LN

Ka

b

c

d

e

a

ARAA

KDRLGNS

1

ioieymiasaswapit

0d

Materials Science and Engineering A 528 (2011) 6616– 6623

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l h o me pa ge: www.elsev ier .com/ locate /msea

ightweight geopolymer made of highly porous siliceous materials with variousa2O/Al2O3 and SiO2/Al2O3 ratios

. Pimraksaa,c,∗, P. Chindaprasirtb, A. Rungcheta, K. Sagoe-Crentsild, T. Satoe

Department of Industrial Chemistry, ThailandSustainable Infrastructure Research and Development Center, Faculty of Engineer, Khon Kaen University, 40002, ThailandMaterials Science Research Center, Faculty of Science, Chiang Mai University, 50200, ThailandManufacturing and Infrastructure Technology, CSIRO, Melbourne 3190, AustraliaInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

r t i c l e i n f o

rticle history:eceived 10 February 2011ccepted 14 April 2011vailable online 22 April 2011

eywords:iatomaceous earthice husk ash

a b s t r a c t

The syntheses of lightweight geopolymeric materials from highly porous siliceous materials viz. diatoma-ceous earth (DE) and rice husk ash (RHA) with high starting SiO2/Al2O3 ratios of 13.0–33.5 and Na2O/Al2O3

ratios of 0.66–3.0 were studied. The effects of fineness and calcination temperature of DE, concentrationsof NaOH and KOH, DE to RHA ratio; curing temperature and time on the mechanical properties andmicrostructures of the geopolymer pastes were investigated. The results indicated that the optimum cal-cination temperature of DE was 800 ◦C. Increasing fineness of DE and starting Na2O/Al2O3 ratio resulted inan increase in compressive strength of geopolymer paste. Geopolymer pastes activated with NaOH gave

ightweight geopolymerlassy phasea2O/Al2O3 ratioiO2/Al2O3 ratio

higher compressive strengths than those with KOH. The optimum curing temperature and time were75 ◦C and 5 days. The lightweight geopolymer material with mean bulk density of 0.88 g/cm3 and com-pressive strength of 15 kg/cm2 was obtained. Incorporation of 40% RHA to increase starting SiO2/Al2O3

and Na2O/Al2O3 ratios to 22.5 and 1.7 and enhanced the compressive strength of geopolymer paste to24 kg/cm2 with only a marginal increase of bulk density to 1.01 g/cm3. However, the geopolymer materialswith high Na2O/Al2O3 (>1.5) were not stable in water submersion.

. Introduction

The manufacture of Portland cement is energy intensive ands associated with carbon dioxide emission of approximately 7%f total global emission [1]. The process also produces pollutionn the forms of dust, noise and vibration with operating machin-ry during blasting in quarries. This has led to efforts in recentears to develop alternative cementitious materials to comple-ent Portland cement production. An example of emerging binders

s to the geopolymer system with inherent low CO2 emissionnd low energy requirement. The reaction process of this binderystem is characterized by dissolution of alumino-silicate oxidesnd followed by the polymerization reaction of those dissolvedpecies, in the presence of alkali ions for charge balancing in frame-ork cavities nearby AlO4 constituents. The reaction produces SiO4

nd AlO4, tetrahedral frameworks linked by shared oxygens as

olysialates or polysialate–siloxo or polysialate–disiloxo depend-

ng on the SiO2/Al2O3 ratio in the system [2,3]. The connection ofhe tetrahedral frameworks is occurred via long range covalent

∗ Corresponding author. Tel.: +66 53 943405; fax: +66 53 892262.E-mail address: kpimrakp@science.cmu.ac.th (K. Pimraksa).

921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2011.04.044

© 2011 Elsevier B.V. All rights reserved.

bonds. Thus, geopolymer structure is perceived as dense amor-phous phase comprising of semi-crystalline 3-D alumino-silicatemicrostructure.

Nowadays, lightweight construction materials are used toreduce the weight of building structures and improve thermal insu-lation efficiency of buildings. This study, therefore, focused on thesynthesis of lightweight geopolymer. Suitable lightweight sourcematerials containing silica and alumina such as diatomaceous earth(DE) and rice husk ash (RHA) are available in large quantity inThailand. DE contains both silica and alumina and RHA containsmainly silica. They both have very high specific surface but particlemorphologies are very different. DE is a sedimentary rock consist-ing principally of highly porous skeletons of diatom, in other word,a loosely packed mineral. The porous silica structure gives DE use-ful characteristics such as unique particulate structure, low bulkdensity, high absorptive capacity and high surface area [4]. Thedeposit of DE in Lampang province in the north of Thailand is quitelarge estimated to be in excess of 100 million tons. The mineralcomposition consists primarily of silica with some clay minerals

viz. montmorillonite, kaolinite and muscovite. DE could readilyundergo chemical reaction when meets Ca(OH)2 resulting in forma-tion of strong binding phase [5]. These clay minerals render DE as asuitable feedstock material for geopolymer synthesis. RHA contains

K. Pimraksa et al. / Materials Science and Engineering A 528 (2011) 6616– 6623 6617

Table 1Chemical compositions of DE and RHA (Si/Al ratio is molar ratio of SiO2/Al2O3).

