-O-Al

bonds belonging to the aluminosilicate contents of the fly ash (Barbosa et al.,

2000). Also, the bands at 794.67 cm-1 and 740.67 cm-1 indicate the stretching

vibration of Al-O and that at 695.20 cm-1 indicates the symmetrically stretching


vibration of Si-O which may all come from the mullite in the fly ash (Padmaja et

al., 2001, Fang, 2013). Moreover, the band at 777.31 cm-1 is considered to be

due to the presence of quartz (Barbosa et al., 2000, Davidovits, 2008).

On the other hand, the geopolymer aggregate sample tested has a different

spectrum to that of fly ash. It presents the same strong band in the range of 950

cm-1 to 1250 cm-1, indicating the presence of amorphous phases containing the

Si-O-Si and Si-O-Al bonds but, at the lower frequency of 999.13 cm-1, implying

an increasing proportion of the tetrahedral aluminium component which is very

possibly due to the involvement of Al components through the

geopolymerisation process (Davidovits, 2008). Moreover, this indicates that

geopolymer gels, mostly likely the N-A-S-H, could be formed in a GA sample as

supported by the findings of other researchers (Fernández-Jiménez and

Palomo, 2005, García-Lodeiro et al., 2007, Rees et al., 2007) which is

reasonable because H2O or OH is able to reside in the aluminosilicate

framework at such a relatively low-temperature (80oC) synthesising condition.

Except for this, there are several less intensive bands found in the range of 690

cm-1 to 800 cm-1 in the GA sample. These bands seem to be related to those

bands that represent crystalline phases in the IR spectrum of fly ash, but display

slightly different wavelength values and intensities. A likely reason may be that,

in a fly ash sample there are only fly ash particles, but in a GA sample there co-

exist some different phases including gels and residual solutes from the added

alkali activator (Rees et al., 2007) which may influence its framework’s

response to an IR spectroscopy. A similar phenomenon in that the detected

bands in such range (690 cm-1 to 800 cm-1) could shift from fly ash to

geopolymers has been discovered (Fang, 2013). Yet further investigation is

169

required. On the other hand, it has been discussed in Chapter 2, section 2.5,

that the mullite in fly ash is likely to be activated by the synthesising conditions

applied in this thesis. For this case, the question whether the shifted bands

observed above also imply a footprint of the activation of mullite must be further

investigated.

Figure 5.20: IR spectra of fly ash and geopolymer aggregate

5.11 Evaluation of Quality of Geopolymer Aggregate

In this chapter, a geopolymer aggregate is proposed as a new option for a FA-

LWA but at a lower cost and with a more eco-friendly manufacturing procedure,

and its relevant properties discussed in previous sections. To substitute for

conventional FA-LWA products in concrete applications, this new geopolymer

aggregate must have sufficient qualities to satisfy requirements in the relevant

standards and be comparable with the usual FA-LWAs in terms of properties

considered in this section.

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

500.01000.01500.02000.02500.03000.03500.04000.0

Tran

smitt

ance

(%)

Wave number (cm-1)

1056

.99

794.

67

777.

31

740.

67

777.

31

798.

53

736.

81

694.

37

999.

13

695.

20

GA

Fly ash


5.11.1 Compatibility with Standards

Firstly, the compatibility of geopolymer aggregates with relevant standards is

investigated. Normally, for a FA-LWA product, density is highly significant for

determining its self-weight and strength. The regulations in the standards

introduced in Chapter 1, subsection 1.1.3, regarding the density range of LWAs

are presented in Table 5.11. Based on previous research, a coarse geopolymer

aggregate has a SSD specific gravity of 1.84, oven-dry specific gravity of 1.66

and dry loose bulk density of 876.8 kg/m3. A fine geopolymer aggregate is

heavier than a coarse one, with a SSD specific gravity of 2.01, oven-dry specific

gravity of 1.77 and dry loose bulk density of 1117.2 kg/m3. According to the

standards listed in Table 5.11, geopolymer aggregates have density values

which qualify them for use as LWAs in concrete structures.

Table 5.11: Density values of geopolymer aggregates in relation to relevant LWA standards

GA coarse

GA fine

AS 2758

ASTM C330

BS EN 13055

Specific gravity (SSD) 1.84 2.01 - - <2.0

Specific gravity (OD) 1.66 1.77 0.5-2.1 - -

Dry loose bulk density (kg/m3) 876.8 1117.2 - Coarse<880

Fine<1120 <1200


Apart from density, grading is another important factor that needs to be

evaluated. Grading of geopolymer aggregates and its requirements in relevant

standards are presented in Table 5.12 from which it has been shown that both

171

coarse and fine geopolymer aggregates can satisfy after undergoing the

crushing and refinement procedures introduced in section 5.5. This indicates a

compact packing of the applied well-graded aggregate samples in concrete

mixes. Therefore, the geopolymer aggregate samples obtained are able to be

applied in actual lightweight concrete manufacture.

Table 5.12: Grading of geopolymer aggregates in relation to relevant LWA standards

Sieve size

Percentage (mass) passing sieves (%)

ASTM C330

BS 3797 grade L1

BS 3797 grade L2 GA

Coarse

9.5 mm 80-100 85-100 85-100 99.7

4.75mm 5-40 - - 1.6

Fine

4.75 mm 85-100 90-100 90-100 100

2.36 mm - 55-100 60-100 -

1.18 mm 40-80 35-90 40-80 80

600 µm - 20-60 30-60 -

300 µm 10-35 10-30 25-40 20

150 µm 5-25 5-19 20-35 5

75 µm - - - -

In addition, geopolymer aggregates have high water absorption capacities of

11.0% for coarse and 13.7% for fine ones. Usually, as high water absorption

means that there is a high proportion of penetrable pores in an aggregate which


can seriously affect its bonding with cement materials in concrete (Lo and Cui,

2004, Kayali, 2008, Zhang and Gjørv, 1990), this value always needs to be

considered. According to US standard ACI 213R-03, the normal capacity value

is suggested to be in the range of 5% to 25% by mass of dry aggregates (ACI,

2003) while it has also been recommended that it be less than 15% (Neville,

1996). According to the requirements of these standards, the absorption

capacity of the geopolymer aggregate is high but is still within the acceptable

range for LWA application while special care should be taken (ACI, 1998, ACI,

2003). The real effect of such a high water absorption capacity is further

investigated during presenting and discussing concrete manufacture in

Chapters 6 and 7.

5.11.2 Comparison with Other FA-LWAs

The properties of the geopolymer aggregates produced are compared with

those of other FA-LWAs, such as Lytag and Flashag aggregates introduced in

Chapter 1, subsection 1.1.5, as shown in Table 5.13. Also, the properties of

NWAs, such as crushed stone and river sand regularly used in the locality of the

author’s institution, are presented for comparison.

From Table 5.13, it is clear that NWA has a higher density and lower water

absorption capacity than the other three FA-LWA products which are detected

to be lightweight with similar specific gravities of 1.4 to 1.8. Of the FA-LWAs, the

geopolymer aggregates have the highest density values followed by the

Flashag, with the Lytag having the lowest.

The light weights of the three FA-LWA samples are related to their backbone

materials and porous structures. Lytag and Flashag share a similar backbone

173

material, sintered fly ash, and a similar porous structure formed by the voids left

from elevated-temperature sintering. Nevertheless, a geopolymer aggregate is

different as it has a unique geopolymer backbone and voids remaining after

geopolymerisation at a mild temperature, as observed by X-ray micro-

tomography (Provis et al., 2012). Therefore, the manufacture of a geopolymer

aggregate can be treated as another way of achieving the lightweight property

desired in FA-LWAs.

Table 5.13: Properties of geopolymer aggregates (GA), Lytag, Flashag and NWAs (crushed stone (coarse) and river sand (fine))

Property NWA GA Lytag Flashag

Coarse Fine Coarse Fine Coarse Fine Coarse & Fine

Specific gravity (SSD)

2.70 2.60 1.84 2.01 1.60 1.76 1.69

Specific gravity (OD)

2.68 2.57 1.66 1.77 1.41 1.60 1.61

Dry loose bulk density

(kg/m3) 1376 1640 877 1117 805 1067 848

Water absorption

(in 24 hours) 0.7% 1.2% 11% 13.7% 13.2% 9.7% 3.4%


Geopolymer aggregates and Lytag appear to have higher open porosity values

in their structures than Flashag, as deduced from their much higher water

absorption capacity values presented in Table 5.13. Compared to Lytag, the low

open porosity in Flashag seems to be due to its modified manufacture which

uses a superplasticiser to reduce the amount of water in the pre-heating mixture


to mitigate the impact of void generation caused by water evaporation. In

contrast, the high open porosity in a geopolymer aggregate is considered to be

caused by the connective structure of the geopolymer material made using

current technology (Provis et al., 2012). Further research is needed to modify

geopolymer synthesis in order to reduce the connectivity of open pores in

hardened geopolymers.

5.12 Conclusions

This chapter proposes a type of artificial FA-LWA. This is a geopolymer

aggregate made based on the principle of geopolymerisation and which has

properties that qualify it for use in lightweight constructions. A practical

manufacturing procedure for it, which can be used to make both coarse and fine

aggregate samples, and can also be repeated for different designs, is proposed

and evaluated.

Also, one specifically designed geopolymer aggregate sample is produced with

its properties such as appearance, grading, density, water absorption capacity,

pore information, crushing value and hardness, measured and its chemical

structure examined using IR spectroscopy. These properties are then evaluated

by comparing them with those of regulations in the standards and other FA-

LWAs. It is found that this new geopolymer aggregate has good qualities

suitable for lightweight concrete application. Concrete and mortar practices

using the geopolymer aggregate sample created as explained in this chapter

are researched and presented in Chapters 6 and 7.

175

Chapter 6

OPC-based Concrete and Mortar using Geopolymer

Aggregate

6.1 Introduction

This chapter investigates the application of a geopolymer aggregate in a

concrete mix which, as it uses OPC as the sole cementitious material, is

referred to as an OPC-based concrete. As OPC is the most widely used

cementitious material in the world, this research on OPC-based concrete is able

to provide necessary information for evaluating the compatibility of geopolymer

aggregates with the current mainstream cementitious system.

Firstly, a normal-weight concrete (NWC) made with normal weight aggregates

(NWAs), such as crushed stone and river sand, is designed and coded as ‘Con-

N-OPC’. Then, a concrete made using geopolymer aggregates instead of

NWAs is referred to as OPC-GA concrete and coded as ‘Con-GA-OPC’. The

mix design of this new OPC-GA concrete is based on that of the Con-N-OPC

using the same volume proportions of aggregates and quantities of the other

ingredients. The research on it aims to determine a workable way of utilising

geopolymer aggregates in an OPC-based concrete mix and investigate the

capability of geopolymer aggregates to realise high-performance lightweight

concrete (LWC) as proposed in Chapter 1, subsection 1.1.3.

Also, a mortar material containing only fine aggregates and a cementitious

material, which can be treated as a special mix of concrete with no coarse

aggregates, is also examined. This aims to evaluate the practicality of a mortar

176 6. OPC-based concrete and mortar using geopolymer aggregate

mix using geopolymer aggregates and, moreover, investigates the quality of a

fine geopolymer aggregate for acting as the sole structural framework in a

construction material.

6.2 NWC

6.2.1 Materials

Table 6.1: Typical sieving analysis of crushed stone and river sand





Coarse

13.20 mm 0.0 0.0 0.0 100.0

9.50 mm 118.3 8.1 8.1 91.9

6.70 mm 1097.0 75.5 83.6 16.4

4.75 mm 203.7 14.0 97.6 2.4

<4.75 mm 34.7 2.4 100.0 0.0

Total 1453.7

Fine

4.75 mm 0.0 0.0 0.0 100.0

2.36 mm 25.7 5.2 5.2 94.8

1.18 mm 74.2 14.9 20.1 79.9

600 µm 128.3 25.8 45.9 54.1

425 µm 63.0 12.7 58.6 41.4

300 µm 77.8 15.7 74.2 25.8

150 µm 112.4 22.6 96.8 3.2

75 µm 14.3 2.9 99.7 0.3

<75 µm 1.4 0.3 100.0 0.0


177

The laboratory-used crushed stone and river sand are selected to represent

NWAs in the NWC mix Con-N-OPC. The aggregates’ grading conditions are

shown in Table 6.1 and Figure 6.1 in which it can be seen that they are well-

graded with an even particle size distribution, suggesting good compaction of a

fresh concrete mix (Neville, 1996).

Figure 6.1: Grading curves of crushed stone and river sand

The density and water absorption capacities of the crushed stone and sand

selected are measured and presented in Table 6.2. The crushed stone has

specific gravities of 2.70 and 2.68 under saturated surface dry (SSD) and oven

dry (OD) conditions respectively. For the river sand, these two values are

slightly different; 2.60 and 2.57 respectively. On the other hand, the water

absorption capacity is as low as 0.7% for crushed stone and 1.2% for river sand.

These density and water absorption characteristics are in accordance with the

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

0.01 0.1 1 10 100

Cum

ulat

ive

perc

enta

ge p

assi

ng (%

)

Sieve size (mm)

Crushed stone

River sand


common understanding of the properties of NWAs (Neville, 1996) and applied

to complete the mix design procedure for Con-N-OPC.

Table 6.2: Characteristics of crushed stone and river sand

Property Crushed stone

River sand

Specific gravity (SSD) 2.70 2.60

Specific gravity (OD) 2.68 2.57

Water absorption capacity (in 24 hours) 0.7% 1.2%


Apart from aggregates, the other ingredients are cement, fly ash, water and

superplasticiser, all of which are common in a normal concrete mix. A GP

cement (Grade 45) regulated in Australian standard AS 3972 (AS, 2010)

produced by Boral Corporation is applied as the major cementitious material in

the proposed Con-N-OPC samples. Also, a fly ash sample is used as a

supplementary cementitious material (SCM) in order to make use of the

pozzolanic property of fly ash for improved durability with a hindered alkali-

aggregate reaction (AAR) (Raask and Bhaskar, 1975, Neville, 1996). This

selected fly ash sample is sourced from the 2012-Dec-13 batch from the Eraring

thermal power station, the basic characteristics of which are provided in

Chapter 2, section 2.2. Deionised water is added to trigger the cement hydration

reaction. Also, a polycarboxylate-based superplasticiser coded as ‘ADVA-142’

obtained from Grace Corporation is used to reduce the water required in a

concrete mix and enhance its fresh and final properties (Uchikawa et al., 1995,

Puertas et al., 2005).

179

6.2.2 Mix Design

The mix design of Con-N-OPC is based on a standard mix design procedure

regulated in British standard BS 5328 (Neville, 1996). Given the scope of this

study as to investigate high-performance concrete manufacture, this Con-N-

OPC mix is designed to be of good mechanical properties. Therefore, it is

specified with a characterised strength of 50 MPa and target strength of 63 MPa.

Also, fly ash is added as a 25% mass proportion of the total cementitious

material and an ADVA-142 superplasticiser applied in the proportion of 0.8 litres

per 100 kg cementitious material. According to the procedure regulated in BS

5328, the water/cement ratio of Con-N-OPC is 0.42 and the mass proportion of

fine aggregates is 43% of all the aggregates. The final mix design for 1 m3 of

fresh concrete then obtained is presented in Table 6.3. In addition, for all

concrete mix designs in this study, including that in Table 6.3, the aggregate

portion is regulated to be in a SSD condition.

Table 6.3: Mix design of Con-N-OPC sample (SSD aggregates condition)

Item Con-N-OPC (kg/m3)

Cement 351

Water 147

Crushed stone 1055

Sand 796

Fly ash 117

Superplasticiser 4

Water/Cement ratio 0.42

Water/Cementitious materials ratio 0.31


6.2.3 Concrete Preparation

The preparation of concrete sample Con-N-OPC is based on a general

procedure for an OPC-based concrete mix (AS, 1993) (Figure 6.2). Firstly, the

saturated surface dry aggregate samples are prepared, by mixing the

aggregates in stock with a certain amount of water necessary for the aggregate

absorption (referred to as SSD water) in a mechanical concrete mixer for

around 2 minutes. Then, cement and fly ash are added and mixed with the wet

aggregates for a further 2 minutes to uniformly distribute the cementitious

material onto the surfaces of the aggregate particles. Following that, a dosage

of superplasticiser, which improved the workability of the fresh pastes produced,

is added along with the mixing water. Then, mixing is continued for 3-5 minutes

until a uniform fresh concrete mixture (Figure 6.3(1)) is formed.

Figure 6.2: Flowchart of preparation of OPC-based concrete

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After mixing, a portion of the fresh concrete produced is tested for its fresh

properties, and the remaining portion used to cast samples in cubic

50mm×50mm×50mm stainless steel moulds. A vibrating table (220-240V AC,

50HZ) is applied to compact the fresh concrete mix into a well-shaped cube

(Figure 6.3(2)). The concrete sample is then sealed and cured in the ER at

20oC for 1 day, demoulded (Figure 6.3(3)), and cured in a 20oC water-spray

room. An image of a 28-day cured Con-N-OPC sample is shown in Figure

6.3(4).

Figure 6.3: Steps in preparation of Con-N-OPC: (1) mixing, (2) casting, (3) immediately after demoulding and (4) after 28-day curing

(1) (2)

(3) (4)


6.2.4 Workability

The workability of a fresh concrete means that the effort needed to cast it into a

mould without any harm to its consistency (Neville, 1996), which is a significant

factor for reflecting its quality, must be carefully considered in its manufacture.

A slump test, which measures the slump distance of a conical pile of fresh

concrete, is used to describe the workability of a fresh OPC-based concrete mix.

A common slump test regulated in Australian standard AS1012 (AS, 1993) is

carried out on the fresh mix of Con-N-OPC. The slump result is shown in Table

6.4 which indicates that the fresh Con-N-OPC has moderate workability with a

slump value of 50 mm.

Table 6.4: Slump result for Con-N-OPC measured on a standard slump cone

Cone type AS1012

Cone height (mm) 300

Cone top diameter (mm) 100

Cone bottom diameter (mm) 200

Slump value (mm) 50


The density values of fresh and hardened Con-N-OPC samples were tested

and their compressive strengths measured using a TECNOTEST compression

machine at a speed of 0.33 MPa/sec, with the final value of each mixing batch

the average of three cubic samples. Two mixing batches with the same design

and mixing procedure were produced in order to ensure that the results

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obtained and shown in Table 6.5 can represent the properties of the designed

Con-N-OPC.

Table 6.5: Density and strength properties of Con-N-OPC

Con-N-OPC (1st batch)

Con-N-OPC (2nd batch)

Fresh density (kg/m3) 2400 2450

Density (AD) (28d) (kg/m3) 2310 2340

Density (SSD) (28d) (kg/m3) 2330 2370

Compressive strength (7d) (MPa) 52.36 (0.70)* 51.77 (1.59)*


SSD - saturated surface dry condition; AD - air dry condition; * the value between brackets is the standard deviation of the relevant result.

As the two batches of Con-N-OPC samples display similar density and strength

values, as seen in Table 6.5, these values are considered representative and

repeatable. The Con-N-OPC mix has a fresh density of 2400-2450 kg/m3 and

hardened densities of 2310-2340 kg/m3 (AD) and 2330-2370 kg/m3 (SSD) which

comply with the regular density ranges of NWC samples (Neville, 1996). The

hardened Con-N-OPC has compressive strengths of approximately 50 MPa and

70 MPa at 7 days and 28 days respectively. Its strength performance complies

with the target strength specified in the mix design procedure introduced in

subsection 6.2.2 and indicates that it is a type of high-strength concrete. This is

also the target strength to be achieved by the OPC-GA concrete created and

discussed in the next section.


