Thermal evolution of metakaolin geopolymers Part 1 – Physical evolution.pdf

8/10/2019 Thermal evolution of metakaolin geopolymers Part 1 Physical evolution.pdf

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Thermal evolution of metakaolin geopolymers:Part 1 Physical evolution

Peter Duxson, Grant C. Lukey, Jannie S.J. van Deventer *

Department of Chemical and Biomolecular Engineering, The University of Melbourne, Vic. 3010, Australia

Received 16 February 2006; received in revised form 4 September 2006

Abstract

The physical evolution of materials during heating is a critical factor in determining their suitability and performance for applicationsranging from construction to refractories and adhesives. The effect of different cations (sodium and potassium) on the physical evolutionof geopolymeric materials derived from metakaolin is investigated for a range of specimens with Si/Al ratios between 1.15 and 2.15. It isobserved that the effect of potassium is to reduce the thermal shrinkage, while thermal shrinkage increases with increasing Si/Al ratio inthe presence of each alkali type. The thermal shrinkage behavior of mixed-alkali specimens is observed to change from a mean of thesodium and potassium specimens at low Si/Al ratio to behave similarly to sodium specimens at high Si/Al ratios. It is clear from thisinvestigation that alkali cations only have a signicant effect on thermal shrinkage of geopolymer at low Si/Al ratios ( 6 1.65), while bothSi/Al ratio and alkali cation have little effect on the extent of thermal shrinkage at Si/Al P 1.65.

2006 Elsevier B.V. All rights reserved.

PACS: 65.60.+a

Keywords: Microstructure; Alkali silicates; Aluminosilicates; Thermal properties

1. Introduction

The alkali aluminosilicate structure of geopolymericgels renders them intrinsically re resistant. The potentialof using geopolymers as a re resistant material withmechanical properties superior to traditional cements hasbeen known for over two decades [1]. More recently,

geopolymers have been suggested as a low cost castableceramic binder [2], with application in some ceramic andhigh-technology applications. The thermal properties of geopolymeric gels have not been fundamentally investi-gated; therefore the effect of gel composition on thermalproperties is unknown yet critical to the tailor-design of

materials to suit applications. The ultimate objective of this research resides in predictive response modelling of geopolymeric structural members under load subjected tore. Such models exist for cements and other binders,and require detailed thermophysical and thermomechani-cal data for their accuracy [3]. Therefore, it is importantto understand rstly the evolution of geopolymer during

thermal exposure, including thermal shrinkage, crystalliza-tion, thermal conductivity and mechanical strength at ele-vated temperature.

Initial investigations of the thermal properties of geo-polymers have explored some of the basic responses of the material to exposure to elevated temperatures [47].The literature centers around simple measures of the abil-ity of geopolymeric materials to resist structural degrada-tion, as measured by crystallization observed in XRDdiffractograms [4,5]. The changes in structure induced byelevated temperature have also been observed to be minor

0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2006.09.019

* Corresponding author. Tel.: +61 3 83446619; fax: +61 3 83444153.E-mail address: [email protected] (J.S.J. van Deventer).

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by NMR [4,5]. Barbosa and MacKenzie [4] suggest thatthe structure of geopolymeric materials is incredibly resil-ient to exposure to high temperatures. Little to no changein the XRD and NMR structure was observed up to1400 C [4]. In contrast, investigation of Na-geopolymersobserved a large extent of linear shrinkage when exposed

to elevated temperature [57]. Two distinct regions of shrinkage have been observed, beginning at approximately100 C and 600 C. These regions of physical change areindicative of structural changes occurring within thematerial.

The physical evolution of geopolymers has been charac-terized recently by a combination of methods includingTMA, DTA, TGA and nitrogen porosimetry [8]. The ther-mal shrinkage and weight loss of Na-geopolymer wasshown to vary signicantly with Si/Al, porosity and heat-ing rate. The behavior of Na-geopolymer was characterizedby four regions of thermal shrinkage and weight lossobserved in all specimens. The initial shrinkage observedby Rahier et al. [7] was linked with a region of capillarystrain resulting from dehydration. The second large charac-teristic region of shrinkage was linked to structural densi-cation by viscous sintering [8]. The region of slowshrinkage and weight loss between the two densicationregions was identied as dehydroxylation and condensa-tion of silanol and aluminol groups on the surface of thegel combined with structural relaxation. The systematicstudy of specimens with 1.15 6 Si/Al 6 2.15 showed thatincreasing the Si/Al ratio decreased the onset temperatureof densication and substantially increased the extent of densication [8]. Investigation of physical evolution at

heating rates between 1 C min1

and 20 C min1

deter-mined that densication was increased as the heating rateincreased, likely due to entrapment of water in the gel,which reduces the energy barrier to viscous sintering.Although alkali is known to affect the structure andmechanical properties of geopolymers [911], the effect of alkali has not yet been investigated on any aspect of thephysical evolution of geopolymers synthesized frommetakaolin. As alkali type is one of the most easily variedcompositional constituents of geopolymer, investigation of the effect of the most common alkali on thermal propertiesis critical.