Raw materials Chemical composition (wt%) Si/Al ratio

SiO2 Al2O3 Fe2O3 Na2O K2O CaO MgO TiO2 P2O5 SO3 LOI

DE 77.46 10.97 9.08 tr 1.07 0.35 0.31 0.41 tr tr 0.35 11.98CDE800 66.48 13.18 18.31 tr 1.12 0.47 tr 0.43 tr tr trMDE800 75.52 12.15 10.27 tr 1.24 0.39 tr 0.44 tr tr trFDE800 79.76 10.81 7.42 tr 1.26 0.33 tr 0.42 tr tr trRHA 85.25 0.11 0.18 tr 1.80 0.79 0.3 0.03 1.10 0.08 10.29 1315

0.75

T

apfd

bf3agosoRotCftr

2

2

I4oho(1

Biib

FRHA 96.47 0.00 0.32 tr 1.50

r = trace (0–0.049%).

pproximately 90% of SiO2 and possesses very high porosity. Theore sizes of RHA are much smaller than those of DE. RHA, is, there-ore, considered beneficial to help enrich dissolved SiO4 speciesuring both dissolution and subsequent polymerization stages [6].

For conventional geopolymer, the SiO2/Al2O3 ratio is normallyetween 2 and 4. At high SiO2/M2O (M = Na, K) ratios, siliconorms various oligomeric ions [7]. It is still not certain whetherD built-up structures of geopolymer made of highly porous silicare impeded due to its characteristic long range bonding of inor-anic polymer chains. Only a few researchers studied the effectf using a high range of SiO2/Al2O3 ratios as the starting compo-itions [8,9]. This research, therefore, aims to study the reactionf highly porous silica obtained from two sources viz. DE andHA. Various starting Na2O/Al2O3 and SiO2/Al2O3 ratios, reductionf unit weight, strength development and microstructure evolu-ion resulting from chemical polymerization reactions are studied.haracterization and understanding of the role of DE and RHAeedstock in geopolymer synthesis reaction should be beneficialo future development of lightweight products based vast naturalesources.

. Materials and methods

.1. Materials

DE was obtained from Lampang province in northern Thailand.t had particle size (d4,3) of 73 �m, specific surface area by BET of6.8 m2/g and specific gravity of 2.55. Mineralogical compositionsf DE consisted of quartz, montmorillonite, muscovite, kaolinite,ematite, some semi-crystalline phase and amorphous phases. Lab-ratory grade sodium hydroxide (NaOH) and potassium hydroxideKOH) and industrial sodium silicate (Na2SiO3) with 32.39% SiO2,3.44% Na2O and 54.17% H2O were used.

RHA with particle size (d4,3) of 62 �m, specific surface area by

ET of 48.7 m2/g was obtained from burning of rice husk at 700 ◦C

n an oxidizing atmosphere to produce amorphous silica [10]. XRDndicated that the mineralogical compositions of RHA composedasically of amorphous phase of silica with only small amount of

Fig. 1. Photomicrographs o

tr 0.02 0.61 tr tr

cristobalite. Chemical compositions of DE and RHA are given inTable 1. Photomicrographs of DE and RHA are shown in Fig. 1.From Table 1 CDE800, MDE800 and FDE800, respectively, repre-sent DE sieved into coarse, medium and fine particle sizes whileFRHA denotes RHA sieved into fine particle size.

DE was calcined at 400, 600, 800 and 1000 ◦C (DE400, DE600,DE800 and DE1000). After calcination, DE was sieved into threelots of coarse, medium and fine particle sizes. Coarse DE (CDE) wasthe portion larger than sieve No. 80. Medium DE (MDE) was theportion larger than sieve No. 200. Fine DE (FDE) was the portionpassed sieve No. 325. Fine portion of RHA was obtained from sievingpassed also sieve No. 325. The chemical compositions of DE, sievedDE800 and RHA are shown in Table 1.