6.3 OPC-GA Concrete

6.3.1 Materials

The OPC-GA concrete uses geopolymer aggregates for both the coarse and

fine aggregate portions in its mix. The geopolymer aggregates applied here are

the ones manufactured following the procedure in Chapter 5, including the

grading refinement procedure for the fine geopolymer aggregates. Their basic

characteristics required for a concrete mix design procedure are introduced in

Chapter 5, sections 5.5 and 5.6. Apart from these, the other ingredients used

are the same as those in a NWC mix, namely, cement, fly ash, water and

superplasticiser.

6.3.2 Mix Design

Absolute Volume Principle

The mix design of Con-GA-OPC is made from that of Con-N-OPC, based on the

so-called absolute volume principle which assumes that the total volume of

fresh concrete equals the sum of the volume proportions of all its ingredients

(Neville, 1996). The specific mix design procedure for Con-GA-OPC is

explained in next paragraph in combination with the details of these two mix

designs presented in Table 6.6.

The volume proportion of each ingredient in the mix design of Con-N-OPC is

based on its known mass and density values, the sum of which is 1 m3 which

obeys the absolute volume principle, with crushed stone and river sand having

volume proportions of 0.391 m3 and 0.306 m3 respectively. In the new mix

design of Con-GA-OPC, the volume proportions of coarse and fine geopolymer

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aggregates are set to be the same as those of Con-N-OPC, which are 0.391 m3

and 0.306 m3 respectively. The mass quantities of these two portions are then

calculated by multiplying their volume proportions by their densities. The mass

and volume values of the other ingredients are the same in both mix designs.

Table 6.6: Masses and volumes of ingredients in mix designs of Con-N-OPC and Con-GA-OPC (SSD aggregates condition)

Item Con-N-OPC Con-GA-OPC

Mass (kg/m3)

Density (kg/m3)

Volume (m3/m3) Mass

(kg/m3) Density (kg/m3)

Volume (m3/m3)

Cement 351 3110 0.113 351 3110 0.113

Water 147 1000 0.147 147 1000 0.147

Crushed stone 1055 2700 0.391 - -

Sand 796 2600 0.306 - -

Coarse GA - - 719.4 1840 0.391

Fine GA - - 615 2010 0.306

Fly ash 117 2210 0.053 117 2210 0.053

Superplasticiser 4 1200 0.003 4 1200 0.003

Water/Cement ratio 0.42 0.42

Water/Cementitious materials ratio 0.31 0.31

The above procedure illustrates a workable way of designing an OPC-GA

concrete mix with the assistance of a known NWC concrete mix design. A

further advantage is that an OPC-GA concrete mix produced this way has the

same composition of the ingredients related to cementitious materials, i.e., the

same quantities of cement, fly ash, water and superplasticiser, as that of the

NWC concrete mix which implies that these two concrete mixes may have


comparable outcomes of the reactions of cementitious materials. For this case,

any variation in the properties of these two concrete mixes is considered mostly

determined by the different types of aggregates used in them.

Final Mix Design

The mix design of Con-GA-OPC presented in Table 6.6 is created by a

theoretical deduction based on that of Con-N-OPC. However, it doesn’t take

into account the different shapes and surface conditions of geopolymer

aggregates and regular NWAs which have the potential to cause workability

differences. Therefore, a further modification, whereby the quantities of water

and aggregates in the mix design are modulated to generate the same

workability as the Con-N-OPC mix, is discussed. It is specified that the

designed Con-GA-OPC and Con-N-OPC mixes have the same amounts of

cement and fly ash, and display the same workability in their fresh stages in

order to compare and assess their properties, as previously suggested (Kayali,

2005).

To realise this, trial mixes of fresh Con-GA-OPC were manufactured and tested

for their slump values, with the amount of water added modified until a mix

reaches a slump value of 50 mm, the same as that of Con-N-OPC. After

confirming the amount of water required, the quantities of the water and the

aggregates in the mix design are modified to suit a 1 m3 fresh concrete mix.

Then, this new mix design is checked by trial mixes to ensure that it results in a

slump value of 50 mm.

The final mix design of Con-GA-OPC created this way is shown in Table 6.7 in

which the water/cement ratio is higher than that in the mix design of Con-N-

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OPC which is most likely due to the higher water requirement of the angular

geopolymer aggregate particles (Shilstone, 1999, Kockal and Ozturan, 2010).

On the other hand, the portion of fine geopolymer aggregates is divided into

several size fractions with narrower ranges, as shown in Table 6.7, to better

express the composition of this portion and help to remind users of its strict

control in practice.

Table 6.7: Final mix design of Con-GA-OPC (SSD aggregates condition)


The preparation of Con-GA-OPC follows the same procedure as that for OPC-

based concrete shown in Figure 6.2. Longer time is needed to mix coarse and

fine geopolymer aggregates because they are LWAs which take a long time to

Item Con-GA-OPC (kg/m3)

Cement 351

Water 178.7

Coarse GA 686.2

Fine GA (1.18-4.75mm) 117.5

Fine GA (300µm-1.18mm) 352.5

Fine GA (150-300µm) 88.1

Fine GA (75-150µm) 29.4

Fly ash 117

Superplasticiser 4

Water/Cement ratio 0.51

Water/Cementitious materials ratio 0.38


fully absorb the added SSD water. According to the author’s observation, it

usually takes 5 minutes for the surfaces of the aggregate samples to be wet

with no clear water drops. Therefore the SSD condition was first achieved with

these geopolymer aggregates, as recommended in ACI 211.2 (ACI, 1998).

Images showing the relevant steps in mixing Con-GA-OPC are presented in

Figure 6.4.

Figure 6.4: Steps in preparation of concrete Con-GA-OPC (1) mixing, (2) casting, (3) immediately after demoulding and (4) after 28-day curing

(1) (2)

(3) (4)

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6.3.4 Workability

As discussed in subsection 6.3.2, the new Con-GA-OPC mix is intentionally

designed to be of the same workability as the Con-N-OPC mix which has a 50

mm slump value based on a standard slump cone.


Using the same methods introduced in subsection 6.2.5, the density and

strength properties of the Con-GA-OPC samples are measured and shown in

Table 6.8. Two mixing batches made at different times are presented to ensure

that the values reported can be representative and repeatable for this type of

concrete.

Table 6.8: Density and strength properties of Con-GA-OPC

Con-GA-OPC (1st batch)

Con-GA-OPC (2nd batch)


Density (AD) (28d) (kg/m3) 1770 1750

Density (SSD) (28d) (kg/m3) 1900 1900




Table 6.8 shows that Con-GA-OPC has a fresh density of approximately 1900

kg/m3, 28-day AD density of 1750-1770 kg/m3 and 28-day SSD density of 1900

kg/m3. Clearly, these values indicate that it is much lighter than the normal-

weight Con-N-OPC whose relevant density values are more than 2300 kg/m3,


as presented in Table 6.5. Due to the same amounts of cement, fly ash, water

and superplasticiser used in these two concrete mixes, this density difference is

very likely to be caused by the use of lightweight geopolymer aggregates.

Otherwise, based on its density values, Con-GA-OPC belongs in the class of

structural LWC products (ACI, 1998, Clarke, 1993) which means that

geopolymer aggregates can be a suitable option for manufacturing LWC

products, one of the main research issues in this thesis.

On the other hand, the hardened Con-GA-OPC achieves 7-day and 28-day

compressive strengths of nearly 38-44 MPa and 57-60 MPa respectively which

indicate a high-strength structure that is very likely qualified for high-

performance construction applications in practice (Hoff, 1990, Kayali, 2005).

Also, compared with the Con-N-OPC that uses NWAs, Con-GA-OPC displays a

lower strength by approximately 10 MPa. This difference in strength should be

studied when deciding which aggregate should be used in a certain construction

work. Without going too much outside the scope of this research, it must be

observed that there are expected to be several economical and structural

compensations and advantages due to reductions in the weight of structural

elements, added to that are environmental and durability advantages.

Nevertheless the difference in strength must be considered in the design of the

relevant structure. Therefore, this indicates that geopolymer aggregates are

able to produce high-quality concretes with good mechanical performances.

The author considers that two factors which make Con-GA-OPC different from

Con-N-OPC are worth discussing. One is the strength of a geopolymer

aggregate sample itself and the other the strength and performance of the

bonding between the geopolymer aggregates and cement matrix. The first

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factor can be reflected by the results from crushing value and rebound testings

discussed in Chapter 5, sections 5.8 and 5.9. Geopolymer aggregates have a

crushing value of 23.43% which is a low value for FA-LWA products. Besides,

they have a rebound index (kPa) of 38.6 based on a Schmidt hammer,

indicating that their hardness can be comparable with several concrete-used

natural rocks. These results indicate good mechanical behaviours of

geopolymer aggregates, which may contribute to the good strength of a

concrete mix using them.

Also, regarding the second factor, the unique properties of the used aggregates,

including density, structure and absorption, can lead to different properties of

subsequent concrete products (Wasserman and Bentur, 1997, Kockal and

Ozturan, 2010). For instance, the angular shape and porous surfaces of

geopolymer aggregates are likely to lead to their enhanced interlocking with a

cement matrix to improve the strength of the bonding between them and the

cement (Kayali, 2008, Zhang and Gjørv, 1990). This may also facilitate the

development of a high-strength concrete, as reflected by the strength results in

Table 6.8.

On the other hand, several other factors can also influence the properties of the

LWA concretes, such as the aggregate proportions and water/cement ratio (Chi

et al., 2003). In this research, however, these factors are here determined by

the selected NWC mix design in section 6.2, which acts as the origin or

template of this newly created OPC-GA mix, and thus other factors that may

affect the properties of the final product are minimised.


More importantly, the OPC-GA concrete made with geopolymer aggregates has

a higher strength/mass ratio than the NWC made with regular NWAs. As seen

in Table 6.5, as the NWC mix Con-N-OPC obtains a compressive strength of

67.70 MPa and AD density of 2310 kg/m3, it has a strength/mass ratio of

67.70/2310=0.0293 (MPa·m3/kg). On the other hand, as shown in Table 6.8,

Con-GA-OPC obtains a compressive strength of 59.32 MPa and AD density of

1770 kg/m3, with a strength/mass ratio of 59.32/1770=0.0335 (MPa·m3/kg).

Therefore, for a unit weight of concrete, OPC-GA has a higher strength than its

NWC counterpart; Or, for a certain strength target, OPC-GA requires less

material. These results demonstrate that OPC-GA concrete is a promising

prospect for fulfilling the concept of modern construction which prefers

reductions in unnecessary materials and foundation requirements (Clarke,

1993). However, further research is required to better understand the principle

and control of the manufacture of such high strength/mass ratio concrete

products.

6.3.6 Fracture Surface

The fracture surfaces of the OPC-GA and NWC concrete samples after

compressive strength testing are observed using a Lynx vision engineering

stereo microscope. Captured images, in which the aggregates are labelled

based on their observed shapes and sizes, are shown in Figure 6.5. The

unlabelled parts are the cement matrices which are composed of hydrated

cement, fly ash and sand.

193

Figure 6.5: Enlarged images (40x) of fracture surface of Con-N-OPC (left) and Con-GA-OPC (right) (the image represents a length of approximately 3 mm)

These images display Con-N-OPC and Con-GA-OPC fractures which represent

two series of interfacial zones, the ‘stone-cement matrix’ and ‘geopolymer

aggregate-cement matrix’. In both images, the aggregates are embedded in the

cement matrix, with a complete but not collapsed interfacial zone detected.

Moreover, little cracking is formed along their interfacial zones which indicates

that the bonding of the aggregates and cement matrices has a certain strength

to resist external pressure. This presents a strong bonding between geopolymer

aggregates and cement matrix, which may be due to an enhanced interlocking

in the interfacial zone (Kayali, 2008, Zhang and Gjørv, 1990). This is likely to be

an important factor that enhances the strength of OPC-GA concrete, along with

the strength of geopolymer aggregate itself (Wasserman and Bentur, 1996),

which may explain the high-strength of Con-GA-OPC in Table 6.8. Also, there is

no clear cracking across the aggregate observed in Con-GA-OPC, although an

LWA is normally easier to suffer a cracking due to its weak strength (Lo and Cui,

2004).

Aggregates

Aggregates


6.4 Comparison of OPC-GA Concrete and Other LWCs

6.4.1 LWC using FA-LWA

The performance of OPC-GA concrete is evaluated by comparison with LWCs

made with other FA-LWAs, such as Lytag and Flashag, two high-quality ones

which have been successfully utilised to create LWCs with good mechanical

and structural properties (Kayali, 2005, Swamy and Lambert, 1981). These

OPC-based concrete mixes are expected to be appropriate references for

assessing the quality of the newly made OPC-GA concrete.

The new Lytag LWC mix is coded as ‘Con-LY-OPC’, whose mix design is

created from that of Con-N-OPC based on the same procedure illustrated in

subsection 6.3.2. The Con-LY-OPC mix is designed with the same amounts of

cementitious materials and same workability as the Con-N-OPC and Con-GA-

OPC mixes, as shown in Table 6.9. Unlike Con-GA-OPC, Con-LY-OPC can

achieve the desired workability with a lower water/cement ratio of 0.43, possibly

due to the spherical shape of Lytag aggregates that require less water to meet

the workability demand (Shilstone, 1999, Kockal and Ozturan, 2010).

The properties of the three concrete mixes are presented in Table 6.10. As

intentionally designed by the author, they all have the same workability with a

slump value of 50 mm. As previously discussed, this workability is suitable for

the normal compaction and casting of a fresh concrete mix. Also, it is found in

practice that the Con-GA-OPC and Con-LY-OPC mixes are relatively more

easily compacted by the applied mechanical vibration than the Con-N-OPC one

despite their having the same slump values. The author suggests that this is

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possibly due to the light self-weights of the first two mixes caused by their

applications of LWAs which make them more vulnerable to a vibration force.

Table 6.9: Mix designs of Con-N-OPC, Con-GA-OPC, and Con-LY-OPC

Ingredient Con-N-OPC Con-GA-OPC Con-LY-OPC

(kg/m3) (kg/m3) (kg/m3)

Cement 351 351 351

Water 147 178.7 150.6

Crushed stone 1055 - -

Sand 796 - -

Coarse GA - 686.2 -

Fine GA - 587.5 -

Coarse Lytag - - 622

Fine Lytag - - 536.1

Fly ash 117 117 117

Superplasticiser 4 4 4

Water/Cement ratio 0.42 0.51 0.43

Water/Cementitious materials ratio 0.31 0.38 0.32

In Table 6.10, Con-LY-OPC has a fresh density of 1810 kg/m3, AD density of

1600 kg/m3 and SSD density of 1770 kg/m3 which are even lower than those of

the Con-GA-OPC mix. This is very likely due to the even lower densities of

Lytag aggregates than those of geopolymer aggregates, as stated in Chapter 5,

section 5.11. Obviously, these density results indicate that, like Con-GA-OPC,

Con-LY-OPC is also a type of LWC product (ACI, 2003).


Table 6.10: Properties of fresh and hardened Con-N-OPC, Con-GA-OPC, and Con-LY-OPC samples

Con-N-OPC Con-GA-OPC Con-LY-OPC

Slump (mm) 50 50 50

Fresh density (kg/m3) 2400 1856 1810

Density (AD) (28d) (kg/m3) 2310 1770 1600

Density (SSD) (28d) (kg/m3) 2330 1900 1770

Compressive strength (7d) (MPa) 52.36 (0.70)* 44.20 (0.46)* 39.97 (1.41)*



On the other hand, the lighter Lytag concrete has a lower strength than the

OPC-GA concrete which agrees with the common idea that a lower self-weight

may lead to a lower load-bearing capability (Neville, 1996). As long as this

finding might be convincing to a certain extent since these results are the

average of a few samples, the variations in strengths of these two concretes are

not significant and, indeed, far smaller than those in their density values, with

both capable of reaching a compressive strength level of 55-60 MPa after 28

days.

The concrete made with Lytag has a strength/mass ratio of 56.14/1600=0.0351

(MPa·m3/kg) which is higher than the 0.0335 (MPa·m3/kg) of OPC-GA concrete.

This may imply that Lytag has slightly better mechanical properties for concrete

197

than geopolymer aggregate. However, the Con-GA-OPC concrete mix can still

achieve the density and strength required for structural LWC application (ACI,

2003, Clarke, 1993). Therefore, the author considers that the variations in

mechanical properties should not be the main reason determining the selection

of either geopolymer or Lytag aggregates for LWCs but that cost, resource and

environmental issues should also be considered. A geopolymer aggregate is

likely to be preferred because of its low production cost, eco-friendly

manufacturing process and suitable properties as discussed in Chapter 5.

In this research, a concrete made with Flashag and coded as ‘Con-FS-OPC’ is

also investigated. However, due to the uniqueness of Flashag, the Con-FS-

OPC mix is made based on the mix design proposed by the inventor of Flashag

which includes using 300 kg/m3 of cement and 300 kg/m3 of fly ash (Kayali,

2008). The obtained LWC sample is considered only as a reference for the

properties of FA-LWA concrete products.

The basic properties of Con-GA-OPC and Con-FS-OPC are presented in Table

6.11. The density values of these two concrete mixes are not much varied, with

a fresh density of 1800-1850 kg/m3 and an AD density of 1770-1780 kg/m3.

Apart from this, these two concretes reach similar strength values of

approximately 60 MPa after 28 days, as seen in Table 6.11. This means that

the concrete made of geopolymer aggregates has the potential to achieve the

same strength level as that of Flashag concrete, and serve as a high-strength

concrete material in future industry (Kayali, 2008, Hoff, 1990). Besides, the new

OPC-GA concrete mix is likely to have a lower production cost compared with

the Flashag concrete, since its aggregate portion is made from a less energy

consumption procedure, as discussed in Chapter 5.


Table 6.11: Properties of Con-GA-OPC and Con-FS-OPC (Kayali, 2008) samples

Con-GA-OPC Con-FS-OPC


Density (AD) (28d) (kg/m3) 1770 1780

Density (SSD) (28d) (kg/m3) 1900 -

Compressive strength (7d) (MPa) 44.20 38.8


SSD - saturated surface dry condition; AD - air dry condition

6.4.2 LWC using Geopolymer-assisted LWA

LWA products made with the assistance of geopolymer technology are

introduced in Chapter 1, subsection 1.2.3, some of which were researched to

make LWCs (Bui et al., 2012, Jo et al., 2007). These LWCs made of the

geopolymer-assisted LWAs are compared with the OPC-GA concrete mix, as

discussed in this subsection.

Three LWC designs reported in the literature are selected and compared with

that of OPC-GA concrete. One LWC is made with aggregates called ALFA

invented by Jo et al. (2007), coded as ‘Con-Jo’ for easy reference, with the

other two reported in Bui’s research referred to as Con-Bui-1 and Con-Bui-2 in

Table 6.12. Con-Bui-1 uses a LWA sample made solely from fly ash which

shares the same origin as a geopolymer aggregate. Con-Bui-2 uses a LWA

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sample created from a combination of 75% fly ash and 25% slag, and displays

the best strength gain of all the mixes tested in Bui’s research (Bui et al., 2012).

Firstly, the mix design of each LWC mix is evaluated. In Table 6.12, it is clear

that Con-GA-OPC is the only mix in which there are both coarse and fine LWA

portions while, in the other three, only coarse LWAs are used, with natural sand

still required to act as the fine fillers. The geopolymer aggregate proposed in the

current research seems to be the first geopolymer-assisted artificial LWA

product capable of including both coarse and fine aggregates in a concrete mix.