This investigation elucidates the physical evolution of K-geopolymers with chemical composition KAlO(SiO) z 5.5H 2 O, where 1.15 6 z 6 2.15, to determine whether thecharacteristic behavior of Na-geopolymers investigated pre-viously applies to geopolymers synthesized with differentcations. Dilatometry, DTA and microscopy are used torelate the different extent and regions of thermal shrinkageexhibited by K-geopolymer to changes in microstructureduring thermal exposure. The thermal shrinkage andweight loss of geopolymers with chemical compositionNa y K 1 y AlO(SiO) z 5.5H2 O, where 0 6 y 6 1, are thencompared to determine the effect of different alkali cationsand heating rates on the physical evolution and the extent

of thermal shrinkage of geopolymers.

2. Experimental procedure

2.1. Materials

Metakaolin was purchased from Imerys (UK) under thebrand name of Metastar 402. The molar composition of

metakaolin determined by X-ray uorescence (XRF) was(2.3:1)SiO2 Al2 O3 with small amounts of a high tempera-ture form of muscovite as an inert impurity. The Brunauer EmmettTeller (BET) surface area [12] of the metakaolin,as determined by nitrogen adsorption on a MicromeriticsASAP2000 instrument, was 12.7 m 2 /g, and the mean parti-cle size (d 50 ) was 1.58 l m.

Alkaline silicate solutions based on three differing ratiosof alkali metal Na/(Na + K) = M (0.0, 0.5 and 1.0) withcomposition SiO 2 /M 2 O = R (0.0, 0.5, 1.0, 1.5 and 2.0)and H 2 O/M 2 O = 11 were prepared by dissolving amor-phous silica in appropriate alkaline solutions until clear.Solutions were stored for a minimum of 24 h prior to useto allow equilibration.

2.2. Geopolymer synthesis

Geopolymer samples were prepared by mechanicallymixing stoichiometric amounts of metakaolin and alkalinesilicate solution to allow Al 2 O3 /M 2 O = 1 to form a homog-enous slurry. After 15 min of mechanical mixing the slurrywas vibrated for a further 15 min to remove entrained airbefore being transferred to cylindrical polyethylene mouldsand sealed from the atmosphere. Samples were cured in alaboratory oven at 40 C and ambient pressure for 20 h

before storage at ambient temperatures in sealed vesselsbeforeuse in DTA/TGA, dilatometric andNitrogen adsorp-tion experiments. Geopolymers synthesized with sodium,mixed-alkali and potassium activating solutions are referredto as NaNaK- and K-geopolymers, respectively.

2.3. Analytical techniques

Simultaneous DTA and TGA measurements were per-formed on a PerkinElmer Diamond DTA/TGA with plat-inum sample crucibles. Experiments were performedbetween 25 C and 1050 C at a heating and cooling scanrate of 10 C/min with a Nitrogen purge rate of 200 mL/min. TMA measurements were performed on a Perkin Elmer Diamond TMA using samples with 5 mm diameterat a constant heating rate of 10 C/min with a nitrogenpurge rate of 200 mL/min. All specimens were run in ran-dom order, with a control specimen run multiple timesthroughout the duration to test reproducibility, whichwas found to be approximately 0.2%.

Microstructural analysis was performed using an FEIXL-30 FEG-SEM. Samples were polished using consecu-tively ner media, prior to nal preparation using 1 l mdiamond paste on cloth. As geopolymers are intrinsicallynon-conductive, samples were coated using a gold/palla-

dium sputter coater to ensure that there was no arching

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or image instability during micrograph collection. Micro-graphs of annealed specimens were taken of specimens thatwere polished prior to annealing, so that any topographicalchanges resulting from thermal exposure could be relatedto structural evolution.