The leachings of Si and Al ions of calcined DE and RHA were mea-sured. The leaching test was carried out by stirring the calcined DEin 10 Molar (M) NaOH solution with solid to liquid ratio of 1:10for 24 h. During storage in order not to allow the precipitation ofthe dissolved ions prior ion measurement, concentrated nitric acidwas dropped to the samples to obtain pH 4 suspension. The dis-solved Si and Al ions were determined quantitatively using AtomicAbsorption Spectroscopy (AAS).

2.2. Details of tests

Mixing was done in an air conditioned room at approximately25 ◦C. The proportion of each mix was stoichiometrically weighedaccording to Table 2. The mixing procedure started with mixing ofNaOH (or KOH) solution, sodium silicate (Na2SiO3) and DE for 5 min.Water was then added and mixed for a further 5 min. The mix-tures were cast in 50 mm × 50 mm × 50 mm acrylic cube mould. Thespecimens were then wrapped with plastic film to prevent mois-ture lost during curing. After delayed time of 1 h, the specimenswere cured in an electric oven. The specimens were cooled downin the oven and then demoulded and kept at 23 ◦C room under

ambient conditions until the scheduled testing. The compressivestrength tests were performed at 7 days in accordance with ASTMC109; density measurements were also conducted at 7 days. Thereported strength and density values were the mean of three tests.

f (a) DE and (b) RHA.

6618 K. Pimraksa et al. / Materials Science and Engineering A 528 (2011) 6616– 6623

Table 2Compositions of FDE800 geopolymer.

Sample no. Hydroxide Molar Mole ratio

SiO2/Al2O3 Na2O/Al2O3 Na2O/SiO2 H2O/Na2O

1 NaOH 4 13.0 0.68 0.05 19.532 6 13.0 0.79 0.06 16.393 8 13.0 0.90 0.07 13.944 10 13.0 1.00 0.08 12.165 12 13.0 1.09 0.08 10.866 KOH 4 13.0 0.66 0.05 19.537 6 13.0 0.76 0.06 16.208 8 13.0 0.84 0.06 13.969 10 13.0 0.94 0.07 11.89

10 12 13.0 1.02 0.08 10.5311 NaOH 10 13.0 1.00 0.08 12.1612 10 13.0 1.50 0.12 11.7713 10 13.0 2.00 0.15 11.5714 10 13.0 3.00 0.23 11.38

Xctw

2

2

soa

2

fiwl

(

(

(

TC

15 NaOH 10 16.6

16 10 22.5

17 10 33.5

RD and SEM analysis were conducted on fractured specimens afterompressive strength test using a Philips X’Celerator diffractome-er with a Cu K�1 radiation and JSM-5910 LV microscope equippedith EDX microanalysis.

.3. Test series

.3.1. Effect of pre-treatmentIn this series, fineness and calcination temperature of DE were

tudied. NaOH with concentration of 10 M and Na2SiO3/NaOH ratiof 1.23 were used. After delayed time of 1 h, specimens were curedt 60 ◦C for a period of 3 days.

.3.2. Effect of synthesized condition on DE geopolymerThe results shown in Section 2.3.1 indicated that the calcined

ne DE with optimum calcination temperature of 800 ◦C (FDE800)as suitable source material. It was, therefore, selected for the fol-

owing tests.

1) Varying concentration of NaOH and KOHCompositions of geopolymer in mole ratios of samples No.

1–10 investigated are shown in Table 2. The concentrations ofNaOH and KOH of 4, 6, 8, 10 and 12 M and Na2SiO3/NaOH ratio of1.23 were used. In this respect, starting Na2O/Al2O3 ratios werevaried from 0.66 to 1.09 with the constant starting SiO2/Al2O3of 13.0 as shown in Table 2. After delayed time of 1 h, specimenswere cured at 60 ◦C for a period of 3 days.

2) Varying alkali contentIn this test, 10 M NaOH; three starting Na2O/Al2O3 ratios of 1,

1.5, 2 and 3; and curing at temperature of 60 ◦C for 3 days werestudied. Compositions of geopolymer in mole ratios of samplesNo. 11–14 are also shown in Table 2.

3) Varying curing temperature and time

able 3ontents of aluminum and silicon ions leached from DE and RHA.

Materials Silicon ion (mg/L)

As-received DE 2090

CDE 2040

MDE 2110

FDE 2750

FDE400 1690

FDE600 3540

FDE800 4140

RHA 4840

FRHA 5310

1.25 0.08 12.161.67 0.07 12.162.46 0.07 12.04

Samples were prepared with 10 M NaOH and Na2SiO3/NaOHratio of 1.23. After 1 h delayed time, specimens were cured inan oven for different time intervals of 1–5 days at curing tem-peratures of 60, 75 and 90 ◦C.