Table 6.12: Mix designs of Con-GA-OPC and LWCs using geopolymer-assisted LWAs reported in literature (adapted from Jo, 2007 and Bui, 2012)

Ingredient

Con-GA-OPC Con-Jo Con-Bui-1 Con-Bui-2

(kg/m3) (kg/m3) (kg/m3) (kg/m3)

Cement 351 352 434.6 436.3

Water 178.7 155 158.7 159.7

Coarse GA 686.2 - - -

Fine GA 587.5 - - -

ALFA - 595 - -

Bui-1 LWA - - 558.5 -

Bui-2 LWA - - - 589.5

Sand - 835 767.8 770.7

Fly ash 117 - 105.5 105.9

Superplasticiser 4 - 3.3 2.9

Water/Cement ratio 0.51 0.45 0.37 0.37

Water/Cementitious materials ratio 0.38 0.45 0.29 0.29


Bui’s concrete mixes require a higher quantity of cement (430 kg/m3) and higher

proportion of fine aggregates than the OPC-GA concrete mix. Similarly, Jo’s

concrete needs a much higher proportion of sand than OPC-GA concrete

despite using a similar cement proportion. As having more cement and fine

aggregates is beneficial for enhancing the properties of a concrete, it is of

concern that the artificial LWAs produced in Bui’s and Jo’s research might

require increased amounts of cement and fine aggregates than geopolymer

aggregates to achieve the desired quality.

Nevertheless, the mix designs of Jo’s and Bui’s concretes have lower

water/cement ratios of 0.37 to 0.45 than that of OPC-GA concrete, as usually

preferred for an enhanced strength of OPC-based concrete mix (Neville, 1996).

However, it is likely that the OPC-GA concrete cannot be designed with a

similar low water/cement ratio, as discussed earlier, because of its resultant bad

workability. Based on the outcomes of the current research, a higher

water/cement ratio is needed for OPC-GA concrete to achieve acceptable

workability for practical applications which is considered to be due to the

angular shapes of geopolymer aggregates. Or, a larger dosage of a

superplasticiser might be useful for achieving the desired workability with a low

water/cement ratio, so as to achieve an improved strength. This is indeed a

subject for future research. In contrast, the LWAs produced in Jo’s and Bui’s

research are spherical due to the application of a pelletisation process (Jo et al.,

2007, Bui et al., 2012) which can facilitate fluidity in a fresh mixture (Shilstone,

1999, Kockal and Ozturan, 2010).

The properties of these LWC mixes are measured and shown in Table 6.13.

The slump results shown in this Table are all based on a normal-sized slump

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cone. Bui’s two mixes present slightly higher fresh density and much higher

slump values than those of Con-GA-OPC. However, these slump values may

be too high to result in the end-product having a strong structure. The density

and slump properties of Jo’s concrete mix are not known.

The hardened concretes Con-Jo and Con-Bui-1 result in low compressive

strengths of less than 30 MPa after 28 days. Although Con-Bui-2, which uses a

LWA made from a combination of fly ash and slag, has a higher compressive

strength of 38.10 MPa, this is still much lower than that of Con-GA-OPC which

is 59.32 MPa after 28 days. Obviously, the OPC-GA concrete generates a

superior high-strength performance to those of LWCs made with other

geopolymer-assisted LWAs. This is considered to be due to the advanced

characteristics of geopolymer aggregates, which have a potential to realise a

high performance geopolymer aggregate concrete manufacture.

Table 6.13: Properties of concretes using different LWA products (some data from Jo, 2007 and Bui, 2012)

Con-GA-OPC Con-Jo Con-Bui-1 Con-Bui-2

Fresh density (kg/m3) 1856 - 1931 2056

Slump (mm) 50 - 260 270

28-day compressive strength (MPa) 59.32 26.70 20.80 38.10


6.5 OPC-GA Mortar

This section studies a mortar material made with fine geopolymer aggregates

which is referred to as OPC-GA mortar and coded as ‘Mor-GA-OPC’. Mortar

can be treated as a special type of concrete without a coarse aggregate portion.

Here, it is considered for making a high-performance OPC-GA mortar with a

similar strength but much lower self-weight than a normal one. Also, this

research can highlight the function of fine geopolymer aggregates working as a

new type of ‘artificial sand’ that may be needed in future construction.

6.5.1 Materials

The same fine geopolymer aggregate sample as in Con-GA-OPC (subsection

6.3.1) is used for Mor-GA-OPC, with the cement grain and water introduced in

subsection 6.2.1 completing the mix. Also, the same natural river sand used in

Con-N-OPC is applied for the manufacture of a normal mortar mix coded as

‘Mor-S-OPC’ for reference purposes.

6.5.2 Mix Design

The mix design procedure for mortars is conducted similar to that for an OPC-

GA concrete introduced in subsection 6.3.2. Firstly, a normal mortar mix (Mor-

S-OPC) using sand and cement is made with volumetric ratio of sand to cement

of 3.0 and mass ratio of water to cement of 0.45 respectively, as shown in Table

6.14. Then, the new Mor-GA-OPC mix is made by replacing the sand in Mor-S-

OPC with the same volume of fine geopolymer aggregates. The volume

proportions of other ingredients are not changed. The mix design of Mor-GA-

OPC is obtained and shown in Table 6.14, with the same cementitious material

as Mor-S-OPC. From trial tests conducted to ensure that the two mortar mixes

203

have the same workability, it is determined that there is no need to modify their

quantities of water.

Table 6.14: Masses and volumes of ingredients in mix designs of Mor-S-OPC and Mor-GA-OPC (SSD aggregates condition)

Ingredient Mor-S-OPC Mor-GA-OPC

Mass (kg/m3)

Density (kg/m3)

Volume (m3/m3) Mass

(kg/m3) Density (kg/m3)

Volume (m3/m3)

Cement 576 3110 0.185 576 3110 0.185

Water 259 1000 0.259 259 1000 0.259

Sand 1445 2600 0.556 - - -

Fine GA - - - 1117 2010 0.556


6.5.3 Mortar Preparation

For consistency, both the Mor-S-OPC and Mor-GA-OPC mixes are prepared

following the same procedure using a semi-automatic mixer. Firstly, the fine

aggregates are mixed with SSD water for 2 minutes to reach the SSD condition,

cement is then added and the mixing continued for a further 2 minutes, followed

by the addition of water and mixing until a uniform mixture is formed (Figure 6.6).


Figure 6.6: Images of fresh mortar mixtures: (1) Mor-S-OPC and (2) Mor-GA-OPC

After mixing, the fresh mortar mixtures are cast into cubic 50mm×50mm×50mm

stainless steel moulds and cured at 20oC for 1 day. Then, the initially hardened

mortar samples are demoulded and placed in a water-spray room at 20oC until

further testing. This curing regime is similar to that for the concrete samples

discussed earlier. Mortar samples using regular sand and the new geopolymer

aggregates after 28 days of curing are shown in Figure 6.7. The samples have

a similar appearance possibly due to the same cement matrix formed.

Figure 6.7: Images of 28-day cured Mor-S-OPC and Mor-GA-OPC samples

(1) (2)

Mor-GA-OPC Mor-S-OPC

205

6.5.4 Workability

The workability performances of the fresh mortars discussed in this section are

evaluated based on the combined slump and mini-flow method discussed in

Chapter 2, subsection 2.4.1. The initial slumps and spreads, and spreads after

10-second vibrations of the mortar samples Mor-S-OPC and Mor-GA-OPC are

measured and presented in Table 6.15 and Figure 6.8.

As seen in Table 6.15, as both the fresh mortar mixes present the same slump

value of 20 mm based on the applied mini-sized slump cone (the cone specified

in subsection 2.4.1 with a top opening of 19 mm) and have good workability with

high fluidity, they can be easily compacted and processed. This high workability

is barely affected by the use of geopolymer aggregates or sand, but more likely

determined by the cement matrix generated in above mortar mixes.

Table 6.15: Slump and spread values of fresh Mor-S-OPC and Mor-GA-OPC

Mortar


Initial slump (mm)



Spread in 3 mins

(cm2)

Spread in 10 mins

(cm2) Mor-S-OPC 20 13.67 61.57 62.46 62.50

Mor-GA-OPC 20 13.13 82.30 82.55 82.56

On the other hand, as seen in Figure 6.8, both the fresh mortar mixes have a

high spread gain after the stimulation of mechanical vibration, with that of Mor-

GA-OPC having higher spread areas than Mor-S-OPC. This is considered to be

due to the use of fine geopolymer aggregates in the former which are much

lighter than sand, thereby reducing the weight of an entire mortar mix and


possibly causing it to be less resistant to the vibration force applied. This

indicates that using lightweight geopolymer aggregates in a mortar mix could

enhance its flow against a mechanical vibration, making it easier for compaction

in practice. This has also been found when processing the previous OPC-GA

concrete samples, and can be a potential benefit for using a lightweight material.

Figure 6.8: Increases in spread areas of fresh Mor-S-OPC and Mor-GA-OPC caused by mechanical vibration


The density and strength properties of the mortar samples produced are

presented in Table 6.16. Firstly, Mor-S-OPC has a fresh density of 2217 kg/m3

and Mor-GA-OPC has a fresh density of only 1903 kg/m3. Similarly, Mor-GA-

OPC sample has an AD density of 1750 kg/m3 and SSD density of 1900 kg/m3

which are much lower than those of Mor-S-OPC. These results show that an

application of geopolymer aggregates instead of sand can significantly reduce

0

10

20

30

40

50

60

70

80

90

initial spread spread atonce

spread in 3mins

spread in 10mins

Spre

ad a

rea

(cm

2 )

Mor-S-OPC

Mor-GA-OPC

207

the density behaviour of mortar, achieving a light self-weight mortar mix just as

what has been done in an OPC-GA concrete.

Table 6.16: Density and strength properties of Mor-S-OPC and Mor-GA-OPC

Mor-S-OPC Mor-GA-OPC


Density (AD) (28d) (kg/m3) 2140 1750

Density (SSD) (28d) (kg/m3) 2220 1900



SSD - saturated surface dry condition; AD - air dry condition

On the other hand, the compressive strength results shown in Table 6.16

indicate that the Mor-GA-OPC with a much lighter density is able to achieve the

same strength level as the Mor-S-OPC mix and its 28-day compressive strength

is even slightly higher. Although this strength difference may not be significant

from a statistical viewpoint, it is at least, a very promising indication that the

utilisation of geopolymer aggregates in mortar will not compromise the strength

of the end-product. Also, as in the previous trend in concrete samples, the Mor-

GA-OPC has a higher strength/mass ratio of 0.034 (MPa·m3/kg) than that of the

Mor-S-OPC of 0.026 (MPa·m3/kg), which is considered mostly related to the

light self-weight of the former one. This indicates the advanced property of fine

geopolymer aggregates in making a high-strength but lightweight mortar


material. For this case, a lightweight mortar product seems to be workable to be

used in replacement of a conventional normal weight one without a concern on

its strength loss.

Clearly, the mortar mix made with geopolymer aggregates is worth further

discussion and research because the current results show that it can be of good

quality, lightweight, a save of natural sand, and more importantly, exhibits no

corresponding strength loss. Although the OPC-GA concrete using geopolymer

aggregates can achieve a high quality, it results in an approximate 10 MPa

strength loss compared with its NWC counterpart using natural aggregates, as

shown in Table 6.5 and Table 6.8. So, the concrete mix with both coarse and

fine geopolymer aggregates seems to not function as well as the mortar mix

with only fine geopolymer aggregates. The author considers that this effect

might be partially due to the more uniform mixture of mortar which can be

treated almost as a two-phase mixture composed of sand (or artificial sand as

fine geopolymer aggregates) and a cement matrix rather than a more

complicated mixture of concrete. Also, the author wonders whether a fine

geopolymer aggregate may present better consistency than a coarse one

because it is subjected to more crushing during its manufacturing stage.

Nevertheless, the concrete mixes discussed in this chapter are not sufficient

enough to conclude that the OPC-GA concrete cannot reach the same strength

as a NWC concrete one and this makes future research in this area, highly

recommended.

209

6.6 Conclusions

In this chapter, the practicality of using geopolymer aggregates in OPC-based

concrete and mortar mixes is researched. It is found that both the concrete and

mortar made with geopolymer aggregates have certain advanced properties,

such as a light weight, high strength/mass ratio and mechanical behaviour

qualified for real-life practice. They are also successful applications of

geopolymer aggregates as a new type of FA-LWA which realise their low-cost

and eco-friendly benefits.

The concrete mix design procedure established in this chapter proves to be

suitable for creating the required mix design of OPC-GA concrete. It can also

help to correlate the OPC-GA concrete and NWC designs which have the same

ingredients for cementitious materials to provide a reference when needed to

assess the performance of the new OPC-GA concrete mix. A similar mortar mix

design procedure is also developed. Apart from being used for the mixes

discussed in this chapter, these design procedures can also be repeatedly

applied in future research for concrete and mortar designs with different

proportions of ingredients.

A workable preparation of OPC-based concrete and mortar made with

geopolymer aggregates is also illustrated, with samples measured and

analysed, and determined to have good fresh and hardened properties.

Therefore, geopolymer aggregates are found to be suited for a Portland cement

system which inspires their potential broad promotion as alternatives to current

FA-LWA products in future construction fields.


Moreover, this research indicates that there might be a promising way of using

fine geopolymer aggregates in the mortar, which can have a higher

strength/mass ratio with no strength loss than the regular mortar made with

sand. It is assumed that this will make the manufacture of OPC-GA mortar of

more interest in the near future.

211

Chapter 7

Geopolymer Binder-based Concrete and Mortar using

Geopolymer Aggregate

7.1 Introduction

This chapter continues the research on the application of a geopolymer

aggregate in a concrete mix. Unlike the OPC-based concrete presented in

Chapter 6, the concrete mix researched in this chapter uses a fly ash-based

geopolymer as the sole cementitious material, and therefore it is referred to as a

geopolymer binder-based concrete. Thus, concrete mix with the aggregates and

cementitious material all made from fly ash-based geopolymer, is researched.

A concrete made with geopolymer aggregates and geopolymer binder is

designed, referred to as Geo-GA concrete and coded as ‘Con-GA-Geo’. A

workable mix design procedure is proposed to create different Geo-GA mixes

for research purposes. The research aims to evaluate the compatibility of

geopolymer aggregates with a geopolymer binder system, and the capability of

the subsequent Geo-GA concrete in realising high-performance geopolymer

LWCs.

Another kind of concrete which uses natural aggregates and geopolymer binder

is also involved in this chapter. This concrete is the one that commonly called

geopolymer concrete and has been researched for decades (Davidovits, 2008)

which is simplified as GC and coded in this chapter as ‘Con-N-Geo’. It is

researched in order to assess the designed geopolymer binders, so as to

provide information needed for a Geo-GA concrete mix. For this reason, the GC

212 7. Geopolymer binder-based concrete and mortar using geopolymer aggregate

mixes included in this chapter are made with the same geopolymer binders as

those of Geo-GA mixes.

Additionally, a mortar material made with a geopolymer binder instead of OPC

is researched. The mortar mixes in this chapter use either the regular natural

sand or fine geopolymer aggregates. It is aimed to evaluate the practicality of

using geopolymer material (both aggregates and binder) in a mortar material.

7.2 Mix Design Procedure for Geopolymer Binder-based

Concretes

7.2.1 Materials

The Geo-GA concrete uses geopolymer aggregates for both the coarse and fine

aggregate portions in its mix. These geopolymer aggregates are from the same

aggregate samples as the ones used in an OPC-GA concrete illustrated in

Chapter 6, section 6.3. On the other hand, the GC mix uses the laboratory-used

crushed stone and river sand whose relevant characteristics are presented in

Chapter 6, subsection 6.2.1.

Otherwise, the common ingredients needed for the synthesis of a fly ash-based

geopolymer, namely fly ash, NaOH solution, Na2SiO3 solution and deionised

water, are applied. The details of these ingredients are illustrated in Chapter 3.

Moreover, two chemical admixtures, which were previously reported to modify

the performance of fresh geopolymer concrete mix (Junaid et al., 2012), are

applied. One is a polycarboxylate ether base mid-range water-reducer (known

commercially as Centrox MWR), and the other is a cellulose base viscosity

modifier (known commercially as Centrox VMA). Both admixtures are added in

a dosage of 1% of the mass of fly ash.

213

7.2.2 Geo-GA Concrete

A mix design procedure is proposed here for creating different Geo-GA mixes

for research purpose. This procedure is to create Geo-GA mix design from the

known OPC-GA mix design, where the latter one can help to determine the unit

weight and aggregate proportion. From this, the proportion of binding material in

a Geo-GA mix is determined. Afterwards, this binding material can be designed

following the geopolymer mix design procedure proposed in Chapter 3. A

detailed procedure is illustrated in following paragraphs.

Firstly, the Con-GA-OPC mix discussed in Chapter 6, section 6.3 is selected as

the base to create the new Con-GA-Geo mix design. The procedure is

explained in combination with Table 7.1 in which it can be seen that Con-GA-

OPC has a unit weight of 1924.4 kg/m3, a coarse aggregate proportion of 686.2

kg/m3, a fine aggregate proportion of 587.5 kg/m3 and a binding material

proportion of 650.7 kg/m3. Since this Con-GA-OPC mix has been proved

workable with a good strength, as shown in Chapter 6, it is considered that the

above values are likely to end up in a workable mix and therefore used as a

reference for Con-GA-Geo. Then, the new mix design of Con-GA-Geo is

designed to have the same unit weight and same proportions of aggregates and

binding material as those of Con-GA-OPC, as seen in Table 7.1. Following this,

some basic information of a mix design can be quickly confirmed, as suggested

in the previous literature (Rangan, 2007).

It should be noted that, in Table 7.1, it is assumed that the 650.7 kg/m3

geopolymer binding material would occupy the same volume as that of 650.7

kg/m3 OPC in the mix. This assumption is only made for the need of mix design


procedure. A necessary modification is required and made later to obtain the

final mix design that suits 1 m3 Geo-GA concrete mix.

Table 7.1: Unit weight and ingredient proportions of mix designs of Con-GA-OPC and Con-GA-Geo (SSD aggregates condition)

Ingredient

Con-GA-OPC

Con-GA-Geo

(kg/m3) (kg/m3)

Binding material 650.7 650.7

Coarse GA 686.2 686.2

Fine GA 587.5 587.5

Unit weight 1924.4 1924.4

Aggregate mass proportion 66% 66%

Later, the ingredients used to compose this 650.7 kg/m3 binding material are

designed. In principle, since the geopolymer binder is also a kind of geopolymer

material, its mix design is completed following the mix design procedure

proposed in Chapter 3. Meanwhile, the two Centrox admixtures are also treated

as parts of this binding material, and needed to be designed for. On the other

hand, as different geopolymer binders may be required to make different Geo-

GA concretes for research purpose, some mix designs for desired geopolymer

binders in current research have been created and presented in Chapter 3,

section 3.7. The detailed mix design procedure for Geo-GA concrete mix is

illustrated in Appendix B. The final mix designs of the Geo-GA concretes

required for the current research are presented in Table 7.2. Following this

procedure, different Geo-GA mixes can be created by selecting different

SiO2/Na2O, H2O/Na2O and W/G ratios. Moreover, users may also design the

215

unit weight and aggregate proportion according to their needs, instead of based

on a known OPC-GA every time.