3. Results

Fig. 1 shows the thermal shrinkage of K-geopolymerswith 1.15 6 Si/Al 6 2.15. The shrinkage of K-geopolymersexhibits the same characteristic trends as those of Na-geo-polymers reported previously [5,7,8]. In particular, theK1.15 specimen exhibits a remarkably small extent of shrinkage beyond 300 C, as observed for Na-geopolymerof the same Si/Al ratio [8]. Therefore, the shrinkage behav-ior of K-geopolymer specimens can be separated into thesame four characteristic regions outlined in the observa-tions of Na-geopolymer [8]. Nominal shrinkage and largefractional weight loss are observed in Region I; Region IIbegins with the onset of initial shrinkage at approximately100 C, lasting until the rate of shrinkage decreases andevaporation of free water is complete at about 300 C;Region III is demarcated by gradual weight loss andshrinkage from dehydroxylation; and Region IV beginswith the onset of densication by viscous sintering. Theonset temperature of Region II for K-geopolymer occursat approximately 70 C, 90 C and 115 C for specimenswith Si/Al ratios of 1.15, 1.40 and P 1.65 respectively.Region II shrinkage can be observed from the onset of shrinkage until about 250300 C. The thermal shrinkage

of the K1.15 specimen in Region III is nominal. A slightregion of expansion observed at approximately 700 C,which is discussed later in this article, may be related toexpansion as a result of crystallization. The specimen isobserved to undergo only a very small extent of densica-tion in Region IV (from 700 C to 1000 C). The specimenswith Si/Al P 1.40 exhibit almost identical thermal shrink-age in Region III, before the onset of a signicant extentof shrinkage during densication in Region IV. The onsettemperature of densication can be clearly observed to

decrease with increasing Si/Al ratio. The overall shrinkageof the specimens also increases with Si/Al ratio, althoughspecimens with Si/Al 6 1.65 exhibit a similar extent of den-sication after heating to 1000 C.

DTA thermograms of K-geopolymer are presented inFig. 2. A large endotherm appears from ambient tempera-

ture until approximately 300 C in all specimens. The endo-therm can be attributed to evaporation of free pore waterand is also observed for Na-geopolymer [8]. The tempera-ture span of the endotherm increases with decreasing Si/Al ratio. In addition, distinct minima appear in the endo-therms at approximately 70 C with increasing Si/Al ratio.

The thermogravimetric data of K-geopolymers are pre-sented in Fig. 3. The weight loss data indicate that the rateof water loss increases with Si/Al ratio, which is observedacross the entire temperature range to 1000 C. The upperbound temperature of weight loss decreases as the Si/Alratio of the specimens increases for 1.15 6 Si/Al 6 1.65,from approximately 750 C to 700 C and 600 C, respec-tively. However, the temperature of nal weight loss of specimens with Si/Al P 1.65 is similar. The loss of weightfrom geopolymer can be assumed to be entirely from waterloss, by either evaporation of free water or condensation of hydroxyl groups. The thermogravimetric data in Fig. 3contain two predominant events of evaporation of uncon-strained pore water (


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condensation/polymerization (>300 C). The temperaturerange of dehydration can be correlated with the endothermin the DTA thermograms in Fig. 2, with the minima in theendotherms of high Si/Al ratio specimens relating to theincreased rate of dehydration in these specimens ( Fig. 3).

Fig. 4 shows the microstructure of K-geopolymer speci-

mens with 1.15 6 Si/Al 6 2.15. The microstructure of K-geopolymer contains large pores in specimens with Si/Al 6 1.40. The large pores in the microstructure of thelow Si/Al ratio K-geopolymer specimens may be attributedto a high degree of gel reorganization during formation,similar to Na-geopolymer [13]. Specimens with Si/Al P1.65 exhibit a more homogeneous microstructure, despitehaving similar nominal porosity, taken up by water. Thechange in microstructure is due to large reductions in thepore size of these specimens as the Si/Al ratio is increased,as a result of the differences in solution chemistry during

0

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Temperature C

Si/Al Ratio

Fig. 3. Weight loss of K-geopolymers measured by TGA from ambient to1000 C. The arrow indicates the increased rate of weight loss withincreasing Si/Al ratios of specimens from 1.15 to 2.15.

Fig. 4. SEM micrographs of K-geopolymer with Si/Al ratios of (a) 1.15, (b) 1.40, (c) 1.65 (d) 1.90 and (e) 2.15.



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reaction [13]. The change in the pore size distributionobserved in Fig. 4 can be correlated with the large increaseof Youngs modulus of K-geopolymer for Si/Al 6 1.65observed in mechanical data presented elsewhere [9].