2.3.3. Effect of incorporation of RHAThe geopolymer pastes with different DE:RHA ratios were used

to obtain various starting SiO2/Al2O3 ratios. The compositions ofsamples No. 15–17 are given in Table 2.

3. Results and discussion

3.1. Dissolution of DE and RHA

Results of dissolution of DE and RHA in 10 M NaOH activatorsolution are shown in Table 3. As expected, the dissolution cor-related well with particle size and fineness of DE. The amount ofleached silicon ion of CDE was 2040 mg/L and increased to 2110and 2750 mg/L, respectively, for finer DE and FDE. The finer DEpossessed high surface area and allowed greater contact with thealkali solutions. Furthermore, fine DE also contained higher silicacontent and less alumina content compared to the coarser ones(Table 1). The SiO2/Al2O3 ratio of dissolved species thus increasedwith an increase in fineness. For the calcined DE, the dissolution ofsilica and especially alumina were significantly affected by the cal-cination temperature, resulting in decrease in SiO2/Al2O3 ratio ofdissolved species as shown in Table 3. The dissolved silicon ions ofthe DE400 (DE calcined at 400 ◦C), DE600 and DE800, respectively,were 1690, 3540 and 4140 mg/L while those of aluminum ions were

158, 1010, 1260 mg/L, respectively. High calcination temperatureof 600 and 800 ◦C were needed for increased dissolution of alu-minum ions. RHA provided a slightly larger amount of the dissolvedsilicon ions than FDE800. This was due to the larger amount of

Aluminum ion (mg/L) SiO2/Al2O3 molar ratio

219 4.7296 3.3230 4.4205 6.5158 5.0

1010 1.71260 1.6

0 ˛0 ˛

K. Pimraksa et al. / Materials Science and Engineering A 528 (2011) 6616– 6623 6619

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0

1

2

3

4

5

6

7

8

R80DE R200 DE P325 DE

Bulk density (g/cm

3)

Com

pres

sive

stre

ngth

(kg/

cm2 )

Fineness (mesh)

Strength

Density

Ffi

smstg

ockdDfdegSctat

FD

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0

2

4

6

8

10

12

1000800600400

Bulk density (g/cm 3)

Com

pres

sive

stre

ngth

(kg/

cm2 )

Calcination temperature (°C)

StrengthDensity

ig. 2. Compressive strength and bulk density of geopolymer pastes with differentnenesses of DE.

ilica containing in RHA than DE. The dissolved ion amount becameuch larger when RHA was finer (FRHA). It was shown here that

tarting material batches calcined under different treated condi-ions and sizes provided different amount of dissolved ions duringeopolymerization reactions.

The silica and alumina contained in clay minerals contributed tobserved increase of dissolved ions. In particular, the silica phaseontained in DE was slightly changed with heat treatment, whileaolinite and montmorillonite could transform to metakaolin andisrupted montmorillonite thereby providing more active surfaces.issolution of silicate molecules at high pH (>10) resulted in the

ormation of silicic acid; H3SiO4−, H2SiO4

2−, HSiO43− and SiO4

4−,ependent on pH of the system [11]. The measured pH of thisxperiment was around 14. Thus, the stable silicic acid species ofeopolymeric system will normally be expected to be HSiO4

3− andiO4

4−. These species are likely to play an important role in sili-ate polymeric condensation. To further investigation these effects,he DE800 was screened through 325 mesh size sieve and useds source material to prepare geopolymeric materials for further

ests.

6050403020100

Inte

nsity

(cou

nts)

2Theta (°)

HQK MMT

M Q Q (a)

Q

Q

(b)

(c)

(d)

(e)

F

F

CC HC F

F

MT=Montmorillonite, M=Muscovite, K=Kaolinite, Q=Quartz, H=Hematite,

F=Magnetite, C=Cristobalite

ig. 3. XRD patterns of calcined DE. (a) Noncalcined DE (b) DE400 (c) DE600 (d)E800 and (e) DE1000.

Fig. 4. Compressive strength and bulk density of geopolymer pastes with differentcalcination temperatures of DE.

3.2. Effect of pre-treatment on DE geopolymer

3.2.1. Fineness of DEThe compressive strength and bulk density of geopolymer

pastes made with the different feedstock finenesses are shown inFig. 2. The strength development at 7 days of R80DE, R200DE andP325DE geopolymer pastes were 2.0, 5.0 and 6.6 kg/cm2, respec-tively. The strength increases with increase in DE fineness. Giventhat DE with higher surface area is likely to undergo a faster dis-solution, thereby increasing the amount of silicon and aluminumions in the solution, a corresponding increase in the paste strengthmay be expected. Data of bulk density clearly testified this trend.