Table 7.2: Mix designs of Con-GA-Geo samples (SSD aggregates condition)

Concrete code Con-GA-Geo-1

Con-GA-Geo-2

Con-GA-Geo-3

Fly ash (kg/m3) 446.1 446.1 431.6

W/G value 0.24 0.26 0.26

SiO2/Na2O (x value) 1.25 1.25 1.25

H2O/Na2O (y value) 11 11 11

Fly ash/Activator (F/A) 2.05 1.86 1.86

Coarse GA (kg/m3) 666.3 652.5 663.7

Fine GA (kg/m3) 565.5 550.6 562.7

NaOH 16M (kg/m3) 47.0 51.8 50.1

Na2SiO3 (kg/m3) 165.7 182.7 176.7

Water (kg/m3) 4.9 5.4 5.2

Centrox MWR (kg/m3) 4.5 4.5 4.3

Centrox VMA (kg/m3) 4.5 4.5 4.3

Unit weight (kg/m3) 1904.5 1898.0 1898.6

Aggregate mass proportion (%) 65% 63% 65%

The expression of the mix designs presented in Table 7.2 is treated as a

standard way for all geopolymer binder-based concrete mixes included in this

thesis. As seen in Table 7.2, four sections are categorised except for the title

section. The first section is the fly ash quantity, which is to highlight the usage of

fly ash since it is the main aluminosilicate source for geopolymerisation


(Davidovits, 2008). This fly ash quantity can also be a key factor to distinguish

each mix throughout this chapter; the second section presents the factors used

to describe the design of geopolymer as detailed in Chapter 3; the third section

lists the other ingredients except fly ash in the mix design; and the last section

shows the unit weight and aggregate mass proportion. This expression of mix

design is promoted by the author since it clearly displays all information needed

for a scientific analysis.

7.2.3 Geopolymer Concrete (GC)

Table 7.3: Mix designs of Con-N-Geo samples (SSD aggregates condition)

Concrete code Con-N-Geo-1

Con-N-Geo-2

Con-N-Geo-3

Fly ash (kg/m3) 446.1 446.1 431.6

W/G value 0.24 0.26 0.26

SiO2/Na2O (x value) 1.25 1.25 1.25

H2O/Na2O (y value) 11 11 11

Fly ash/Activator (F/A) 2.05 1.86 1.86

Crushed stone (kg/m3) 977.7 957.5 973.9

River sand (kg/m3) 731.6 712.2 727.9

NaOH 16M (kg/m3) 47.0 51.8 50.1

Grade D Na2SiO3 (kg/m3) 165.7 182.7 176.7

Deionised water (kg/m3) 4.9 5.4 5.2

Centrox MWR (kg/m3) 4.5 4.5 4.3

Centrox VMA (kg/m3) 4.5 4.5 4.3

Unit weight (kg/m3) 2382.0 2364.6 2374.0

Aggregate mass proportion (%) 72% 71% 72%

217

Although the main objective of this chapter is the Geo-GA concrete, a research

on geopolymer concrete (GC) using natural aggregates is needed to be firstly

conducted, for assessing the effects of applied geopolymer binders. For this

reason, these GC mixes use the same geopolymer binders as those of Geo-GA

mixes. The mix designs of GC are presented in Table 7.3, which are made by

replacing the geopolymer aggregates in the Geo-GA mixes in Table 7.2 with the

same volume proportions of natural aggregates.

7.3 Geopolymer Concrete (GC)

The geopolymer concrete (GC) samples designed in Table 7.3 are firstly

manufactured prior to Geo-GA samples. The properties of GC samples are

measured to evaluate the performance of designed geopolymer binders, to

provide useful information for subsequent Geo-GA mixes.


The preparation of concrete sample Con-N-Geo is shown in Figure 7.1, based

on the general geopolymer concrete mixing procedure (Hardjito et al., 2004,

Junaid et al., 2012, Kong and Sanjayan, 2010) with slight modification

according to the author’s experience. Firstly, the selected crushed stone and

sand are mixed with the SSD water using a mechanical concrete mixer for

around 2 minutes. Then, fly ash and Na2SiO3 solution are added and mixed with

the wet aggregates for a further 2-3 minutes. This step is to uniformly distribute

the cementitious material (which is fly ash) onto the surfaces of the aggregates,

which is like the step of adding cement to wet aggregates in an OPC-based

concrete mixing. Following that, NaOH solution, water and admixtures are


added. Then, the mixing is continued for 2-3 minutes until a uniform fresh

concrete mixture (Figure 7.2(1)) is formed.

Figure 7.1: Flowchart of preparation of fly ash-based geopolymer concrete

It needs to be noticed that, for a geopolymer concrete mix, the addition of

Na2SiO3 solution is prior to the addition of NaOH solution and water, which is

different from a neat geopolymer paste mix. This is because fly ash must be

well distributed onto the aggregate surfaces before the formation of geopolymer

binder. To fulfil this, Na2SiO3 solution is added first to enhance the bonding of fly

ash and aggregates, and later, NaOH and water are added to generate the

geopolymer binder. Also, to prevent the possible deactivation of the applied

admixtures that may occur as a result of a direct mixing of them with NaOH

(Palacios and Puertas, 2005), these admixtures are added along with water

around 1 minute after the addition of NaOH solution.

219

Figure 7.2: Steps in preparation of concrete Con-N-Geo (1) mixing, (2) casting, (3) after heating and demoulding and (4) after 28-day curing

After mixing, a portion of the fresh Con-N-Geo mix is tested for its fresh

properties, and the remaining portion used to cast samples in cubic

50mm×50mm×50mm steel moulds with polyethylene sheets stuck onto the

inner surface to facilitate demoulding. A vibrating table (220-240V AC, 50HZ) is

applied to compact the fresh mix into well-shaped cubes (Figure 7.2(2)). Since

the fresh Con-N-Geo mixes produced here are very stiff in workability, a longer

time of vibration in assistance with manual tamping is usually required. The

Con-N-Geo sample is then sealed and cured in the ER at 20oC for 1 day, and

(1) (2)

(3) (4)


later cured at 80oC for 3 days in an electric oven. After heating, the sample is

placed back in the ER, and demoulded after it cools down (Figure 7.2(3)). The

demoulded sample remains in the ER, exposed to a low-moisture atmosphere

(20oC and 50% RH), until further testing. An image of a 28-day cured Con-N-

Geo sample is shown in Figure 7.2(4).

7.3.2 Workability

The same slump test that has been used for OPC-based concretes was applied

on the fresh Con-N-Geo mixes described in this section. However, it turned out

that all tested mixes had no slump at all, displaying an extremely stiff workability.

Actually, these fresh mixes were so stiff that the mixed material could not be

properly piled in the slump cone by rodding only, but also required manual

tamping and pressing. This is considered to be due to the rather high viscosity

of geopolymer material (Criado et al., 2009). In addition, the Con-N-Geo mixes

designed here are to pursue a better strength outcome, so their W/G ratios are

set to be very low which can influence their workability (Rangan, 2007).

To deal with these viscous fresh mixes, the author also tried to use a mini-flow

test that has been introduced in Chapter 2, subsection 2.4.1, to assess their

spread increases under vibration, but based on a larger sized slump cone to

suit a concrete mix. However, there was no significant flow spread detected on

the tested Con-N-Geo samples, despite that several previously published

articles have stated the suitability of flow test for measuring the workability of

geopolymer products (Criado et al., 2009, Chindaprasirt et al., 2007, Nath and

Sarker, 2014). It is thought that the mix is too stiff, so it is hard to flow freely on

the vibrating table.

221

Obviously, this stiff workability would cause problems in a real concrete practice

situation, which is already noticed during the preparation of Con-N-Geo mixes in

this research. But for the current need of a strong geopolymer binder in the

following Geo-GA concrete manufacture, this study would rather focus on the

mechanical properties and compromise the workability requirement. Otherwise,

to well compact this concrete mix in the moulds, a long time mechanical

vibration in assistance with a certain degree of manual tamping is applied with

caution as not to affect the uniformity of the mixture.


The density values of fresh and hardened Con-N-Geo samples were tested and

their compressive strengths measured using a TECNOTEST compression

machine at a speed of 0.33 MPa/sec, with the final value of each mixing batch

taken as the average of three cubic samples as shown in Table 7.4.

The densities of the three tested Con-N-Geo mixes are not much varied, with a

fresh density of 2300-2350 kg/m3 and an air dry density of 2150-2170 kg/m3.

For these mixes, their 28-day air dry densities are lower than their 7-day ones,

but the disparities are not significant. The density range of Con-N-Geo is close

to the range of NWC, and much higher than that of LWC (Clarke, 1993), which

is reasonable since Con-N-Geo still uses the normal weight aggregates as

crushed stone and sand.

The tested Con-N-Geo samples have 7-day compressive strengths of 55-64

MPa, which are nearly 90% of their 28-day strengths. This means that Con-N-

Geo can achieve a high early strength which agrees with the previous findings

(Rangan, 2007, Hardjito et al., 2004). Otherwise, these Con-N-Geo samples


have 28-day compressive strengths of 60-70 MPa, which are high-strength

concrete and may be preferred in real practice (Hoff, 1990).

Table 7.4: Density and strength properties of Con-N-Geo

Con-N-Geo-1 Con-N-Geo-2 Con-N-Geo-3


Density (AD) (7d) (kg/m3) 2170 2167 2170

Density (AD) (28d) (kg/m3) 2160 2150 2160



AD - air dry condition; * the value between brackets is the standard deviation of the relevant result.

Con-N-Geo-1 has a 28-day compressive strength of 70.42 MPa which is the

highest among the above three mixes. It is a high-strength concrete and

comparable to the NWC samples presented in Chapter 6, subsection 6.2.5.

Therefore, it is estimated that the geopolymer binder used in this mix must be

strong enough for a Geo-GA mix. On the other hand, Con-N-Geo-2 and Con-N-

Geo-3 result in lower 28-day strengths of around 60 MPa, very possibly due to

their higher W/G ratios (0.26) of the mix (Rangan, 2007). Nevertheless, these

two samples also have strong geopolymer binders to make them reach a high-

strength level. Moreover, they are found to have a better workability than Con-

N-Geo-1 because of their higher W/G ratios. For this case, the geopolymer

223

binders used in these two mixes are also applied for making Geo-GA concretes

described in the next section.

7.4 Geo-GA Concrete


Con-GA-Geo mixes are made based on the selected mix designs shown in

Table 7.2. The preparation of Con-GA-Geo follows the same procedure as that

for GC mix shown in Figure 7.1. As the workability of these Con-GA-Geo mixes

is not much improved, a long time vibration with several manual tamping is also

required for compaction. Meanwhile, a strict control suggested in the manual

ACI 211.2 is required to assure the SSD conditions of applied lightweight

geopolymer aggregates (ACI, 1998). Images showing the relevant steps in

mixing Con-GA-Geo are presented in Figure 7.3.


The density and strength properties of the Con-GA-Geo samples are measured

using the same methods introduced in subsection 7.3.3, and shown in Table 7.5.

Similar to the Con-N-Geo mixes, the densities of produced Con-GA-Geo

samples are not significantly varied, with a fresh density of 1870-1890 kg/m3, a

7-day air dry density of 1670-1690 kg/m3, and a 28-day air dry density of 1630-

1640 kg/m3. It can be seen that these Con-GA-Geo samples are lightweight (AS,

1998a, Clarke, 1993) after the application of lightweight geopolymer aggregates.

Through this way, one can produce a geopolymer binder-based LWC, which is

a new kind of concrete material that has been at the focus of studies in recent

years (Arellano Aguilar et al., 2010, Hu et al., 2009). Nevertheless, the LWC


proposed in this chapter is based on fly ash-based geopolymer aggregates and

binder.

Figure 7.3: Steps in preparation of concrete Con-GA-Geo (1) mixing, (2) casting, (3) after heating and demoulding and (4) after 28-day curing

Also, it can be observed that there is a slight density loss from 7-day Con-GA-

Geo samples to 28-day ones, which is possibly due to the release of water from

the geopolymer binder cured at a low-moisture condition (50% RH in the ER),

as previously discussed in Chapter 4, section 4.4.

On the other hand, the tested Con-GA-Geo samples achieve 7-day and 28-day

compressive strengths of around 25-29 MPa and 40-46 MPa respectively.

(1) (2)

(3) (4)

225

These results indicate that they can achieve a medium-strength and qualified

for structural LWC applications (ACI, 1998, Clarke, 1993). Among the samples,

Con-GA-Geo-1 has the highest compressive strength of 45.34 MPa after 28

days of curing, which is in accordance with the trend in strength results of Con-

N-Geo mixes, possibly due to its lowest W/G ratio (0.24) (Rangan, 2007).

Besides, the other two mixes with the same W/G ratio share a similar strength

result, even though they have different amounts of ingredients in the mix. For

this case, W/G ratio seems to be more important regarding the strength

development of Geo-GA concrete.

Table 7.5: Density and strength properties of Con-GA-Geo

Con-GA-Geo-1

Con-GA-Geo-2

Con-GA-Geo-3


Density (AD) (7d) (kg/m3) 1680 1690 1670

Density (AD) (28d) (kg/m3) 1630 1630 1640


28.67 (0.92)*

26.18 (1.39)*

25.77 (1.27)*


45.34 (1.41)*

40.04 (0.59)*

41.32 (0.75)*

AD: air dry condition; * the value between brackets is the standard deviation of the relevant result.

Compared to the Con-N-Geo samples, which are made with the same

geopolymer binders and curing regimes, Con-GA-Geo samples result in

approximately 20 MPa lower compressive strengths. Part of this strength

deficiency is considered due to using the lightweight geopolymer aggregates


instead of normal weight aggregates. Apart from this, it is not certain whether

some other factors may also affect the strength development of the concrete

using geopolymer aggregates. It can be found that the strength difference of

Geo-GA and GC (around 20 MPa) is more significant than that of OPC-GA and

NWC (around 10 MPa). It seems that the strength loss by using geopolymer

aggregates is exaggerated in a geopolymer binder system compared to that in

an OPC system. Except from the lower self-strength of geopolymer aggregate,

it is estimated that this may also be relevant to the bonding strength of

geopolymer aggregates and geopolymer binder (compared to the bonding of

geopolymer aggregates and OPC matrix), and this possibility requires further

research.

Moreover, it is not observed that Con-GA-Geo will have a high early strength

that has been found in Con-N-Geo. The 7-day strengths of Con-GA-Geo mixes

are 25-29 MPa and only achieve 62-66% of their 28-day strengths.

Nevertheless, Con-GA-Geo mixes have strength gains of approximately 15

MPa during the period of 7-28 days, where there is even no heat treatment.

This strength gain is found special because it is clearly higher than those of

Con-N-Geo mixes (4-7 MPa). It is also higher than the normal strength gain

within this period reported in the literature (Hardjito et al., 2004). As the Con-N-

Geo and Con-GA-Geo mixes share the same geopolymer binders and curing

regimes, this special strength gain is considered much relevant to the presence

of geopolymer aggregates, which needs further investigation.

The research on Geo-GA concrete in this section proves that a workable

concrete mix made with geopolymer aggregates and binder can be realised with

good mechanical properties. For this case, an ‘absolute geopolymer concrete’

227

which is almost 100% from geopolymer material is manufactured. This will

provide a new concept of concrete mix in the field of geopolymer concrete, and

a new kind of LWC material with the promising quality preferred in future

construction. However, to realise the desired mechanical properties, the

workability of Geo-GA concrete has been compromised. The Geo-GA concrete

samples produced here have stiff workability, which is even difficult to be

accurately measured by normal workability tests. The addition of admixtures

cannot significantly improve the workability, but only modify the fresh

performance to make them better compacted. This Geo-GA concrete can be

promising for its strength, but still needs further research to modify its

workability performance.

7.4.3 Fracture Surface

The fracture surface of the Geo-GA sample after compressive strength testing

is observed using a Lynx vision engineering stereo microscope. The captured

image is shown in Figure 7.4 in which the geopolymer aggregates and

geopolymer binder matrix are labelled based on their detected shapes and

sizes.

In the image, the geopolymer aggregates are embedded in the geopolymer

binder matrix, with a complete interfacial zone observed. Little cracking is

formed along the detected interfacial zone, which indicates a good strength

performance of the bonding of the aggregates and binder. Also, no cracking is

formed across the aggregates. It can be seen that Con-GA-Geo sample has a

similar fracture surface as that of Con-GA-OPC presented in Chapter 6,

subsection 6.3.6.


Figure 7.4: Enlarged image (40x) of fracture surface of Con-GA-Geo (the image represents a length of approximately 3 mm)

On the other hand, from this image, there appears no indication that

geopolymer aggregates and geopolymer binder would merge during the

concrete mix. These two phases are still clearly distinguished after 28 days of

curing, possibly due to their different manufacturing histories, despite that they

share the same nature of geopolymer material. Also, it is not found that the

hardened geopolymer aggregates would react again, even under the alkali

activator and heat treatment applied in the manufacture of Con-GA-Geo sample.

Still, the above discussion is only based on an image observation which is not

sufficient enough to clarify the situation of the interfacial zone of Geo-GA. If the

above discussion is pointing to the true explanation, it means that the

geopolymer aggregates and geopolymer binder will be, later on in their service,

inert and independent for each other. Nevertheless, the author wonders that if a

reaction on the boundary (interfacial zone) may happen it may therefore

enhance or disrupt the bonding of aggregates and binder, leading to an

improvement or deterioration in the strength of concrete, depending on the

nature of such reaction. And if a reaction can happen, it will be interesting to

Geopolymer Aggregates

Geopolymer Matrix

229

find out if the reactant from geopolymer aggregates is either the fly ash residue

(if any) or the hardened geopolymer structure, or both.

7.5 Comparison of Geo-GA Concrete and Other LWCs

The performance of the Geo-GA concrete produced in this research, which is a

new type of LWC, is evaluated by comparison with other LWCs. It is firstly

compared with the OPC-GA concrete discussed in Chapter 6, which uses the

same geopolymer aggregates but a different cementitious material. After this, it

is compared with a geopolymer binder-based LWC which uses a conventional

FA-LWA Lytag aggregate.

7.5.1 Geo-GA versus OPC-GA

Both Geo-GA and OPC-GA mixes are investigated in this thesis, whose

measured properties are presented in Table 7.6. The three mixes have similar

fresh densities of around 1880-1890 kg/m3. However, the two Con-GA-Geo

samples have 28-day air dry densities of 1630 kg/m3 which are around 15%

less than that of Con-GA-OPC. This difference in density is thought to be

caused by the different features of applied cementitious materials. In a Con-GA-

Geo mix, the water gets released when the concrete is cured at a low-moisture

condition after heating, which will reduce the weight and is likely to reduce the

density, as discussed in Chapter 4. However, in a Con-GA-OPC mix, water is

held in for a hydration process (Lea, 1970). Besides, further water may be

absorbed from the atmosphere during the curing of Con-GA-OPC in a water-

spray room, to take part in the cement hydration, and may increase the weight.

This may explain the higher density result of Con-GA-OPC.


Table 7.6: Density and strength properties of Con-GA-OPC and Con-GA-Geo samples

Concrete type Con-GA-OPC

Con-GA-Geo-1

Con-GA-Geo-2

Curing regime 20oC, water-spray

80oC, 3 days

80oC, 3 days


Density (AD) (7d) (kg/m3) - 1680 1690

Density (AD) (28d) (kg/m3) 1770 1630 1630


44.20 (0.46)*

28.67 (0.92)*

26.18 (1.39)*


59.32 (0.71)*

45.34 (1.41)*

40.04 (0.59)*


On the other hand, Con-GA-Geo samples have 15-20 MPa lower compressive

strengths in comparison with the Con-GA-OPC sample which is thought quite

significant. This is considered relevant to the lower density of Con-GA-Geo,

since a concrete with a lower density is likely to have a less resistance to

external pressure (Neville, 1996). Apart from this, it is observed that geopolymer

aggregates do not perform well in a geopolymer binder system, where the

geopolymer binder-based concrete using geopolymer aggregates (Con-GA-Geo)

has a significant strength loss than the one using natural aggregates (Con-N-

Geo). This may explain the strength difference found in Con-GA-OPC and Con-

GA-Geo.

