The Relationship between Water Absorption and Porosity for

Geopolymer Paste

Z. F. Farhana1, a, H. Kamarudin1, Azmi Rahmat1, and A. M. Mustafa Al Bakri1

1Center of Excellence Geopolymer and Green Technology (CEGeoGTech), School of Materials

Engineering, Universiti Malaysia Perlis (UniMAP), P.O. Box 77, D/A Pejabat Pos Besar, 01000

Kangar, Perlis, Malaysia

aEmail: farahfarhanazainal@yahoo.com

Keywords: Water absorption, porosity, geopolymer, fly ash

Abstract: This paper presents an experimental study of the relationship between water absorption

and porosity for geopolymer paste. In order to reduce the carbon dioxide (CO2) emissions to the

environment, alternatively fly ash was used as binders in making concrete paste. Fly ash is a waste

materials produced by combustion of coal at power plant. Geopolymer paste prepared from class F

fly ash was obtained from coal power plant and mixed with an alkaline activator. The combination

of sodium silicate (Na2SiO3) solution and sodium hydroxide (NaOH) solution were used as alkaline

activator. The alkaline activator was prepared 24 hrs prior to use with the ratio of the mixture of

Na2SiO3/NaOH is 2.5. Mixed fly ash and alkaline activator placed in moulds and compacted. The

samples were kept at ambient temperature in the moulds until it hardened. All the samples were

removed from the moulds after 24 hrs. Then, the samples were cured at 60˚C in the oven for 24 hrs.

All twelve samples were prepared for water absorption and porosity measurement. The samples

were examined after 7, 14, 28 and 90 days in terms of water absorption test and porosity test. It was

observed that after day 90 the sample had the lowest water absorption of 3.81% and porosity 3.77%.

The sample had the highest water absorption and porosity of 4.65% and 11.95% respectively at day

7. The pore size decreases and the structures became denser. The morphological structure of

geopolymer paste pores can be observed by Scanning Electron Microscope (SEM). The porosity

and permeability also decreased hence the durability potentially improved.

1. Introduction

Over the last few decades, there was some trend in thinking that a durable concrete is a

hardened concrete. However, the durability and strength of concrete have to be established at the

design stage. The improper use of concrete in the environment wrongly intended for will result in

premature failure [1]. In most cases, the durability of concrete is related to its permeability to fluids,

the porosity of the aggregates and hardened cement paste and the quality of aggregate/cement paste

interface. The porosity is the proportion of the total volume occupied by pores and is usually stated

in percent. If the pores are interconnected, then the permeability tends to be high and the porosity is

high, vice versa if the porosity is low and the pores are disconnected, then the permeability is

relatively low [2].

Neville [1] also stated that there are three kinds of fluids that affect the durability. They are

water (pure or carrying deleterious ions), carbon dioxide and oxygen. The fluids can penetrate

through the pores into the concrete and cause its deterioration. On the other hand, Jayeshkumar et.

al [3] described that concrete is a porous material which interacts with the surrounding

environment. The movement of water through concrete structure is the main factor that will affect

the durability of mortar and concrete. Due to this problem, the water absorption test is done to

measure the absorb water in the concrete. Water absorption indicates the degree of porosity of a

material by determining the percentage of water absorbed under prescribed conditions. The water

absorption test was carried out by immersing the samples in water for 4 days at 25˚C and the

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percentage water absorbed was measured. The experiment was carried out after 7 days and after 28

days from initial curing [4].

Since decades ago, the fields of geopolymer composites have attracted some attention. The

use of waste materials such as fly ash and blast furnace slag to produce of geopolymers is

increasing. The manufacture of geopolymer composite consumes little energy and little impact on

the environment [5]. Karthik [6] stated that every 1 ton of Portland cement produced will released

about 0.9 tons of carbon dioxide (CO2) to the environment. Thus, the reduction of CO2 emissions by

using fly ash and slag in replacing cement used in construction is an area of research to be

developed. Fly ash is a waste materials produced by combustion of coal obtained from the coal

power plant. It is rich in silica and alumina which needs a highly alkaline solution for reaction to

produce aluminosilicate gel, acted as the binding material for the concrete. Geopolymer concrete is

an innovative construction material and can be produced without adding ordinary Portland Cement

(OPC) [7].

Suresh et. al [5] have conducted the study on the effect of water absorption, apperent

porosity and sorptivity on durability of fly ash based geopolymer mortars. The results in this study

found that water absorption, porosity and sorptivity of geopolymer mortars specimens influence the

durability of geopolymer mortars. The samples with higher water absorption, porosity and water

sorptivity having lowest compressive strength. It can be observed that the calcium content present

in fly ash played an important role in developing the compressive strength. The presence of calcium

ions produced a faster reactivity. Thus, the geopolymer yields quick hardening in shorter curing

time [8].

Besides, Bakharev et. al [9] reported that the geopolymer composites performed better than

Ordinary Portland Cements after conducted durability tests on alkali activated slag. Furthermore,

the reduction of ionic diffusion and permeability have been observed in GP concrete since GP

concrete binders have a nano-porous pore system [10].