3.2.2. Calcination temperature of DEXRD patterns for different calcination temperatures of DE are

shown in Fig. 3. The transformation of chemical structures ofmonmorillonite and kaolinite containing in 600, 800 and 1000 ◦Ccalcined DE were observed from the disappearances of peaks at 2�of 6.5◦ and 12.5◦. Hematite (Fe2O3) was transformed into magnetite(Fe3O4). Fig. 4 shows the results of compressive strength and bulkdensity of geopolymer pastes made with calcined DE. The strengthof DE800 paste was significantly higher than those of noncalcinedDE, DE400, DE600 and DE1000 ◦C. At lower calcined temperatures,only limited dissolution of silicon and aluminum ions occur. At hightemperature of 1000 ◦C, quartz was transformed to cristobalite andthe magnetite phase increased accompanied by reduction in reac-tivity of the DE1000 material, as evidenced by the lower strengthof geopolymer made with DE1000 shown in Fig. 3.

3.3. Effect of synthesized condition on DE geopolymer

3.3.1. Concentration of NaOH or KOHFig. 5 shows the results of compressive strength and bulk den-

sity of geopolymer samples made with NaOH and KOH as alkalineactivators. The strength increased along with an increase of concen-tration of NaOH or KOH. In other words, the high Na2O/Al2O3 andNa2O/SiO2 ratios promoted the strength enhancement. Alkali solu-tions with higher concentration provided better dissolution of DEparticles generating more reactive bonds for the monomer, which,in turn increased geopolymerization of pastes [12]. When the con-centration of hydroxide exceeded 10 M, the setting time was veryfast. At such high alkali concentration, excess hydroxide ion con-

centration caused aluminosilicate gel precipitation at the very earlystages, resulting in lower characteristic strength [13]. The DE sam-ples made with NaOH gave higher strength than that with KOH.

6620 K. Pimraksa et al. / Materials Science and Engineering A 528 (2011) 6616– 6623

0.85

0.87

0.89

0.91

0.93

0.95

0.97

0.99

0

2

4

6

8

10

12

1210864

Bulk density (g/cm

3)

Com

pres

sive

stre

ngth

(kg/

cm2 )

Alkali ne concentration (mol/l )

NaOH strength KO H stre ngth

NaOH density KO H density

Fc

Cb

3

fo4pstdgaaa7w

3

g

Ft

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

0

10

20

30

40

50

60

70

1.0 1. 5 2. 0 3.0

Bulk density (g/cm

3)

Com

pres

sive

stre

ngth

(kg/

cm2 )

Na2O/Al2O3 ratio

StrengthDensity

ig. 5. Compressive strength and bulk density of geopolymer with different con-entrations of NaOH or KOH.

ation size of Na+ was smaller than K+, thus Na+ provided higherond strength than that of K+.

.3.2. Curing temperature and timeThe results of compressive strength and bulk density with dif-

erent curing temperatures and times are given in Fig. 6. Theptimum curing temperature and time were found to be 75 ◦C and–5 days. The strengths increased with the increases in curing tem-erature and time. For the high curing temperature of 90 ◦C, thetrength after 5 days curing dropped. At high curing temperature,he sample could experience a substantial loss of moisture and theeterioration of strength resulted [14]. As expected, the density ofeopolymer decreased with the increases in curing temperaturend time. With the progress of polymerization, water was gradu-lly removed and the SiO4 and AlO4 tetrahedral were linked to formmorphous network geopolymer structures. The sample cured at5 ◦C for 5 days has bulk density of 0.880 g/cm3 while the strengthas reasonably high at 15 kg/cm2.

.3.3. Alkali contentFig. 7 shows the compressive strength and bulk density of

eopolymer pastes with varying Na2O/Al2O3 molar ratios of 1–3.

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0

2

4

6

8

10

12

14

16

54321

Bulk density (g/cm 3)

Com

pres

sive

stre

ngth

(kg/

cm2 )

Curing time (days )

60 C strength 75 C Strength 90 C strength

60 C density 75 C density 90 C density

ig. 6. Compressive strength and bulk density of geopolymers with different curingemperatures and times.

Fig. 7. Compressive strength and bulk density of geopolymer with different Na/Alratios.

The strength increased considerably with an increase in start-ing Na2O/Al2O3 ratio. However, high alkali content (Na2O/Al2O3ratios of 2 and 3), makes polymerized networks less stable thanNa2O/Al2O3 ratio of 1 [15]. The density of geopolymer increasedwith an increase in Na2O/Al2O3 ratio. The increase in the density ofgeopolymer paste to this level was undesirable. Fig. 8 shows insta-bility of DE geopolymeric materials with Na2O3/Al2O3 ratios of 2and 3. The samples rapidly disintegrated after submersion in waterfor 24 h while the material with Na2O3/Al2O3 ratio of 1 withstoodthe immersion test. The materials with high Na2O/Al2O3 encom-passed high load bearing resistance that contradicted to its watersubmersion behavior. Considerably was known on the geopolymerstructures but still less was known about the relation.