231

7.5.2 Geo-GA versus FA-LWA Geopolymer Concrete

Table 7.7: Mix designs of Con-GA-Geo and Con-LY-Geo (SSD aggregates condition)

Concrete type Con-GA-Geo

Con-LY-Geo

Fly ash (kg/m3) 446.1 446.1

W/G ratio 0.26 0.26

SiO2/Na2O (x value) 1.25 1.25


Fly ash/Activator (F/A) 1.86 1.86

Coarse GA (kg/m3) 652.5 -

Fine GA (kg/m3) 550.6 -

Coarse Lytag (kg/m3) - 567.4

Fine Lytag (kg/m3) - 482.1

NaOH 16M (kg/m3) 51.8 51.8

Na2SiO3 (kg/m3) 182.7 182.7

Water (kg/m3) 5.4 5.4

Centrox MWR (kg/m3) 4.5 4.5

Centrox VMA (kg/m3) 4.5 4.5

Unit weight (kg/m3) 1898 1744.4

Aggregate mass proportion (%) 63% 60%

Another kind of geopolymer binder-based LWC made with a FA-LWA is

manufactured and used as a reference for assessing the quality of Geo-GA

concrete. The selected FA-LWA is Lytag aggregate which is a high-quality FA-

LWA as introduced in Chapter 5, subsection 5.11.2. The produced concrete is


coded as ‘Con-LY-Geo’ whose mix design is created based on that of Con-GA-

Geo by using the same volume proportion of aggregates, as shown in Table 7.7.

This way, the mixes of Con-GA-Geo and Con-LY-Geo use the same

geopolymer binder.

The properties of Con-GA-Geo and Con-LY-Geo samples are measured and

presented in Table 7.8. Con-LY-Geo has a fresh density of 1671 kg/m3, a 7-day

AD density of 1500 kg/m3 and a 28-day AD density of 1460 kg/m3, which are

even lower than those of lightweight Con-GA-Geo. It is assumed that this is due

to its application of Lytag aggregate, which has a lower density than the

geopolymer aggregate as stated in Chapter 5, subsection 5.11.2. The density

results of Con-LY-Geo also indicate that it is a type of LWC according to

relevant standards (AS, 1998a, ACI, 1998).

Table 7.8: Properties of Con-GA-Geo and Con-LY-Geo

Con-GA-Geo Con-LY-Geo


Density (AD) (7d) (kg/m3) 1690 1500

Density (AD) (28d) (kg/m3) 1630 1460




233

The two mixes end up in a similar 28-day compressive strength of

approximately 40 MPa, which is a medium-strength level and suitable for

serving as a type of structural LWC in real practice (ACI, 2003). Besides, it is

found that the geopolymer concrete using Lytag can achieve a similar strength

but an even lower self-weight compared to the one using geopolymer aggregate.

This may imply a more advanced performance of Lytag than geopolymer

aggregate. Nevertheless, as discussed in Chapter 6, subsection 6.4.1,

geopolymer aggregate can also achieve a good quality with much lower

production cost and a more eco-friendly manufacture, and therefore is

considered to be preferred compared to Lytag in future construction.

7.6 Geopolymer Binder-based Mortar

This section studies a new type of mortar material which is made of geopolymer

binder in replacement of conventional OPC. The mortars studied here include a

mortar mix using geopolymer aggregates and geopolymer binder, which is

referred to as Geo-GA mortar and coded as ‘Mor-GA-Geo’, and a mortar mix

using natural sand coded as ‘Mor-S-Geo’. The mortar using sand and

geopolymer binder has been previously researched (Chindaprasirt et al., 2007,

Kong and Sanjayan, 2010) to take advantage of geopolymer as what has been

done in concretes. In this section, apart from the geopolymer mortar using sand

(Mor-S-Geo), it also investigates the mortar mix using fine geopolymer

aggregates. The latter one is expected to be a new lightweight mortar material.

7.6.1 Materials

The same fine geopolymer aggregate sample and natural sand used in Con-

GA-Geo and Con-N-Geo mixes are used. Also, the ingredients required for a


geopolymer synthesis introduced in earlier sections, such as fly ash, NaOH

solution, Na2SiO3 solution and deionised water are applied.

7.6.2 Mix Design

The mortar mix designs are created based on the same mix design procedure

of Geo-GA and GC mixes, as illustrated in section 7.2. The mix design of Mor-

GA-Geo is firstly made based on that of Mor-GA-OPC introduced in Chapter 6,

subsection 6.5.2. Later, the mix design of Mor-S-Geo is created from Mor-GA-

Geo by replacing geopolymer aggregates with the same volume of sand. The

final mix designs of above mixes are modified to suit 1 m3 mortar mix, and

presented in Table 7.9. The detailed mix design procedure of above mortar

mixes is included in Appendix B.

Table 7.9: Mix designs of Mor-S-Geo and Mor-GA-Geo (SSD aggregates condition)

Mortar type Mor-S-Geo Mor-GA-Geo

Fly ash (kg/m3) 580.5 580.5

W/G ratio 0.26 0.26




River sand (kg/m3) 1372.5 -

Fine GA (kg/m3) - 1061.1

NaOH 16M (kg/m3) 67.4 67.4

Na2SiO3 (kg/m3) 237.7 237.7

Water (kg/m3) 7.0 7.0

235

7.6.3 Mortar Preparation

Both the Mor-S-Geo and Mor-GA-Geo mixes are prepared following the same

procedure, based on a semi-automatic mixer. Firstly, the fine aggregates are

mixed for 2 minutes with the amount of water necessary to make the aggregate

arrive to its SSD condition. Fly ash and Na2SiO3 solution are then added and

the mixing continues for a further 2 minutes, followed by the addition of NaOH

solution and water and mixing until a uniform mixture is formed (Figure 7.5).

Figure 7.5: Images on fresh mortar mixtures: (1) Mor-S-Geo and (2) Mor-GA-Geo

After mixing, the fresh mortar mixtures are cast into cubic 50mm×50mm×50mm

steel moulds, which have polyethylene sheets stuck onto the inner surfaces.

The samples are then cured in the ER at 20oC for 1 day, and heated at 80oC for

3 days. After heating, these samples are placed back in the ER and demoulded

to expose to the atmosphere (50%) when they cool down. This curing regime is

similar to that of Geo-GA and GC mixes, and found optimum for an enhanced

strength gain as discussed in Chapter 4. Mortar samples using sand and fine

geopolymer aggregates after 28 days of curing are shown in Figure 7.6. The

(1) (2)


samples display a similar appearance as the previous geopolymer concrete

samples.

Figure 7.6: Images on 28-day cured Mor-S-Geo and Mor-GA-Geo samples

7.6.4 Workability

The workability performances of the fresh mortars discussed in this section are

evaluated based on the combined slump and mini-flow method discussed in

Chapter 2, subsection 2.4.1. The initial slumps and spreads, and spreads after

10-second vibrations of the mortar samples; Mor-S-Geo and Mor-GA-Geo, are

measured and presented in Table 7.10 and Figure 7.7.

Table 7.10: Slump and spread values of fresh Mor-S-Geo and Mor-GA-Geo

Mortar


Initial slump (mm)



Spread in 3 mins

(cm2)

Spread in 10 mins

(cm2) Mor-S-

Geo 1 11.34* 13.58 13.92 13.92

Mor-GA-Geo 0 11.34* 14.52 14.65 14.65

*11.34cm2 is the bottom area of the mini-sized cone which indicates that there is no spread in this case.

Mor-S-Geo Mor-GA-Geo

237

Figure 7.7: Increases in spread areas of fresh Mor-S-Geo and Mor-GA-Geo caused by mechanical vibration

As seen in Table 7.10, both mortar mixes almost have no initial slump or spread

which indicates their stiff workability similar to the concrete samples reported in

previous sections. As mentioned before, this is relevant to the high viscosity of

the geopolymer binders used here. Nevertheless, as seen from Figure 7.7,

these mortars can have some detectable spreads after the stimulation of

mechanical vibration, even though these spreads are less significant than those

of OPC-based mortars presented in Chapter 6, subsection 6.5.4. The spread

results indicate that these geopolymer mortars can have better workability than

the concretes, when coarse aggregates are not present. Also, the mortar using

geopolymer aggregate results in higher spread values compared to the one

using sand, meaning that it is more vulnerable to the vibration force, possibly

due to its lower self-weight.

4

6

8

10

12

14

16

initial spread spread atonce

spread in 3mins

spread in 10mins

Spre

ad a

rea

(cm

2 )

Mor-S-Geo

Mor-GA-Geo



The density and strength properties of the mortar samples produced are

presented in Table 7.11. Mor-GA-Geo has a fresh density of 1819 kg/m3, a 7-

day AD density of 1710 kg/m3 and a 28-day AD density of 1650 kg/m3. These

densities are much lower than those of Mor-S-Geo, which are all above 2000

kg/m3. An application of geopolymer aggregate in replacement of natural sand

can significantly reduce the density behaviour of mortar mix, as seen in Table

7.11, which is similar to the Geo-GA concrete. Meanwhile, the density range of

Mor-GA-Geo seems low enough to serve as a lightweight construction material.

On the other hand, Mor-S-Geo has a high early strength of 64.48 MPa after 7

days of curing, following the general trend that a geopolymer concrete will have

a high early strength as discussed in section 7.3. It also reaches a 28-day

compressive strength of 68.53 MPa which indicates a high-strength property.

However, Mor-GA-Geo only results in a 7-day compressive strength of 37.65

MPa, but it reaches a 28-day compressive strength of 57.58 MPa which is also

high enough for practical use, and is even comparable with the OPC-based

mortars discussed in Chapter 6, section 6.5.5 (55-59 MPa).

Otherwise, Mor-GA-Geo displays a similar trend in strength development as the

concrete sample Con-GA-Geo in that it does not have a high early strength but

receives a high strength gain at an ambient curing from 7 to 28 days after

mixing. This strength gain might be partly benefited from the low-moisture

ambient curing, as discussed in Chapter 4. But there might also be other

reasons to make this happen, considering that this strength gain is so high

(nearly 20 MPa) for an ambient-cured geopolymer. It seems that this high

239

strength gain would rather happen in a structure of ‘geopolymer binder-

geopolymer aggregates’, which needs an investigation based on further

experimental results.

Table 7.11: Density and strength properties of Mor-S-Geo and Mor-GA-Geo

Mor-S-Geo Mor-GA-Geo


Density (AD) (7d) (kg/m3) 2060 1710

Density (AD) (28d) (kg/m3) 2000 1650



AD: air dry condition

7.7 Conclusions

In this chapter, the practicality of using geopolymer aggregates in geopolymer

binder-based concrete and mortar mixes is reported and discussed. The

discussion is part of a continuous research following Chapter 6, and

concentrates on a geopolymer binder system.

This chapter establishes a workable concrete mix design procedure for creating

the required mix design of Geo-GA concrete and mortar. It also illustrates a

practical preparation of Geo-GA concrete and mortar, with samples tested and

analysed. The Geo-GA samples (concrete and mortar) are found to have

lightweight and good mechanical properties, which can be used as new kinds of


lightweight materials. The produced Geo-GA concrete can be qualified LWC

used in structural application. The Geo-GA mortar can reach a high-strength

level which is even comparable with the conventional OPC-based mortar.

However, to pursue a good strength, the workability of above Geo-GA mixes is

compromised.

A concrete/mortar mix made from 100% fly ash-based geopolymer material with

sufficient mechanical properties for structural application is realised based on

the research of this and previous chapters. The produced material is treated as

a new way of low-cost and eco-friendly concrete/mortar manufacture, since it

consumes less energy, and uses the aggregates and cementitious material both

of which originating from industrial by-products.

241

Chapter 8

Room Temperature-cured Geopolymer Aggregate

(RTGA) and its Application in Concrete

8.1 Introduction

This chapter concentrates on a special type of geopolymer aggregate (GA),

which is manufactured at a room-temperature condition (20oC) and therefore

referred to as a room temperature-cured geopolymer aggregate (RTGA).

As introduced in Chapter 1, a target of the current study is to take advantage of

geopolymer technology so as to manufacture a low-cost, eco-friendly

geopolymer aggregate. Based on the research work in previous chapters, a

geopolymer aggregate, which only requires a mild-temperature heating

operated at tens of degrees instead of a conventional elevated-temperature

heating at above a thousand degrees, is successfully created with an advanced

quality for a concrete mix. Nevertheless, this geopolymer aggregate still needs

a mild-temperature heating, whereby a concept of RTGA is promoted here to

investigate the possibility of a geopolymer aggregate made without the extra

heat treatment. Compared to the former heated geopolymer aggregates, the

development of RTGA is anticipated to be able to further reduce the energy

consumption, and also simplify the manufacturing procedure by saving the

equipment and effort for heat treatment.

A specific manufacturing procedure for RTGA, which is based on a prolonged

curing period at a low-moisture condition as suggested in Chapter 4, is firstly

illustrated in this chapter. Then, the basic properties of RTGA are measured

242 8. Room temperature-cured geopolymer aggregate (RTGA) and its application in concrete

which indicate that RTGA is a kind of FA-LWA with a low specific gravity and

high water absorption capacity. Moreover, the performance of RTGA sample in

a concrete manufacture is examined. Both OPC-based and geopolymer binder-

based concretes using the coarse and fine RTGAs are made and tested. The

results indicate that RTGA can be qualified for making a structural LWC, even

though it is not as good as its heated counterpart.

8.2 Manufacturing Procedure of RTGA

The manufacturing procedure for RTGA is similar to that for the geopolymer

aggregate manufacture previously introduced in Chapter 5, section 5.3, with

several steps changed to suit a room-temperature manufacture. For simplicity,

the previous geopolymer aggregate that has been through heat treatment is

referred to as ‘heated GA’ throughout this chapter. A flowchart of the

manufacturing procedure for RTGA is presented in Figure 8.1.

The geopolymer mix design used for the manufacture of RTGA is shown in

Table 8.1 which is the same one used for heated GA. Apart from this, the entire

mixing and casting steps for RTGA are also similar to those for the heated GA,

which are detailed in Chapter 5, subsection 5.3.1.

243

Figure 8.1: Flowchart of manufacturing procedure for RTGA

Table 8.1: Geopolymer mix design for RTGA manufacture


W/G value 0.24




Fly ash (kg/m3) 1283.42



Water (kg/m3) 14.03


A special step of the manufacture of RTGA is curing, which is completed in a

controlled environment room (ER) at 20oC and 50% RH. In the first 7 days, the

prepared fresh RTGA sample is sealed in the moulds to assure sufficient

moisture for the initial stage of geopolymerisation. After this, this RTGA sample

is unsealed, but still kept in the moulds to maintain its shape, and exposed to a

low-moisture condition of 50% RH in the ER for further 77 days. The total curing

time for RTGA is 84 days, as suggested in Chapter 4, where it is found that

such a long-term ambient-curing at a low-moisture atmosphere can enhance

the strength of room temperature-cured geopolymer and make it achieve a

similar strength level as the short-term heated one.

Another step of the manufacturing procedure that needs to be noticed is

washing. The washing of heated GA is needed to remove the hazardous alkalis

for a user-friendly purpose. However, this washing step in the manufacture of

RTGA has an extra mission to terminate (at the chosen time marking the end of

curing) the potential geopolymerisation that may happen in a RTGA sample.

This is done at the 84th day by removing the residual alkali content by water, so

as to clear out the reactant of geopolymer reaction. Otherwise, the residual

alkali content may continue to take part into the geopolymerisation after 84 days

(like during the period of storage) and the produced RTGA sample cannot then

be referred to as the ‘84-day room temperature-cured’ because its structure

would then be formed through a geopolymerisation process longer than 84 days.

The washing step for the RTGA sample is the same as that for the heated GA

as illustrated in Chapter 5, subsection 5.3.3. In this procedure, 5 kg of RTGA

sample is soaked in 20 litres of water in a water tank. The duration of washing is

decided by the pH variation of the solution in the water tank. As seen from

245

Figure 8.2, it takes 5-6 days of washing to reduce the pH value to a neutral

range (pH=5-9) when it is considered that most alkali content has been

removed from the RTGA sample.

Then, the RTGA samples are air-dried and crushed using a jaw crusher. The

crushing program used for RTGA is the same as that for heated GA as detailed

in Chapter 5, subsection 5.3.4. The prepared coarse (4.75-10 mm) and fine

(<4.75 mm) RTGA samples are maintained in an air dry condition and then

stored in sealed containers.

Figure 8.2: pH value after every day’s washing

Compared with the heated GA, the manufacturing procedure of RTGA does not

include any extra heat treatment, which is beneficial for reducing the energy

consumption and simplifying the practical work. For these reasons, it is

assumed that this newly made RTGA can be preferred for its low-cost, eco-

friendly and simple manufacture. It is also a kind of cold-bonded FA-LWA, which

0

2

4

6

8

10

12

14

1 2 3 4 5 6

pH v

alue

Washing period (day)


has been proposed in recent literature for a reduced energy consumption of FA-

LWA manufacture (Gesoğlu et al., 2007, Manikandan and Ramamurthy, 2008),

but realised based on a geopolymer technology.

8.3 Characteristics of RTGA

8.3.1 Appearance

The air-dried coarse and fine RTGAs produced by the procedure in section 8.2

are displayed in Figure 8.3. RTGAs display a unique light-grey appearance, with

angular particle shapes, similar to the heated GA described in Chapter 5,

section 5.4.

Figure 8.3: Coarse (>4.75mm) (left) and fine (<4.75mm) (right) RTGAs in air-dried condition

8.3.2 Grading

The grading of the original coarse and fine RTGA samples, which are the ones

directly collected after the jaw crushing, is tested based on the standard sieve

analysis, with the results shown in Table 8.2 and subsequent grading curves in

Figure 8.4. Meanwhile, the grading curves of the heated GAs are also

presented in Figure 8.4 for comparison.

247

The coarse RTGA sample displays a good grading that is considered suitable

for a concrete mix. As seen from Figure 8.4, the grading curves of coarse RTGA

and coarse heated GA are almost identical. This could be reasonable since

these two aggregate samples are manufactured through the same crushing

program.

Table 8.2: Typical sieving analysis of coarse and fine RTGAs





Coarse

13.20 mm 0.0 0.0% 0.0% 100.0%

9.50 mm 3.0 0.0% 0.0% 100.0%

6.70 mm 396.2 0.4% 0.4% 99.6%

4.75 mm 270.4 58.3% 58.7% 41.3%

<4.75 mm 10.0 39.8% 98.5% 1.5%

Total 679.6

Fine

4.75 mm 1.8 0.3% 0.3% 99.7%

2.36 mm 94.7 13.5% 13.8% 86.2%

1.18 mm 127.3 18.2% 31.7% 68.3%

600 µm 223.5 31.9% 63.6% 36.4%

425 µm 55.7 7.9% 71.5% 28.5%

300 µm 64.4 9.2% 80.7% 19.3%

150 µm 116.2 16.6% 97.3% 2.7%

75 µm 15.0 2.1% 99.4% 0.6%

<75 µm 3.9 0.6% 100.0% 0.0%



Nevertheless, from the same Figure, it can be seen that the fine RTGA sample

results in a finer particle size distribution with more particles passing 425 µm

and 300 µm, compared to the fine heated GA sample. However, the fine RTGA

still requires a further refinement before applying it to a concrete mix, because it

does not have a sufficient percentage of particles passing 150 µm according to

the standard ASTM C330 (ASTM, 2009). This refinement is done following the

same procedure for the fine heated GA as illustrated in Chapter 5, subsection

5.5.2.