Experimental Methods

2.1 Materials

Geopolymer paste in this study was made from fly ash class F based geopolymer mixed with

the alkaline activators which were sodium silicate (Na2SiO3) solution and sodium hydroxide

(NaOH) solution. The American Society for Testing and Materials (ASTM) defines fly ash as a

residue resulting from the combustion of coal in power plants [11]. Fly ash has the advantage of

reducing concrete permeability and mitigates against corrosion of reinforcement, Class F fly ash is

highly pozzolanic material and contains almost 70% pozzolanic compounds (silica oxide, alumina

oxide and iron oxide) [12].

Fly ash is highly cost effective and environmental friendly due to low carbon dioxide

emissions to environment. Table 1 shows the chemical composition of fly ash class F between

ASTM C618 and fly ash in this study. The chemical composition of the fly ash was determined by

an X-Ray Fluorescence (XRF) analysis. From the table, the fly ash class F in this study has met the

standard specification of the ASTM C618.

Materials Science Forum Vol. 803 167

Table 1: Class F fly ash chemical composition

Chemical Composition

(Mass %)

ASTM C618 Requirements,

(%)

Fly ash used in this study

(%)

SUM

(SiO2+Al2O3+Fe2O3)

70.0 min 76.78

Sulfur Trioxide (SO3) 5.0 max 1.72

Moisture Content 3.0 max 0.76

Loss on Ignition (LOI) 6.0 max 2.30

2.2 Sample Preparation

Samples were prepared according to the twelve same mix designs. Firstly, the NaOH

solution was prepared by dissolving the NaOH pellets in distilled water. Then, the NaOH solution

was mixed with Na2SiO3 solution. An alkaline activator was prepared 24 hrs prior to use with the

ratio of the mixture of Na2SiO3/NaOH is 2.5. The ratio is very important in order to obtain a

homogeneous solution. The alkaline activator was later mixed with the fly ash for about 30 minutes.

The prepared mixtures were placed in moulds and compacted. The samples were kept at

ambient temperature in the moulds until it becomes hardened. All the samples were taken out from

the moulds after 24 hrs. Then, the samples were cured at 60˚C in the oven for 24 hrs [13].

2.3 Water Absorption Tests

The water absorption tests were conducted according to ASTM C642-06 [14]. The percent

of water absorption for these samples were determined by Eq. (1).

% Water Absorption = Wet weight – Dry weight x 100% (1)

Dry weight

2.4 Porosity Tests

The porosity tests were conducted according to ASTM D7063/D7063M-11 [15]. The

percent of porosity for these samples were determined by Eq. (2).

% Porosity = Wu-Wk x 100% (2)

Wu-Wa

where:

Wu = mass of specimen in air

Wk = mass of dry specimen

Wa = mass of specimen under water

168 Geopolymer and Green Technology Materials

Results and Discussions

Table 2 shows the results of water absorption in units of percent (%), porosity in units of

percent (%) and SEM micrographs for geopolymer paste after day 7, day 14, day 28 and day 90.

From Table 2, it was observed that the percent of water absorption were reduced after day 7 until

day 90 where the average absorption were 4.65%, 4.27%, 4.16% and 3.81% respectively.

Meanwhile, the percent of porosity also showed a reduction for the samples tested after day 7 until

day 90 from 11.95%, 11.02%, 7.65% and 3.77% respectively. Both samples for water absorption

and porosity decreased uniformly after day 7 until day 90.

It shows that the porosity affects the water absorption. When the porosity decreased, the

pore size also decreased. Thus, the water absorbed also decreased. The structure of samples

becomes denser, harder and improved crystallinity after day 7 until day 90. The pore size decreases

and the structures are closer to each other. The morphological structure of geopolymer paste pores

were shown by Scanning Electron Microscope (SEM) morphology. The porosity and permeability

also decreased hence the durability would be improved.

Table 2: Results of water absorption (%), porosity (%) and SEM micrographs view of a concrete

pore for geopolymer paste after day 7, day 14, day 28 until day 90.

Sample Water

Absorption

(%)

Porosity

(%)

SEM morphology

Average

7 days

4.65 11.95

Materials Science Forum Vol. 803 169

Average

14 days

4.27 11.02

Average

28 days

4.16 7.65

Average

90 days

3.81 3.77

Figure 1 shows the graph of average water absorption represents in percentage (%) against

average porosity also in percentage (%). It was observed that the relationship between water

absorption and porosity increase exponentially with a relatively high correlation coefficient (R2) of

0.888. The samples with higher water absorption and porosity are potentially less durable.

170 Geopolymer and Green Technology Materials

Figure 1: Relationship between average water absorption and average porosity after day 7, day 14,

day 28 until day 90.

Conclusions

The geopolymer paste samples at day 90 showed the lowest percentage of water absorption

and porosity while the geopolymer paste samples at day 7 showed the highest water absorption and

highest porosity. This is due to the structure of samples becomes denser, harder and improved

crystallinity from day 7 until day 90. The pore size decreases and the structure became denser. It

can be proved by Scanning Electron Microscope (SEM) morphology. The porosity and permeability

also decreased hence the durability potentially be improved.

Acknowledgements

The authors wish to acknowledge the staff of School of Materials Engineering, Universiti Malaysia

Perlis (UniMAP) for their involvement in the discussions, contributions and valuable advice. This

work was financially supported by the Center of Excellence Geopolymer & Green Technology,

UniMAP.

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