Fig. 9 demonstrates morphological aspects of geopolymerpastes at different Na2O/Al2O3 ratios. The geopolymer paste withNa2O/Al2O3 of 1 was less dense than those with Na2O/Al2O3 ratiosof 2 and 3. Some voids and unreacted DE could be observed inthe sample with Na2O/Al2O3 of 1 (Fig. 8(a)). The dense amorphousglassy geopolymer matrices could be seen for Na2O/Al2O3 of 2 and3 geopolymer (Fig. 9(b) and (c)). The results of EDX analyses ofthe DE geopolymers are also shown in Fig. 9(a)–(c). The major ele-ments are Si and Al with the presence of a small amount of Na, Kand Ca. Although the starting SiO2/Al2O3 ratios of DE geopolymerswere similar at 13.0, the ratios of SiO2/Al2O3 for DE geopolymersobtained with different Na2O/Al2O3 ratios were significantly differ-ent. The Si/Al ratios for this suite of DE geopolymer samples withNa/Al ratios of 1, 2 and 3 were 9.9, 13.9 and 17.4, respectively. Theresults suggested that the dissolubility of silica and alumina weredependent on Na/Al ratios [16]. It was shown here that using lowNa/Al ratio (∼1), the dissolubility of aluminate phases such clayminerals in DE were favorable. The higher Na/Al ratios, the silicatephases were more susceptible to waster dissolution. In addition, itwas shown that excess sodium ions were likely more localized inthe geopolymer structure compared to samples with higher Na/Alratio. The characteristic of EDX peaks of geopolymer pastes withNa/Al ratio of 2 was very similar to that of zeolite.

Fig. 10 shows XRD-patterns of DE geopolymeric pastes with dif-ferent Na/Al ratios. Alkaline activator solutions with Na/Al ratiosof 1–3 did not produce any of new crystalline phases, althoughincreasing the Na/Al ratios resulted in decreased intensities ofquartz and muscovite peaks. In addition, increased Na/Al ratiosappears to induce higher degree of amorphization observed by abroad band at 2� = 25–27◦ [17]. The reduction of muscovite con-

tent is probably due to greater aluminum ions in aluminosilicatechain condensation reactions.

The degree of amorphous phase formation detected using IR-spectrum is shown in Fig. 11. The IR bands of highly ordered

K. Pimraksa et al. / Materials Science and Engineering A 528 (2011) 6616– 6623 6621

stes a

s[ofd

Fig. 8. States of DE geopolymer pa

tructures are typically sharp and have no spectral splitting

18]. The identification of the IR spectral bands was carriedut using IR bands of calcined diatomaceous earth (DE800)or reference. The IR spectra of DE800, a SiO2 based pow-er with quartz as primary composition revealed the presence

Fig. 9. SEM–EDX analyses of geopolymer pastes. (a) Na2O/

fter submersion in water for 24 h.

of three key vibrational modes of Si–O–Si groups (tetrahedra)

in the region between 400 and 1300 cm−1 [19] 450–465 cm−1

of in-plane Si–O bending, 790–820 cm−1 of Si–O–Si symmetricstretching, and 1000–1200 cm−1 of Si–O–Si asymmetric stretching.The strong IR spectrum around 1000–1200 cm−1 was broadened to

Al2O3 = 1, (b) Na2O/Al2O3 = 2 and (c) Na2O/Al2O3 = 3.

6622 K. Pimraksa et al. / Materials Science and Engineering A 528 (2011) 6616– 6623

0 10 20 30 40 50 60 70 80 90

Na/Al = 3

Na/Al = 2

Q QQ Q

Q

QM

M H

Inte

nsity

(cou

nts)

2Theta

M

QFM

Na/Al = 1

M=Muscovite, Q=Quartz, F=Magnetite, H=Hematite

Fig. 10. XRD patterns of DE geopolymer pastes with different Na/Al ratios.

Table 4Peak area ratio from IR spectra of pastes at Si–O–Si stretching.