Figure 8.4: Grading curves of RTGAs and heated GAs

8.3.3 Density and Water Absorption

The density and water absorption capacity regarding the RTGA produced as

described in section 8.2 are measured according to Australian standard AS

1141 (AS, 1999) and shown in Table 8.3. Here, both coarse and fine RTGAs

are tested when they are still in air-dried condition right after the jaw crushing

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

0.01 0.1 1 10 100

Cum

ulat

ive

perc

enta

ge p

assi

ng (%

)

Sieve size (mm)

RTGA coarse

RTGA fine

GA coarse

GA fine

249

procedure. Also, the relevant properties of the heated GA measured in the

same air-dried condition are presented in this Table.

Table 8.3: Densities and water absorption capacities of RTGA and heated GA

RTGA Heated GA

Coarse (84-day)

Fine (84-day) Coarse

(28-day) Fine

(28-day)

Specific gravity (SSD) 1.78 1.85 1.84 2.01

Specific gravity (OD) 1.59 1.69 1.66 1.77

Dry loose bulk density (kg/m3) 841.2 1097.5 876.8 1117.2

Water absorption capacity (in 24 hours) 10.0% 9.0% 11.0% 13.7%


From Table 8.3, a coarse RTGA has specific gravities of 1.78 and 1.59 under

saturated surface dry and oven dry conditions respectively. A fine RTGA has a

similar result but of a higher value, with specific gravities of 1.85 and 1.69 under

saturated surface dry and oven dry conditions respectively. These results

indicate that RTGA should be a type of LWAs (AS, 1998a). Meanwhile, the dry

loose bulk densities of coarse and fine RTGAs are 841.2 kg/m3 and 1137.5

kg/m3 respectively which also indicate the lightweight of RTGA as LWAs (ASTM,

2009). Therefore, RTGA is another type of FA-LWA product created by

geopolymer technology, same as the heated GA previously introduced in

Chapter 5.


Also, Table 8.3 indicates that the RTGA have lower density values than the

heated GA. Considering the same mix design and mechanical crushing applied,

this detected density difference is most likely determined by the different curing

regimes of RTGA and heated GA. It is considered that the RTGA cured at room

temperature has a less densified geopolymer structure, in comparison with the

heated GA. This complies with the finding based on SEM observations in

Chapter 4, subsection 4.5.4, that a heat treatment is found beneficial for the

structural densification of fresh geopolymer. Nevertheless, the density values of

RTGA are still in the regular range of LWAs for concretes (ASTM, 2009, Clarke,

1993).

On the other hand, both coarse and fine RTGAs have high water absorption

capacities of 10% and 9% respectively. These absorption capacities, although

slightly lower than those of the heated GA, are still much higher than usual

normal weight aggregates (Neville, 1996) and should be rather noticed during

the practical concrete mix (ACI, 1998, ACI, 2003).

It was once thought that RTGA would have a higher water absorption capacity

than the heated GA. This is because a geopolymer structure cured at room

temperature usually ends up in a less densified structure than the one cured by

heating (as discussed in Chapter 4, section 4.5). This suggests that the RTGA

is expected to have a higher porosity. However, this was not the case in the

current test. The measured lower water absorption capacity of RTGA indicates

that it has a lower content of penetrable pores, compared to the heated GA,

even though RTGA may have a higher porosity. Indeed, a large proportion of

these pores must be impenetrable to water. This indicates that there must be

microstructural differences between these two aggregate samples that have

251

been caused by their different curing regimes. A rigorous study of the total

porosity as well as the pore size distribution is therefore needed to shed more

light on this phenomenon. This is, no doubt, a subject for future research.

The properties discussed in this subsection reflect the basic information of the

newly made coarse and fine RTGA samples. Moreover, these properties are

needed for completing the concrete mixes using RTGA, which will be later

researched in this chapter.

8.3.4 Infrared Spectroscopy

An infrared (IR) spectroscopy test is used to analyse the chemical structure of

the produced RTGA sample. This is conducted using a SHIMADZU infrared

spectrophotometer equipped with an attenuated total reflectance (ATR)

accessory, from 4000 cm-1 to 650 cm-1 with a 4 cm-1 resolution, single bounce

and total of 32 scans. The IR spectrum of RTGA is presented in Figure 8.5,

where the IR spectra of fly ash and heated GA are also included for comparison.


Figure 8.5: IR spectra of fly ash, heated GA and RTGA

For the RTGA sample, there is a strong band at 1001.06 cm-1 which indicates

the presence of an amorphous phase that normally lies in the range of 950 cm-1

to 1250 cm-1 (Lee and Van Deventer, 2003) and could be Si-O-Si and Si-O-Al

bonds of amorphous aluminosilicate contents (Barbosa et al., 2000). This band,

as the one referring to the amorphous Si and Al contents, is also found in the

spectra of fly ash and the heated GA, but at different frequencies. Compared to

that of fly ash (1056.99 cm-1), this band shifts to a lower frequency (1001.06 cm-

1) in the spectrum of RTGA. As said in Chapter 5, section 5.10, this shift is likely

due to an increasing proportion of the tetrahedral aluminium component as a

result of geopolymerisation and is indicative of N-A-S-H formation (Fernández-

Jiménez and Palomo, 2005, Rees et al., 2007, Davidovits, 2008). Otherwise,

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

500.01000.01500.02000.02500.03000.03500.04000.0

Tran

smitt

ance

(%)

Wave Number (cm-1)

Fly Ash

Heated GA

RT GA

1056

.99 79

4.67

77

7.31

74

0.67

79

8.53

77

7.31

73

6.81

69

4.37

999.

13

796.

60

777.

31

694.

37

734.

88

1001

.06

695.

20

253

this band of RTGA still locates at a higher frequency than that of heated GA

(999.13 cm-1), implying that the increase of the tetrahedral aluminium in RTGA

is not as much as that in heated GA. For this case, it is estimated that RTGA

may result in a lower degree of geopolymerisation compared to the heated GA,

which possibly explains the lower density of RTGA found in subsection 8.3.3.

On the other hand, within the range of 690 cm-1 to 800 cm-1 which covers the

crystalline phases, RTGA has a band at 777.31 cm-1 which is considered to be

due to the presence of quartz (Barbosa et al., 2000, Davidovits, 2008). This

band is detected at the same frequency of all three tested samples in Figure 8.5,

indicating that quartz is relatively stable during geopolymerisation, confirming

previous reporting (Chen-Tan et al., 2009). Nevertheless, the other bands at

796.60 cm-1, 734.88 cm-1 and 694.37 cm-1 found in the spectrum of RTGA,

which refer to the mullite (Padmaja et al., 2001), are slightly different from those

found in the spectra of fly ash and heated GA. As discussed in Chapter 5,

section 5.10, this difference is considered relevant to the use of different

synthesising conditions and needs further research.

8.4 RTGA in Concrete Manufacture

In this section, the application of RTGA in a concrete mix is researched. The

concretes tested here include both OPC-based concrete and geopolymer

binder-based concrete, to investigate the suitability of RTGA in above two

cementitious systems. The properties of concrete samples using RTGA are

measured, and also compared with those of the concretes made with the

heated GA discussed in Chapters 6 and 7.


8.4.1 OPC-based Concrete

Firstly, an OPC-based concrete mix made with both coarse and fine RTGAs is

researched, to assess the compatibility of RTGA with the OPC system as the

current mainstream cementitious system. This concrete is referred to as OPC-

RTGA concrete and coded as ‘Con-RTGA-OPC’.

Table 8.4: Mix designs of Con-RTGA-OPC and Con-GA-OPC (SSD aggregates condition)

Item Con-RTGA-OPC Con-GA-OPC

(kg/m3) (kg/m3)

Cement 351 351

Water 182.9 178.7

Coarse RTGA 659.7 686.2

Fine RTGA (1.18-4.75mm) 107.4 117.5

Fine RTGA (300µm-1.18mm) 322.3 352.5

Fine RTGA (150-300µm) 80.6 88.1

Fine RTGA (75-150µm) 26.9 29.4

Fly ash 117 117

Superplasticiser 4 4


Water/Cementitious materials ratio 0.39 0.38

The mix design of Con-RTGA-OPC is created based on the mix design of a

previous concrete mix Con-GA-OPC discussed in Chapter 6. It is specified that

Con-RTGA-OPC has the same cementitious material content and the same

workability (50 mm slump based on a standard slump test) as Con-GA-OPC.

255

This is in order to better compare the effects of RTGA and heated GA for a

OPC-based concrete mix, as previously suggested (Kayali, 2005). The final mix

design is created following the mix design procedure detailed in Chapter 6,

subsection 6.3.2, and presented in Table 8.4.

The preparation of Con-RTGA-OPC follows the normal mixing procedure of

OPC-based concrete as presented in Chapter 6, subsection 6.2.3. Same as the

preparation of Con-GA-OPC, longer time is needed to mix coarse and fine

RTGAs because they are LWAs which take a long time to fully absorb the

added SSD water. Images showing the relevant steps in mixing Con-RTGA-

OPC are presented Figure 8.6. As regulated, the fresh Con-RTGA-OPC mix

has a slump value of 50 mm based on a standard slump test (AS, 1993).

The density values of fresh and hardened Con-RTGA-OPC samples were

tested and their compressive strengths measured using a TECNOTEST

compression machine at a speed of 0.33 MPa/sec, with the final value of each

mixing batch taken as the average of three cubic samples. The density and

strength results are shown in Table 8.5. Meanwhile, the relevant results of Con-

GA-OPC are presented in the same Table for comparison.

Table 8.5 shows that Con-RTGA-OPC has a fresh density of 1921 kg/m3, 28-

day AD density of 1630 kg/m3 and 28-day SSD density of 1860 kg/m3. As using

lightweight RTGAs, Con-RTGA-OPC displays a low self-weight and belongs in

the class of structural LWC products (Clarke, 1993, ACI, 2003), which is similar

to the Con-GA-OPC.


Figure 8.6: Steps in preparation of Con-RTGA-OPC (1) mixing, (2) casting, (3) immediately after demoulding and (4) after 28-day curing

Also, despite that Con-RTGA-OPC has a slightly higher fresh density, the AD

and SSD densities of its hardened sample are lower than those of Con-GA-

OPC. This is very likely due to the fact that RTGA has a lower specific gravity

compared to the heated GA, as shown in Table 8.3.

On the other hand, the hardened Con-RTGA-OPC achieves 7-day and 28-day

compressive strengths of 35.88 MPa and 42.03 MPa respectively, as seen from

Table 8.5. This indicates that Con-RTGA-OPC is close to achieving a medium-

strength and also qualified for a structural LWC (which needs a minimum of 17

(1) (2)

(3) (4)

257

MPa) (ASTM, 2009, ACI, 2003). However, Con-RTGA-OPC has much lower

compressive strengths in comparison with Con-GA-OPC, where the latter one

has 7-day and 28-day compressive strengths of 44.20 MPa and 59.32 MPa

respectively. The difference in 28-day compressive strength of above two

concrete mixes is as large as 17 MPa which is considered significant. Since

these two samples are made with the same cementitious material content and

similar water/cement ratios (only varied by 0.01), and designed with the same

workability, it is considered that the above strength difference is very likely

caused by the different qualities of RTGA and heated GA.

Table 8.5: Density and strength properties of Con-RTGA-OPC and Con-GA-OPC

Con-RTGA-OPC Con-GA-OPC


Density (AD) (28d) (kg/m3) 1630 1770

Density (SSD) (28d) (kg/m3) 1860 1900




From the strength results, it is estimated that RTGA has a lower mechanical

property than the heated GA, which is likely due to a weaker structure formed

without the enhancement of heat treatment (Bakharev, 2005a, Jiang and Roy,

1990). Although the strength of RTGA should have been improved by a

prolonged curing period at a low-moisture condition, it may still be not as good

as that of the heated GA which is cured at a very effective curing regime of


80oC for 3 days. This is in accordance with the previous finding in Chapter 4,

section 4.6, that a 84-day low-moisture room temperature-cured geopolymer

cubic sample results in an around 10 MPa lower compressive strength than a 7-

day geopolymer cubic sample that has been heated at 80oC for 3 days.

Therefore, although RTGA has a good quality to be suitable for making a

structural LWC product, it is still not as good as the heated GA which can

realise a high-strength LWC. Nevertheless, using RTGA instead of the heated

GA implies further low-cost and eco-friendly benefits in the manufacture of LWC

product. Also, the produced OPC-RTGA concrete can be used for the

applications that require a low-strength concrete. The exact advantages or

disadvantages of one type compared to the other depend largely on the

particular purposes and the specificities and economy of production.

8.4.2 Geopolymer Binder-based Concrete

Afterwards, a geopolymer binder-based concrete mix using both coarse and

fine RTGAs is made and researched, which is referred to as Geo-RTGA

concrete and coded as ‘Con-RTGA-Geo’. This is to evaluate the performance of

the RTGAs produced here in the fly ash-based geopolymer binder system.

The mix design of Con-RTGA-Geo is created based on one of the Con-GA-Geo

mixes introduced in Chapter 7 which is designed with a fly ash quantity of 446.1

kg/m3 and a geopolymer binder with SiO2/Na2O=1.25, H2O/Na2O=11 and

W/G=0.26. This mix design is obtained by using the same volumes of RTGAs to

replace the heated GAs in the mix design of Con-GA-Geo, as presented in

Table 8.6.

259

Table 8.6: Mix designs of Con-RTGA-Geo and Con-GA-Geo (SSD aggregates condition)

Con-RTGA-Geo Con-GA-Geo

Fly ash (kg/m3) 446.1 446.1

W/G value 0.26 0.26




Coarse RTGA (kg/m3) 631.3 652.5

Fine RTGA (kg/m3) 506.7 550.6

NaOH 16M (kg/m3) 51.8 51.8

Na2SiO3 (kg/m3) 182.7 182.7

Water (kg/m3) 5.4 5.4

Centrox MWR (kg/m3) 4.5 4.5

Centrox VMA (kg/m3) 4.5 4.5

Unit weight (kg/m3) 1832.9 1898.0

Aggregate mass proportion (%) 62% 63%

The preparation of concrete sample Con-RTGA-Geo follows the same

procedure as that of Con-GA-Geo illustrated in Chapter 7, subsections 7.3.1

and 7.4.1. Images showing the relevant steps in mixing Con-GA-Geo are

presented in Figure 8.7.


Figure 8.7: Steps in preparation of Con-RTGA-Geo (1) mixing, (2) casting, (3) after heating and demoulding and (4) after 28-day curing

The density and strength properties of the Con-RTGA-Geo samples are

measured using the same methods introduced in subsection 8.4.1, and shown

in Table 8.7. Con-RTGA-Geo has a fresh density of 1819 kg/m3, a 7-day air dry

density of 1580 kg/m3 and a 28-day air dry density of 1540 kg/m3. As expected,

by using lightweight RTGAs, this Con-RTGA-Geo sample end up in a type of

LWC (AS, 1998a). Meanwhile, the above density values of Con-RTGA-Geo are

lower than those of Con-GA-Geo, very possibly due to the lower specific gravity

of RTGA than the heated GA.

(1) (2)

(3) (4)

261

Table 8.7: Density and strength properties of Con-RTGA-Geo and Con-GA-Geo

Con-RTGA-Geo Con-GA-Geo


Density (AD) (7d) (kg/m3) 1580 1690

Density (AD) (28d) (kg/m3) 1540 1630




From Table 8.7, the hardened Con-RTGA-Geo sample has 7-day and 28-day

compressive strengths of 19.85 MPa and 23.60 MPa respectively. Although this

strength is not very good based on the common knowledge of concrete

materials, it is still slightly higher than 17 MPa and therefore qualified for

structural LWC applications (ASTM, 2009, ACI, 2003).

Otherwise, the strength values of Con-RTGA-Geo are much lower than those of

Con-GA-Geo. As the situation of OPC-based concretes discussed in subsection

8.4.1, it is estimated that this detected strength difference is due to the weaker

structure of RTGA than that of the heated GA. Moreover, the strength of Con-

RTGA-Geo is so low that may hinder the promotion of this type of Geo-RTGA

concrete. Further research will be needed to make a Geo-RTGA concrete with

an improved strength quality.

8.5 Conclusion

This chapter proposes a new type of artificial geopolymer aggregate. This one

is also manufactured based on the geopolymer technology but processed at a


room-temperature curing regime, and referred to as RTGA. The most dominant

difference of RTGA compared to the heated GA discussed in Chapters 5, 6 and

7 is that, there is no extra heat treatment used in the manufacture of RTGA.

In this chapter, it is aimed to produce a cold-bonded geopolymer aggregate

based on geopolymer technology, which can have sufficient properties required

for a concrete mix. This has been realised through the research reported in this

chapter, where a low-density RTGA sample is made and can be qualified for

producing structural LWCs, even though the generated LWCs only have

low/medium-strengths. Through the used methods described here, a room

temperature-cured geopolymer aggregate with further possible lower-cost and

eco-friendly benefits compared to the previous heated geopolymer aggregate

may be realised. However it must be emphasised that the latter type has a

better quality and can generate a high-strength LWC.

263

Chapter 9

Conclusions and Recommendations for Future

Research

A high-performance lightweight concrete (LWC) made with fly ash-based

lightweight aggregate (FA-LWA) is preferred because of its advanced physical,

mechanical and structural features. The promotion of FA-LWAs in LWC

manufacture is also advocated as a workable solution to problems of the

depletion of natural aggregates and recycling of large amounts of fly ash waste.

However, to ensure the quality of FA-LWAs, high-powered mechanical and

thermal treatments that consume large amounts of energy are normally required

for its manufacture which can significantly affect costs, resources and the

environment, thereby hindering the promotion of conventional FA-LWAs.

Therefore, it is of interest whether an alternative technology capable of

producing a high-quality FA-LWA, but under mild- or room-temperature

conditions to significantly reduce the amount of energy consumed during its

manufacture, is possible. Then, a new type of low-cost, eco-friendly FA-LWA

which removes the current financial and environmental burdens, and better

utilises the features of FA-LWA for a high-performance LWC in the future

concrete industry could be realised. These were the initial motivations for the

research discussed in this thesis.

The geopolymer technology adopted in this study to realise the low-cost, eco-

friendly manufacture of a FA-LWA, with the generated product called

‘geopolymer aggregate (GA)’, was introduced in Chapter 1. The advantage of

geopolymer technology is that it is capable of generating strong and durable

264 9. Conslusions and recommendations for future research

construction materials based on chemical synthesis under mild-temperature

(40-80oC) conditions. However, whether it is a suitable mechanism for making a

type of FA-LWA qualified for utilisation in construction was a promising

hypothesis worthy of the in-depth research. Another creative aspect of this

study has been how to synthesize coarse and fine GAs that are both qualified

for LWCs based on fly ash-derived geopolymer technology. A feasible

procedure for manufacturing the proposed GAs which aimed to obtain a high-

quality product was researched in this study.

Another subject addressed in this thesis was whether the GAs produced using

geopolymer technology could attain a sufficient quality for high-performance

LWC manufacture. Conventional high-quality FA-LWAs can result in LWC

products with advanced mechanical and structural properties. Therefore, it was

considered that only when the newly made GA achieved a comparable quality

to that of conventional FA-LWAs could it be promoted as an alternative with

low-cost and eco-friendly features. In this research, the quality of the GA was

evaluated according to relevant engineering standards and compared with other

commonly used FA-LWAs. Moreover, its performances in concrete and mortar

mixes, in both OPC- and geopolymer binder-based cementitious systems, were

examined.