Sample Location ofSi–O–Si (cm−1)

Corrected area Peak area ratio

DE800 1040 8.41 1Na2O/Al2O3 = 1 1044 8.70 1.03

1Tw[Swdtom3ht1N

F

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

0

5

10

15

20

25

30

0 20 40 60

Bulk density (g/cm

3)

Com

pres

sive

stre

ngth

(kg/

cm2 )

Quantity of Rice-husk ash (%mass)

Stre ngthDensity

acted as defect sites and exerted a negative effect on strengthof geopolymeric materials. Although, the strength increased, thedensity values were low at 0.92–1.01 g/cm3 which were highly

Na2O/Al2O3 = 2 1022 10.45 1.24Na2O/Al2O3 = 3 1062 15.14 1.80

000–1340 cm−1 when DE800 was activated with alkali solution.his system contained a short range ordered aluminosilicate phasehich was likely to undergo vibration at around 1070–1080 cm−1

20]. This vibrational band overlapped with the original crystallineiO2 phase. The increase in the IR band areas as shown in Table 4,ith an increase in Na/Al ratios can be attributed to the higheregree of aluminosilicate condensation [16]. The broad absorp-ion band around 3400–3700 cm−1 corresponded to the vibrationf hydroxyl (OH) groups and H2O molecules [21]. The band of waterolecules associated with silanol groups are normally found at

450 cm−1 while the band of hydrogen bonded silanol (SiO–H)aving adjacent pairs of Si–OH groups with hydrogen bonded

o each other was detected around 3660 cm−1. The band around620–1660 cm−1 arose from the adsorbed water. An increase ofa/Al ratio from 1 to 2 and 3 increased these vibrational modes

ig. 11. IR spectra of diatomaceous earth geopolymer with different Na/Al ratios.

Fig. 12. Compressive strength and bulk density of geopolymer pastes with differentRHA contents.

corresponding to presence of larger silanol groups. It was shownhere that there was a certain amount of residual silicate phase leftover which was not involved in the aluminosilicate condensationprocess. For the DE800 powder, there was no single hydroxyl banddetected.

3.4. Effect of proportion of DE and RHA

The RHA was used to adjust the composition of silica phase.The Si/Al ratios of 16.6–33.5 of samples No. 15–17 are shown inTable 2. Fig. 12 shows the compressive strength and bulk densityof geopolymer pastes made of the mixtures of DE800 and RHA.The strengths increased with the increase in the amount of RHA upto 40% of solid powder which corresponded to Si/Al ratio of 22.5.The strength increased to 24 kg/cm2 due to the proper obtained–Si–O–Al– bonds in geopolymeric structures. An increase in thereplacement level of RHA beyond this level resulted in decreaseof strength. This is due to the presence of –Si–O–Al– bonds ofhigh Si/Al ratio (too less AlO4 species) which does not favor acondensation of long chain polymeric structure [22]. Thus, they

desirable.

0 10 20 30 40 50 60 70 80 90

60% RHA

40% RHA

20% RHA

QQQ

C

M

Q

F

M

Inte

nsity

(Cou

nts)

2Theta

Q

0% RHA

M=Muscovite, Q=Quartz, F=Magnetite, C=Cristobalite

Fig. 13. XRD pattern of geopolymer pastes with 0, 20, 40 and 60% RHA.

K. Pimraksa et al. / Materials Science and Engineering A 528 (2011) 6616– 6623 6623

r past

4whtismidbostpgNirr

4

cpfisptDiilpstcitit

[

[[

[

[[[[[

[[

[

Fig. 14. States of DE–RHA geopolyme

Fig. 13 shows XRD patterns of geopolymer pastes with 0, 20,0 and 60% RHA. The degree of crystalline structure decreasedith the replacement of DE with RHA. This could be seen at theump peaks around 18–30 degree increased with the increase inhe RHA content or in SiO2/Al2O3 and Na2O/Al2O3 as a result ofncreases in the glassy phase. The geopolymer pastes containedimilar crystalline phases to those of original DE and RHA. Quartz,uscovite, magnetite and cristobalite which were originally found

n DE and RHA were undissolvable. Quartz content was loweredue to RHA replacement. No new crystalline phase was observed,ut for the increase in RHA content. A broad below 8 degreef 2� was evident with the increase in RHA content. This peakuggested the formation of a mesoporous material of poorly crys-alline nature [23]. The increase in the glassy phase of geopolymerastes appears responsible for observed increase in strength. Theeopolymer materials with high RHA addition (high SiO2/Al2O3 anda2O/Al2O3) were not stable after submersion in water for 24 h as

ndicated in Fig. 14. However, proportions with high SiO2/Al2O3atios which were different from conventional geopolymer ratiosesulted in the lightweight geopolymeric structures.