The main findings of this thesis are summarised in sections 9.1 to 9.4, and

several recommendations for future research are presented in section 9.5.

265

9.1 Research on Geopolymer for Manufacture of Geopolymer

Aggregate

Firstly, as the GA proposed in this study was composed of a fly ash-based

geopolymer material, research was needed to produce one with features

required for a FA-LWA, as discussed in Chapters 2, 3 and 4.

The manufacture of GAs based on a fly ash-derived geopolymer technology

was investigated, with the fly ash a significant factor of concern as it is the main

aluminosilicate source for geopolymerisation. The author found that fly ash

samples from different batches could cause significant differences in the fresh

properties of their geopolymer products even though they were from the same

coal power station. This was considered to be due to the different early reaction

rates of fly ash samples caused by their slightly varied contents of amorphous

SiO2, Al2O3 and CaO. Therefore, it was necessary to strictly control the

consistency of the fly ash which was achieved by selecting one specific fly ash

sample and using it for all the geopolymer mixes. It was an ASTM F fly ash with

an air dry density of 2210 kg/m3, loss on ignition value of 0.97% and fine

particle size distribution, with 79.4% of its particles passing through a 45 µm

sieve. Furthermore, from an XRD test supplemented by a Rietveld quantification

process, it was composed of quartz (9.9%, wt.%), mullite (19.0%, wt.%) and an

amorphous content (71.1%, wt.%). Also, the fresh properties of geopolymers

affected by different fly ash samples were examined, so as to ensure that the

selected fly ash could result in a desired fresh geopolymer suitable for the

processing of GA manufacture.


The geopolymer synthesis in this study was undertaken by activating fly ash

with a combination of NaOH and Na2SiO3 solutions, with the final geopolymer’s

properties determined by the proportions of these ingredients. To control these

proportions and establish a systematic method for creating different geopolymer

mixes, the mix design procedure described in Chapter 3 was proposed. A

unique feature of this procedure was that, as it concentrated particularly on the

relationships among the chemical components involved in geopolymerisation, it

used the criteria of SiO2/Na2O, H2O/Na2O and W/G to distinguish between each

geopolymer mix and calculate the final mix design. In this way, it was able to

adopt previous knowledge of the effects of the chemical components for

geopolymer synthesis to help determine the geopolymer mixes needed for this

research. Then, a geopolymer mix with SiO2/Na2O=1.25, H2O/Na2O=11 and

W/G=0.24 was proposed as an optimum one for GA manufacture owing to its

high alkali and silicate, and low water contents that could enhance a GA’s

strength and structural development. Moreover, some other geopolymer mixes,

with SiO2/Na2O=1.25, H2O/Na2O=11-13 and W/G=0.22-0.26, were designed for

the geopolymer pastes, concretes and mortars investigated. The proportions of

these mixes’ ingredients were then confirmed based on the mix design

procedure in Chapter 3. Also, the factors conventionally related to a geopolymer

mix, such as the molarity of the NaOH and mass ratio of the Na2SiO3/NaOH

solutions could be obtained from the outcome of this procedure and applied to

guide the practical work.

As its curing regime is another significant factor that can influence the

development of a geopolymer, this was investigated and discussed in Chapter 4.

Firstly, the effect of a traditional heat curing regime was researched and found

267

to be better than a room-temperature one for strength and structural

development providing both methods had the same duration. As has been

reported, heat can help the development of a fly ash-based geopolymer (a

thermoset inorganic polymer) which requires high energy to activate the fly ash.

Heat curing at 80oC for 3 days, which resulted in the best strength outcome for

the geopolymer mixes tested, was selected and used for the manufacture of

GAs and geopolymer binders for concrete and mortar mixes.

This study also investigated the manufacture and development of a room

temperature-cured geopolymer, as discussed in Chapter 4. As it has generally

been agreed in recent years that heat treatment is not necessary for

geopolymerisation but is more akin to an acceleration method, the author

wondered whether a high-strength geopolymer made under room-temperature

conditions was achievable. If a geopolymer could achieve high strength with a

stable structure without heat, this would imply that it may be possible to realise

a cold-bonded GA with further low-cost, eco-friendly benefits than those of one

requiring a mild-temperature heat treatment. As the initial motivation of this

research was to lessen the amount of energy consumed in FA-LWA

manufacture, realising a cold-bonded GA was likely to be its ultimate aim. From

the study reported in Chapter 4, it was found that the strength of a room

temperature-cured geopolymer could be significantly improved with a low

moisture condition and prolonged curing period, and was able to obtain a

comparable strength to that of one heated for a short period. As a curing regime

operating at 20oC for 84 days with a low-moisture curing condition of 50% RH

enabled a cubic geopolymer sample to achieve a compressive strength of 44


MPa, which was considered sufficient for a LWA, this was applied to a room

temperature-cured geopolymer aggregate (RTGA), as discussed in Chapter 8.

9.2 Geopolymer Aggregate

Then, the synthesis of GAs, which aimed to successfully produce ones based

on a low-energy geopolymer technology, and enable them to maintain a

comparable quality to usual FA-LWAs, was studied (Chapter 5). This was

conducted through the feasible manufacturing procedure designed in this

research which could be used to make both coarse and fine aggregate samples,

and also be repeated for different designs.

Based on the outcomes of previous chapters, a geopolymer material was

produced with sufficient structure and strength quality to serve as the material

base for GA manufacture. For simplicity in the curing and crushing steps in this

procedure, the geopolymer material produced was cast into chunks with

dimensions of approximately 100mm×50mm×10mm. Also, to produce

aggregate particles of the desired sizes, a mechanical jaw crushing process

with a suitable crushing program was determined and applied, with both coarse

and fine GAs created. For this case, the costs, energy requirements and efforts

needed in the pelletisation process in traditional LWA manufacture were saved.

In addition, this jaw crushing was applied in order to create angular aggregate

particles which were considered beneficial for enhanced bonding with the

cement matrix in subsequent LWC manufacture. Compared with that for

conventional FA-LWAs, this manufacturing procedure was low-cost, simple and

eco-friendly, and meets the nature of geopolymer technology. This suggests a

feasible way to create GAs from fly ash based on the geopolymer technology.

269

The quality of the GA produced was assessed based on its physical and

mechanical properties to determine whether it could still achieve good quality

after undergoing the above low-energy manufacture. It displayed a lightweight

density in the range of 1660 to 1770 kg/m3 under an oven dry condition which

indicated that it was a type of FA-LWA. Also, it was of an adequately good

physical quality to satisfy relevant engineering standards and comparable with

that of conventional FA-LWAs. In addition, a standard crushing value test

indicated that it had a crushing value of 23.4%, higher than those of some

conventional FA-LWAs, which implied that it had good strength for resisting

external pressure. Also, based on a rebound test using the Schmidt hammer, it

was found that this GA had good hardness with a rebound index of 38.6 (kPa),

which is similar to those of some types of natural rocks used as aggregates in

concrete. These findings indicated that it had mechanical properties sufficiently

suitable for high-performance LWC manufacture. This meant that it could be

possible to produce a FA-LWA based on geopolymer technology with the

desired density and strength qualities but manufactured with low energy

consumption.

On the other hand, as the above density and strength performances were most

likely determined by the microstructure of the GA, as this was considered an

important aspect, it was investigated. Using a Lynx stereo inspection

microscope, it was observed that the GA had many porous craters on its

surface which were likely to generate a high number of penetrable pores, as

reflected by its detected high water absorption capacity (11.0%-14.0%). These

findings led to further research on the internal structure of the GA conducted

using an optical microscope with the assistance of image-processing software.


As well as its porous surface, the GA was found to have a porous internal

microstructure with a large range of pores of different shapes and sizes which

was thought to be the main reason for its lightweight property. The pores

detected were micron-sized in a range of 1 µm to 250 µm, with most less than

50 µm and only 0.1% larger than 100 µm, which is beneficial for realising a

strong structure for LWAs and could possibly explain the good mechanical

properties of GAs measured. What needs to be highlighted is that this porous

microstructure was formed mainly due to the process of geopolymerisation, as

supported by the results obtained from infrared (IR) spectroscopy which

identified that the GA’s structure contained newly formed amorphous Si-O-Si

and Si-O-Al bonds that should belong to geopolymer molecules. Therefore,

geopolymerisation was considered a new mechanism for creating an advanced

porosity system consisting of homogeneously distributed small pores, as

preferred in lightweight materials.

9.3 OPC-based Concrete and Mortar using Geopolymer

Aggregate

This study then examined whether this newly made GA could qualify for the

manufacture of high-performance LWC. As OPC is the current mainstream

cementitious system widely applied throughout the world, this research firstly

concentrated on an OPC-based concrete using GAs (OPC-GA concrete,

Chapter 6). The results showed that such a concrete had a lightweight property

with a 28-day air dry density of 1750-1770 kg/m3 and high 28-day compressive

strength of 57-60 MPa which indicated that this type of GA had a sufficiently

high quality to generate a high-strength LWC. Moreover, the OPC-GA concrete

271

mix produced, achieved a high strength/mass ratio of 0.03 (MPa·m3/kg) which

was higher than that of a normal concrete made with the same cementitious

material content and same workability but normal weight crushed stone and

sand. More importantly, both the coarse and fine aggregate portions of this

high-strength LWC were from GAs which the author considers a breakthrough

in the area of geopolymer-assisted LWA manufacture.

It was estimated that the high-strength of the lightweight OPC-GA concrete

should have come from two main aspects related to GA’s application, its self-

strength and strength of bonding with the cement matrix. The first was proven

by its good mechanical properties based on the results from crushing value and

rebound tests. To assess the second, a study of the interfacial zone of the

OPC-GA concrete’s fracture after compressive strength testing was conducted.

It was observed that the GAs were embedded in the cement matrix with a

complete and not collapsed interfacial zone. This indicated that the bonding of

GAs with the cement matrix had a certain strength for resisting external

pressure which was very likely due to their enhanced interlocking due to their

unique shape and porous surfaces.

On the other hand, as the fine GA produced in this research was found to be

qualified to serve as an aggregate portion instead of natural sand in a high-

strength LWC mix, the author undertook further research that focused on only

the performance of fine GAs. This was achieved by assessing the properties of

a mortar material using fine GAs as its sole aggregate portion (referred to as

OPC-GA mortar). The results showed that this OPC-GA mortar had a

lightweight property with a 28-day air dry density of 1750 kg/m3 and high 28-day

compressive strength of 58.86 MPa. It had a similar strength to a mortar made


from natural sand provided both had the same cementitious material content.

Based on this, fine GAs seem to be practical for use as a new type of eco-

friendly artificial sand capable of replacing natural sand in a mortar mix without

any strength loss.

9.4 Geopolymer Binder-based Concrete and Mortar using

Geopolymer Aggregate

Apart from the study of an OPC-GA concrete, discussed in Chapter 7, using

GAs in a geopolymer binder-based concrete (referred to as Geo-GA concrete)

was investigated. This was because, in recent decades, using a geopolymer

binder as a new green cementitious material instead of OPC in the concrete

industry has been recommended. Also, this study explored the possibility of

producing a LWC material made from a 100% fly ash-based geopolymer

material which could be considered a new type of eco-friendly concrete

because it used both aggregates and cementitious materials from industrial by-

products.

Firstly, research on the mix design of the geopolymer binder in Geo-GA

concrete was conducted based on the research work discussed in Chapter 3, to

determine workable geopolymer binders for the desired Geo-GA concrete mixes.

Also, for the better strength development of these binders, a powerful heat

curing regime of 80oC for 3 days was applied. As each of the Geo-GA concretes

produced had a lightweight property (an air dry density of 1630-1640 kg/m3) and

medium-strength (a 28-day compressive strength of 40-46 MPa), they were

suitable for structural LWC applications. Therefore, a practical ‘absolute

geopolymer LWC’ made from 100% geopolymer materials was obtained.

273

Nevertheless, the application of GAs in a geopolymer binder system could

significantly weaken the subsequent Geo-GA concretes, the strengths of which

were 20 MPa lower than those of ones using natural aggregates with the same

geopolymer binders and curing regimes. Moreover, their workability which

seemed to be necessary to obtain better strength outcomes for the samples

used in this research, was rather stiffer in the Geo-GA mixes than the

workability of the usual OPC-based ones. Based on the test results, it was

concluded that the performance of GAs in a geopolymer binder-based concrete

was not as good as that in an OPC-based one.

Later, research on a geopolymer binder-based mortar mix was investigated.

The mortar samples were made from either natural sand or fine GAs, with the

same binder, SiO2/Na2O=1.25, H2O/Na2O=11 and W/G=0.26, and same curing

regime, 80oC for 3 days. The mortar using natural sand resulted in a high 28-

day compressive strength value of 68.5 MPa which indicated that the

geopolymer mix and curing regime used were sufficient to realise a strong

geopolymer binder. The mortar using fine GAs also resulted in a high strength

with a 28-day compressive strength of 57.6 MPa and rather light weight with a

28-day air dry density of 1650 kg/m3. Apart from the strong geopolymer binder

applied, the fine GAs seemed to perform better when used alone in a mortar

mix than in combination with coarse ones. Again, this proved that the fine GAs

produced were of sufficiently high quality for use as an alternative kind of

artificial sand in future lightweight construction. Moreover, it suggested that fine

GAs may be more suitable than coarse ones for a geopolymer binder system.


9.5 Room Temperature-cured Geopolymer Aggregate (RTGA)

Finally, this study researched RTGAs and their applications in concretes

(Chapter 8). A RTGA is a special type of GA made under room-temperature

conditions (20oC), a type of cold-bonded FA-LWA with no extra heat treatment

applied. The idea of using a RTGA came from the promising strength results for

the room temperature-cured geopolymer samples investigated in Chapter 4 and

aimed to further reduce the energy consumption and simplify the manufacture

of GAs by eliminating the heating step. Based on this, a modified manufacturing

procedure which depended on a prolonged curing period (84 days) plus a low-

moisture curing condition (50% RH), as suggested by the research in Chapter 4,

was developed.

The RTGAs produced, ranged in density between 1590 and 1690 kg/m3 at oven

dry condition which indicated that they belonged in the class of FA-LWAs and

had a high water absorption capacity (9.0%-10.0%). Their physical properties

were similar to those of the heated GAs but with slightly lower values. Their

performances were directly evaluated based on their concrete products which

were found to be qualified structural LWCs but with much lower compressive

strengths (approximately 15-20 MPa) than those using heated GAs. This meant

that, although RTGAs must have already been improved by their prolonged

curing periods under low-moisture conditions, they were still not as good as the

GA that underwent heat curing at 80oC for 3 days. Therefore, a heated GA still

has better quality and is the primary one for generating a high-strength LWC

while a RTGA has its own benefits of being low-cost and eco-friendly, and is

suitable for low- or medium-strength LWC manufacture.

275

9.6 Recommendations for Future Research

This research investigated a low-cost, eco-friendly geopolymer aggregate (GA)

and its application in OPC-based and geopolymer binder-based lightweight

concretes and mortars. From the in-depth findings and discussions presented in

this thesis, the following issues need to be further explored in future work.

1) – the porous structure of a GA needs further research because its formation

is relevant to the geopolymerisation process, and also determines the density,

absorption and strength of a GA sample produced. In the microscopic results

obtained, it was detected that there was a large scale of micron-sized pores

present in both the surface layer and internal structure of a GA which were

thought to be a result of geopolymerisation. However, further clarification of the

mechanism of pore formation by geopolymerisation and, moreover, a technique

for controlling the porosity of a geopolymer structure to produce an acceptable

GA for the aggregate industry is required. Although several achievements of

other researchers have already revealed some information about the porosity of

a geopolymer material, further research is needed to correlate it with the quality

of a GA.

It is also recommended that a further study based on X-ray micro-tomography

(micro-CT) technology be conducted on the pore structure and tortuosity of GAs

produced in order to obtain more information on the development of porosity

under various synthetic conditions.

Moreover, further research for dealing with the high proportion of penetrable

pores in a GA, as reflected by its high water absorption capacity (around 10%

based on a standard test), is required because one that is too high may affect


the durability of a concrete under severe conditions (such as freezing and

thawing). The GA proposed in this research was already the one with the lowest

water absorption capacity among all designs based on the current method. In

future, improvements are needed to modify the porosity of the structures of

generated GAs so as to block their penetrable pores and prevent their

connection with the atmosphere.

A rigorous study of porosity would also reveal the different porous structures of

heated GAs and RTGAs which, it is anticipated, would better explain the

variations in water absorption capacities detected in these two aggregate

products. Also, the total porosity and proportions of penetrable and

impenetrable pores of each of these aggregates should be investigated.

2) – research is needed to explain whether a heat treatment applied on a

geopolymer that has already been hardened by heating could lead to any

further reaction and modification of its structure. As the GA considered in this

research was produced from an incomplete geopolymerisation process of fly

ash, theoretically, further heating could lead to a change in its structure which

could change its strength and porosity. This issue is important since it

determines the method of drying required for the manufacture and testing of

GAs. It is also important when it comes to assessing concretes using GA in fire

situations.

3) – further testing of the properties of concretes and mortars made with GAs is

needed. The scope of this study was limited to concentrating on only the

fundamental properties of these concretes and mortar mixes, such as

workability, fresh and hardened density, compressive strength and fracture

277

surface. Further evaluations of other properties, including tensile strength,

elasticity, shrinkage, and the durability when exposed to aggressive internal

and/or external agents, could demonstrate the benefits and/or shortcomings of

using GAs in concretes and mortars.

4) – the author considers it reasonable to propose a further investigation of the

interfacial zone between GAs and the cement matrix of a concrete. Due to the

unique nature of GAs, it is likely that a special interfacial zone in a concrete

using them would be seen which could explain their density and strength

performances.

Moreover, it is considered necessary to launch an in-depth investigation of the

interfacial zone of a ‘GA-geopolymer binder matrix’ to determine whether a

reaction could happen between these two geopolymers produced using

different manufacturing processes. This would establish whether a GA could

react during the mix of a geopolymer binder-based concrete. And if it does, how

would the reaction be and how it would affect the concrete’s performance.

A scanning electron microscope study of the interfacial zone would also be

required, to better determine the nature and composition, and physical-chemical

interaction, between the GAs and either OPC or geopolymer cementitious

matrix.

5) – The phenomenon of the specific strength gain detected in the Geo-GA

mixes in this research needs to be explained by further investigations. It

demonstrated that Geo-GA mixes (both concretes and mortars) could achieve

high strength gains of around 15-20 MPa during a period of 7-28 days after

mixing with no extra heat treatment, which were much higher than those


achieved by the mixes using natural aggregates. This may be better explained

by more specific monitoring of the strength development of these Geo-GA

samples. Also, it is considered that research on the changes in the chemical

and physical structures of these mixes that occur during 7-28 days after mixing

is essential.

6) - research is still required to complete the study of a RTGA based on the

initial results reported in this thesis and it is expected that a modified

manufacturing method would improve its quality. Although the RTGA made in

this study could qualify for structural LWC manufacture, the LWCs produced

had only low to medium strengths whereas the manufacture of high-strength

LWCs based on RTGAs could be a challenge but promising work in future

research.

7) – Future researches are also needed to study the mineralogical and

microstructural changes taking place during the formation process of

geopolymers. The formation of geopolymer gels (like N-A-S-H) and the

performance/change of crystalline phases during a fly ash-based

geopolymerisation process should be more deeply investigated in addition to

the XRD and FTIR performed in the work of this thesis. Such studies can be

correlated with the mechanical and physical properties of GAs analysed and

discussed in this thesis.