. Conclusions

Based on the results of investigation, the following conclusionsould be drawn. DE appears to be a good candidate material forroducing lightweight geopolymeric material. It was further con-rmed that calcined and sieved DE produced fine reactive particlesuitable for use as a source material. The optimum calcination tem-erature of DE was 800 ◦C and the finer DE was more reactive dueo the increase in the surface area. This allowed greater contact ofE particle surface with alkali solution and hence a faster leach-

ng of silica and alumina. The XRD patterns of the calcined DEndicated the transformation of chemical structures of montmoril-onite and kaolinite. With regards to the types of alkali, geopolymerastes activated with 10 M NaOH possessed higher compressivetrength than that with 10 M KOH. Curing temperature and dura-ion also affected the properties of the geopolymers. The optimumuring temperature and time were 75 ◦C and 5 days. The start-

ng Na2O/Al2O3 ratios of mixtures also affected the properties ofhe geopolymer pastes. At starting SiO2/Al2O3 ratio of 13.0, thencrease in starting Na2O/Al2O3 ratios from 1.0 to 3.0 increasedhe compressive strength from 11 to 60 kg/cm2 but the samples

[

[

es after submersion in water for 24 h.

with Na2O/Al2O3 ratios of 2.0 and 3.0 were not stable as indi-cated in water immersion test. However, the bulk density valuesalso increased from 0.93 to 1.5 g/cm3. RHA was incorporated inmixtures maintain the low density of geopolymer paste samples.High SiO2/Al2O3 ratios used resulted in the lightweight geopolymermaterials.

Acknowledgements

Research fund given by Thailand Research Fund (TRF)(MRG5180350) is gratefully acknowledged and we wish to thankalso the National Research University Project under Thailand’sOffice of the Higher Education Commission for financial support.

References

[1] V.M. Maholtra, ACI Concrete Int. 24 (7) (2002) 22.[2] J. Davitdivit, J. Mater. Ed. 16 (2) (1994) 91–139.[3] H. Xu, J.S.J. Van Deventer, Int. J. Miner. Process. 59 (2000) 247–266.[4] A. Chaisena, K. Rangsriwatananon, Mater. Lett. 59 (12) (2005) 1474–1479.[5] K. Pimraksa, P. Chindaprasirt, Ceram. Int. 35 (1) (2008) 471–478.[6] S. Detphan, P. Chindaprasirt, Int. J. Miner. Metal Mater. 16 (6) (2009) 720–726.[7] R.K. Iler, The Chemistry of Silica, Wiley-Interscience, New York, 1979.[8] K. Sagoe-Crentsil, Proceeding of the Second National Concrete and Geopolymer,

Khon Kaen University, Thailand, 2008.[9] R.A. Fletcher, K.J.D. MacKenzie, C.L. Micholson, S. Shimada, J. Eur. Ceram. Soc.

25 (2005) 1471–1477.10] P. Chindaprasirt, P. Kanchanda, A. Sathonsaowaphak, H.T. Cao, Constr. Build.

Mater. 21 (2007) 1356–1361.11] M. Dietzel, Geochim. Cosmochim. Acta 64 (19) (2000) 3275–3281.12] H. Wang, H. Li, F. Yan, Colloids Surf. A: Physicochem. Eng. Aspects 268 (2005)

1–6.13] W.K.W. Lee, J.S.J. Van Deventer, Colloids Surf. A: Physicochem. Eng. Aspects 211

(2–3) (2002) 115–126.14] T. Bakharev, Cem. Concr. Res. 35 (6) (2005) 1224–1232.15] J.W. Phair, J.D. Smith, J.S.J. Van Deventer, Mater. Lett. 57 (2003) 4356–4367.16] U. Rattanasak, P. Chindaprasirt, Miner. Eng. 22 (12) (2009) 1073–1078.17] J. Temuujin, K. Okada, K.J.D. MacKenzie, Appl. Clay Sci. 21 (2002) 125–131.18] J. Zhang, J.L. Provis, D. Feng, J.S.J. van Deventer, J. Hazard. Mater. 157 (2008)

587–598.19] J.W. Phair, J.S.J. Van Devanter, Int. J. Miner. Process. 66 (2002) 121–143.20] W.K.W. Lee, J.S.J. Van Deventer, Colloids Surf. A: Physicochem. Eng. Aspects 211

(2002) 49–66.21] B. Smith, Infrared Spectral Interpretation: A Systematic Approach, CRC Press,

New York, 1999.

22] P. Duxson, J.L. Provis, G.C. Lukey, S.W. Mallicoat, W.M. Kriven, J.S.J. Van Deven-

ter, Colloids Surf. A: Physicochem. Eng. Aspects 269 (2005) 47–58.23] E. Alvarez-Ayuso, X. Querol, F. Plana, A. Alastuey, N. Moreno, M. Izquierdo, O.

Font, T. Moreno, S. Diez, E. Vazquez, M. Barra, J. Hazard. Mater. 154 (1–3) (2008)175–183.