279

Appendix A

Calculation on Crystalline and Amorphous Contents in

Fly Ash based on Rietveld Quantification Data

As introduced in Chapter 2, subsection 2.2.5, the crystalline and amorphous

contents in fly ash are calculated based on the Rietveld quantification data. The

specific calculation steps are presented in this Appendix.

From Chapter 2 it is known that, a test sample composed of 80% of fly ash and

20% of corundum (α-Al2O3, 99.99% purity) is scanned using an X-ray

diffractometer. Using a certain software called FullProf suite (Rodríguez-

Carvajal, 2001), a Rietveld quantification process is carried out based on the

obtained X-ray diffraction patterns to quantify the mass proportions of the

detected crystals in this test sample (Diaz et al., 2010, Ward and French, 2006).

The Rietveld primary quantification results of the three test samples

corresponding to the three selected fly ash batches discussed in Chapter 2, are

shown in Table A.1.

Table A.1: Rietveld quantification preliminary results (which served as the data for the consequent calculations) of the tested samples

Item TS1 (wt.%) TS2 (wt.%) TS3 (wt.%)

Corundum 46.0 52.5 53.7

Quartz 18.2 12.6 14.0

Mullite 35.0 34.9 32.2 Note: TS1, TS2 and TS3 refer to the samples containing the fly ash FA1, FA2 and FA3 respectively. Each of these samples was constructed such that it contains 20% corundum and 80% fly ash. Besides, due to the probable presence of error, the summation of these mass proportions may not be 100%.

280 A. Calculation on crystalline and amorphous contents in fly ash based on Rietveld quantification data

These primary results obtained by the application of the Rietvield process are

now used to calculate the proportions of the crystalline and amorphous contents.

The calculations process is explained in the following paragraphs.

From Table A.1, three types of crystals as corundum, quartz and mullite are

distinctively detected in the X-ray diffraction patterns. Among them, quartz and

mullite are obviously derived from the original fly ash, while corundum was not

present in the fly ash as it was intentionally added. Based on the Rietveld

quantificaiton results shown in Table A.1, the proportions of crystalline and

amorphous contents in fly ash can be calculated, as illustrated by the following

example:

As chosen, in the test sample TS1, 80% in mass is fly ash FA1 and 20% in

mass is corundum. Meanwhile, the contents of quartz and mullite are totally

from the fly ash.

The mass proportions of quartz and mullite in fly ash are calculated by dividing

their masses by the mass of fly ash, as shown below:

mass proportion (quartz) =mass (quartz)mass (fly ash)

mass proportion (mullite) =mass (mullite)mass (fly ash)

Since fly ash occupies 80% in mass of the test sample, the above equations

can be trasferred into the following ones:

mass proportion (quartz) =mass (quartz)

80% ∗ mass (test sample)

281

mass proportion (mullite) =mass (mullite)


On the other hand, in the test sample TS1, there are 46.0% of corundum, 18.2%

of quartz and 35.0% of mullite, as seen from Table A.1. From this, the ratio of

quartz to corundum and the ratio of mullite to corundum, can be known as

shown below:

mass(quartz)mass(corundum)

=18.2%46.0%

mass(mullite)mass(corundum)

=35.0%46.0%

In this example the 46.0% of corundum occupy 20% in mass of the test sample

TS1. From this, we can get the following equaitons:

mass(quartz)mass(corundum)

=mass(quartz)

20% ∗ mass(test sample)=

18.2%46.0%

mass(mullite)mass(corundum)

=mass(mullite)

20% ∗ mass(test sample)=

35.0%46.0%

Therefore,

mass(quartz) = 20% ∗ mass(test sample) ∗18.2%46.0%

mass(mullite) = 20% ∗ mass(test sample) ∗35.0%46.0%

Hence, the mass proportions of quartz and mullite in the fly ash (of FA1) can be

calcuated as follows:

282 A. Calculation on crystalline and amorphous contents in fly ash based on Rietveld quantification data

mass proportion (quartz) =mass (quartz)


=20% ∗ mass(test sample) ∗ 18.2%

46.0%80% ∗ mass (test sample)

=20% ∗ 18.2%

46.0%80%

= 9.9%

mass proportion (mullite) =mass (mullite)


=20% ∗ mass(test sample) ∗ 35.0%

46.0%80% ∗ mass (test sample) =

20% ∗ 35.0%46.0%

80%= 19.0%

After this, the mass proportion of the amorphous content in the fly ash (of FA1)

can be calucated as:

100% − 9.9% − 19.0% = 71.1%

Hence, it is calculated that the fly ash FA1 is composed of 9.9% of quartz, 19.0%

of mullite and 71.1% of amorphous content. Following the former calculation

steps, the crystalline and amorphous contents of fly ashes FA2 and FA3 can

also be obtained, as shown in Table A.2. These results are later used for this

research work as discussed in Chapter 2.

Table A.2: Quantification of crystalline and amorphous contents in the three fly ash batches

Item FA1 (wt.%) FA2 (wt.%) FA3 (wt.%)

Quartz 9.9 6.0 6.5

Mullite 19.0 16.6 15.0

Amorphous 71.1 77.4 78.5

283

Appendix B

Design of Geopolymer Binder for Geopolymer Binder-

based Concretes and Mortars

In Chapter 7, several geopolymer binder-based concretes and mortars are

designed for research purposes. The design of the geopolymer binder (matrix)

in these concretes and mortars is detailed in this Appendix, which is completed

based on the mix design procedure and results reported in Chapter 3.

B.1 Geo-GA concrete

In Chapter 7, subsection 7.2.2, a Geo-GA concrete is designed with the same

unit weight and aggregate mass proportion as an OPC-GA concrete proposed

in Chapter 6, as shown in Table B.1. Afterwards, the 650.7 kg/m3 binding

material in this Geo-GA concrete is required to be designed.

Table B.1: Ingredient proportions of mix designs of OPC-GA and Geo-GA concretes (SSD aggregates condition)

Ingredient OPC-GA Geo-GA

(kg/m3) (kg/m3)


Coarse GA 686.2 686.2

Fine GA 587.5 587.5



284 B. Design of geopolymer binder for geopolymer binder-based Concretes and mortars

As introduced in Chapter 7, this 650.7 kg/m3 binding material is composed of a

neat geopolymer mix to serve as the binder, and also two chemical admixtures.

The design of the ingredients used to compose this binding material is

explained based on the following example:

Suppose that a geopolymer mix with SiO2/Na2O=1.25, H2O/Na2O=11 and

W/G=0.24 is selected to be the binder for this Geo-GA concrete. From the mix

design results in Chapter 3, section 3.7, for 1 m3 of such geopolymer mix, it

requires 1283.42 kg/m3 of fly ash, 135.00 kg/m3 of a 16M NaOH solution,

475.88 kg/m3 of a Na2SiO3 solution and 14.03 kg/m3 of deionised water.

Suppose the amount of each chemical admixture in the mix design is a function

of ‘𝑎𝑎’. Since each chemical admixture is added in a dosage of 1% of the mass

of fly ash, it is able to define the amount of fly ash which is:

a1%

= 100a

Then, the amounts of other ingredients can be defined, based on their mass

relations with the fly ash in a 1 m3 geopolymer mix, which are:

16M NaOH solution:

100a ∗135.00

1283.42

Na2SiO3 solution:

100a ∗475.88

1283.42

Deionised water:

285

100a ∗14.03

1283.42

As this 650.7 kg/m3 binding material is composed of fly ash, 16M NaOH solution,

Na2SiO3 solution, deionised water, and two chemical admixtures, the following

equation is obtained:

100a + 100a ∗135.00

1283.42+ 100a ∗

475.881283.42

+ 100a ∗14.03

1283.42+ a + a

= 650.7 kg/m3

The value of ‘𝑎𝑎’ is solved to be:

a = 4.316 kg/m3

This value is the amount of the chemical admixture used in the mix design.

From this, the amount of fly ash is:

4.316 ∗ 100 = 431.6 kg/m3

Then, the amounts of other ingredients are:

16M NaOH solution:

4.316 ∗ 100 ∗135.00

1283.42= 45.5 kg/m3

Na2SiO3 solution:

4.316 ∗ 100 ∗475.88

1283.42= 160.3 kg/m3

Deionised water:

4.316 ∗ 100 ∗14.03

1283.42= 4.7 kg/m3


Hence, for the 650.7 kg/m3 binding material, it requires 431.6 kg/m3 of fly ash,

45.5 kg/m3 of 16M NaOH solution, 160.3 kg/m3 of Na2SiO3 solution, 4.7 kg/m3 of

deionised water, 4.3 kg/m3 of Centrox MWR and 4.3 kg/m3 of Centrox VMA.

After this, considering that this designed geopolymer binder could have a

different volume compared to that of the OPC matrix in Table B.1, a

modification is made on the aggregate proportions to make the final mix design

fit 1 m3 Geo-GA concrete mix.

From known information, the densities of the ingredients used to compose the

binding material of Geo-GA concrete are presented in Table B.2.

Table B.2: Densities of ingredients for binding material of Geo-GA concrete

Item Density (kg/m3)

Fly ash 2210

16M NaOH 1465

Na2SiO3 solution 1520

Deionised water 1000

Centrox MWR 1030

Centrox VMA 1030

The volume of the binding material is considered equal to the summation of the

volume of each ingredient present in Table B.2, based on the absolute volume

principle, which is:

431.62210

+45.51465

+160.31520

+4.7

1000+

4.31030

+4.3

1030= 0.345 m3

On the other hand, from known information, the SSD densities of coarse and

fine geopolymer aggregates are 1840 kg/m3 and 2010 kg/m3 respectively.

287

Therefore, if the aggregate proportions are still the original ones regulated in

Table B.1, the volumes of coarse and fine geopolymer aggregates are:

Coarse geopolymer aggregate:

686.21840

= 0.373 m3

Fine geopolymer aggregate:

587.52010

= 0.292 m3

For this case, the volume of total aggregates is:

0.373 + 0.292 = 0.665 m3

Nevertheless, if the design aims to make a 1 m3 mix containing 0.345 m3

binding material, the volume left for the aggregates should be:

1 − 0.345 = 0.655 m3

From this we can see, the volume of aggregates is now 0.01 m3 more than the

ideal volume they should occupy, provided that the final mix is 1 m3. So, a slight

modification is made on the aggregates, by reducing this 0.01 m3 volume from

them.

Otherwise, the volumetric proportions of coarse and fine aggregates in the total

aggregates are not changed.

The volumetric proportion of coarse aggregates in total aggregates is:

0.3730.665

= 56.1%


The volumetric proportion of fine aggregates in total aggregates is:

0.2920.665

= 43.9%

So, the new volumes of geopolymer aggregates are calculated as:

Coarse geopolymer aggregates:

0.655 × 56.1% = 0.368 m3

Fine geopolymer aggregates:

0.655 × 43.9% = 0.287 m3

Then, the amounts of coarse and fine geopolymer aggregates therefore, are:

Coarse geopolymer aggregates:

0.368 ∗ 1840 = 677.0 kg/m3

Fine geopolymer aggregates:

0.287 ∗ 2010 = 577.3 kg/m3

Therefore, the final mix design for this Geo-GA concrete example is made and

presented in Table B.3.

It needs to be noticed that, this mix design proposed here is based on the

theoretical calculations, and a further modification with trial tests is normally

required to confirm that the proposed mix design is actually for 1 m3 concrete

mix. Meanwhile, the effects of mixing factors, like the air content, should also be

taken into consideration during this modification. Nevertheless, the above mix

design procedure is only applied as a workable way for designing binders in

289

Geo-GA concretes included in current research. Different Geo-GA mixes can be

created by selecting different SiO2/Na2O, H2O/Na2O and W/G ratios. Moreover,

users may also design the unit weight and aggregate proportion according to

their needs or experiences, instead of based on a known OPC-GA concrete

every time. Three Geo-GA concrete mixes required for this research are

subsequently made and applied in Chapter 7, section 7.4.

Table B.3: Final mix design of Geo-GA example (SSD aggregates condition)

Fly ash (kg/m3) 431.6

W/G value 0.24




Coarse GA(kg/m3) 677.0

Fine GA (kg/m3) 577.3

NaOH 16M (kg/m3) 45.5

Na2SiO3 (kg/m3) 160.3

Water (kg/m3) 4.7

Centrox MWR (kg/m3) 4.3

Centrox VMA (kg/m3) 4.3

Unit weight (kg/m3) 1924.4

Aggregate mass proportion (%) 66%


B.2 Geopolymer Concrete (GC)

The geopolymer concrete (GC) mix required in Chapter 7, section 7.3 is

designed based on the Geo-GA concrete, by using the same volumes of natural

aggregates, namely crushed stone and river sand, to replace the geopolymer

aggregates in the mix design of Geo-GA concrete. This is explained based on

the previous Geo-GA example (Table B.3).

From the above example, it is known that the volumes of coarse and fine

geopolymer aggregates in the mix design are 0.368 m3 and 0.287 m3

respectively.

From known information, the SSD densities of crushed stone and river sand are

2700 kg/m3 and 2600 kg/m3 respectively.

As the crushed stone and river sand in the GC mix have the same volumes as

the coarse and fine geopolymer aggregates in the Geo-GA mix, their mass

proportions can thus be calculated, which are:

Crushed stone:

0.368 ∗ 2700 = 993.4 kg/m3

River sand:

0.287 ∗ 2600 = 746.7 kg/m3

Hence, the mix design for the GC mix is obtained and presented in Table B.4.

Following the same procedure discussed in this section, the mix designs for

other GC mixes required in Chapter 7, section 7.3 can also be created.

291

Table B.4: Final mix design of GC example (SSD aggregates condition)


W/G value 0.24




Crushed stone (kg/m3) 993.4

River sand (kg/m3) 746.7

NaOH 16M (kg/m3) 45.5

Na2SiO3 (kg/m3) 160.3

Water (kg/m3) 4.7

Centrox MWR (kg/m3) 4.3

Centrox VMA (kg/m3) 4.3

Unit weight (kg/m3) 2390.8

Aggregate mass proportion (%) 73%

B.3 Geopolymer Binder-based Mortar

In this section, the design of geopolymer binder for the mortar mixes is

introduced. At first, the mortar using fine geopolymer aggregates (referred to as

Geo-GA mortar) is discussed. As said in Chapter 7, section 7.6, Geo-GA mortar

is made based on an OPC-GA mortar by sharing the same binding material

content, as shown in Table B.5.


Table B.5: Ingredient proportions of mix designs of OPC-GA and Geo-GA mortars (SSD aggregates condition)

Ingredient OPC-GA Geo-GA

(kg/m3) (kg/m3)


Fine GA 1116.8 1116.8



These mixes are different from the Geo-GA concrete discussed in section B.1,

in that the 835.2 kg/m3 binding material of this Geo-GA mortar is a neat

geopolymer binder without any chemical admixture. For this reason, the design

of this binding material can be directly made from the results of Chapter 3,

section 3.7, as shown below:

Suppose that a geopolymer binder with SiO2/Na2O=1.25, H2O/Na2O=11 and

W/G=0.22 is selected for this Geo-GA mortar. From Chapter 3, it is known that

this binder has a fly ash/activator (F/A) ratio of 2.28.

Therefore, the amount of fly ash in the 835.2 kg/m3 binding material is:

835.2 ∗2.28

2.28 + 1= 580.5 kg/m3

From Chapter 3, section 3.7, for 1 m3 of the selected geopolymer binder, it

requires 1338.98 kg/m3 of fly ash, 127.01 kg/m3 of 16M NaOH solution, 447.68

kg/m3 of Na2SiO3 solution and 13.20 kg/m3 of deionised water. As the mass

proportions among the above ingredients are not changed when using this

293

geopolymer as a binder in the Geo-GA mortar mix, the amounts of the other

three ingredients can be calculated based on the known amount of fly ash,

which are:

16M NaOH solution:

580.5 ∗127.01

1338.98= 55.1 kg/m3

Na2SiO3 solution:

580.5 ∗447.68

1338.98= 194.1 kg/m3

Deionised water:

580.5 ∗13.20

1338.98= 5.7 kg/m3

Also, to make the final mix design fit 1 m3 mortar mix, the mass proportion of

fine geopolymer aggregates must be modified, as what has been discussed for

a Geo-GA concrete mix design in section B.1. However, since a mortar only

contains one type of aggregate portion, this modification is not complicated.

To fulfil this, the volume of the binding material is firstly calculated to be:

580.52210

+55.11465

+194.11520

+5.7

1000= 0.434 m3

So the volume of fine geopolymer aggregates is:

1 − 0.434 = 0.566 m3

And then, the mass of fine geopolymer aggregates in the final mix design can

be directly calculated as:


0.566 ∗ 2010 = 1137.6 kg/m3

Therefore, the final mix design of this Geo-GA mortar example is obtained and

presented in Table B.6.

Table B.6: Final mix design of Geo-GA mortar example (SSD aggregates condition)

Mortar type Geo-GA


W/G ratio 0.22





NaOH 16M (kg/m3) 55.1

Na2SiO3 (kg/m3) 194.4

Water (kg/m3) 5.7

Moreover, it is possible to specify the quantity of one or more items, and use the

above steps to obtain a new mortar mix. The Geo-GA mortar mix discussed in

Chapter 7, section 7.6 is made by specifying the fly ash quantity, as introduced

below:

It is required that this Geo-GA mortar still uses 580.5 kg/m3 of fly ash as the last

example, but the geopolymer mix with SiO2/Na2O=1.25, H2O/Na2O=11 and

W/G=0.26 are now the required proportions.

295

From Chapter 3, section 3.7, it is known that for 1 m3 of the geopolymer mix

with SiO2/Na2O=1.25, H2O/Na2O=11 and W/G=0.26, it requires 1230.98 kg/m3

of fly ash, 142.65 kg/m3 of 16M NaOH solution, 502.82 kg/m3 of Na2SiO3

solution and 14.83 kg/m3 of deionised water.

Following the calculation steps of the last example, the amounts of other

ingredients can be calculated based on the amount of fly ash, which are:

16M NaOH solution:

580.5 ∗142.65

1230.98= 67.4 kg/m3

Na2SiO3 solution:

580.5 ∗502.82

1230.98= 237.7 kg/m3

Deionised water:

580.5 ∗14.83

1230.98= 7.0 kg/m3

Then, the volume of the binding material is:

580.52210

+67.41465

+237.71520

+7.0

1000= 0.472 m3

The mass proportion of fine geopolymer aggregates is thus calculated as:

(1 − 0.472) ∗ 2010 = 1061.1 kg/m3

Hence, the final mix design of the Geo-GA mortar required in Chapter 7, section

7.6 can be obtained, as shown in Table B.7.


Table B.7: Mix design of Geo-GA mortar

Mortar type Geo-GA


W/G ratio 0.26





NaOH 16M (kg/m3) 67.4

Na2SiO3 (kg/m3) 237.7

Water (kg/m3) 7.0

Table B.8: Mix design of geopolymer mortar

Mortar type Geopolymer mortar


W/G ratio 0.26




River sand (kg/m3) 1372.5

NaOH 16M (kg/m3) 67.4

Na2SiO3 (kg/m3) 237.7

Water (kg/m3) 7.0

297

Similar to the mix design procedure for GC mix, the mix design of geopolymer

mortar using natural sand can be made based on the Geo-GA mortar in Table

B.7, by using the same volume of natural sand to replace the fine geopolymer

aggregates, as shown in Table B.8.

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