Fig. 5 shows the microstructures of K1.15 geopolymerafter being exposed to temperatures of 300 C, 600 C

and 1000 C. The microstructure of the K1.15 specimendoes not appear to undergo signicant microstructural evo-lution as a result of exposure to temperatures up to1000 C, which may be expected from the small extent of shrinkage observed in Fig. 1. The microstructures of K1.65 geopolymer specimens after being exposed to thesame temperatures are presented in Fig. 6. The K1.65 spec-imen undergoes signicant extent of thermal shrinkageafter exposure to 300 C and 1000 C (Fig. 1), and can beobserved to undergo some extent of microstructural evolu-tion during heating, especially at 1000 C (Fig. 6c). TheK1.65 specimen without heat treatment ( Fig. 4c) can beobserved to exhibit a largely homogeneous microstructure.After heating to 300 C (Fig. 6a), the polished surface of the specimen can be observed to exhibit a less smooth sur-face, with numerous small pores and small cracks observedacross the cross-section of the microstructure. The poresobserved after heating to 300 C are in the order of 100 nm in size, while the cracks are similarly wide, withlengths ranging from several hundred nanometers to sev-eral microns ( Fig. 6a).

After heating to 600 C the K1.65 specimen can beobserved to exhibit a reduced number of cracks in themicrostructure, though the remaining cracks are larger

(Fig. 6b). The small pores readily seen in the microstruc-ture at 300 C are no longer observed, and the topographyof the gel phase appears smoother. The reduction in thenumber of cracks, pores and smooth topography of themicrostructure observed in Fig. 6b implies that the gelundergoes a signicant level of thermal relaxation and

healing of small cracks, which is consistent with reductionin surface area and joining of surfaces that occur as a resultof condensation during dehydroxylation [14]. After heatingto 1000 C (Fig. 6c), the microstructure of the K1.65 spec-imen can be observed to exhibit a more textured surface,with dark regions several hundred nanometers in size seenin the gel phase. The exposed edges of large pores appearsmoothed in comparison to the pores in the microstruc-tures of specimens exposed to lower temperatures, implyingthe material has softened and the surface roughness hasbeen reduced, driven by surface tension. The dark regionsin the gel appear evenly dispersed in the gel phase, butdo not appear within unreacted material. Therefore, thedark regions suggest a phase separation occurs within thegel, which may be indicative of nucleated crystallization.

Fig. 7 shows XRD diffractograms of the K1.15 andK1.65 specimens subjected to the same conditions as thosein the micrographs in Figs. 5 and 6. It can be observedclearly in Fig. 6b that the K1.65 specimen appears amor-phous at 600 C, but peaks correlating to leucite (KAlSiO 6 )and kaliophilite (KAlSiO 4 ) appear in the specimen exposedto 1000 C. The appearance of leucite and kaliophilite inthe K1.65 specimen only after exposure to 1000 Csupports the interpretation of the dark regions in the

Fig. 5. SEM micrographs of K1.15 geopolymer after heating to (a) 300 C, (b) 600 C and (c) 1000 C at a constant heating rate of 10 C min 1 before

quenching.

P. Duxson et al. / Journal of Non-Crystalline Solids 352 (2006) 55415555 5545


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micrographs in Fig. 7b. Although it can not be observed inthe micrograph presented in Fig. 5c, from Fig. 7a it can beobserved that the K1.15 specimen also develops crystallinecontent correlating to kaliophilite after exposure to1000 C. Crystallization in geopolymeric gel and the identi-cation and quantity of phases formed at high temperatureare analysed in greater detail in the second part of this arti-cle by quantitative XRD [15].

Fig. 8 presents comparisons of the thermal shrinkage of Na-, NaK- and K-geopolymer with 1.15 6 Si/Al 6 2.15.The specimens generally exhibit thermal shrinkage in theorder Na > NaK > K in Region II, with similar rates of shrinkage in Region III, while the onset temperature of

Region IV appears to follow K > NaK Na. The

NaK1.15 specimen appears to exhibit thermal shrinkagethat is the mean of the Na1.15 and K1.15 specimens. Ingeneral, the behavior of NaK-specimens tends more toNa-geopolymer than K-geopolymer as the Si/Al isincreased.

The derivative of the dilatometric data is shown inFig. 9, which allows for clearer observation of changes inthe rate of axial shrinkage and the onset temperatures of each region of thermal shrinkage. The onset temperatureof Region II is known to increase with Si/Al ratio for K-geopolymer ( Fig. 1) and Na-geopolymer [8], but the effectof alkali on the onset temperature of Region II is yet tobe identied. The onset temperature of Region II is higher

for K-geopolymer than Na-geopolymer at all Si/Al ratios.

Fig. 6. SEM micrographs of K1.65 geopolymer after heating to (a) 300 C, (b) 600 C and (c) 1000 C at a constant heating rate of 10 C min 1 beforequenching.

Fig. 7. XRD diffractograms of (a) K1.15 and (b) K1.65 geopolymer at (i) ambient temperature and after annealing at (ii) 300 C, (iii) 600 C and (iv)1000 C.



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exhibits characteristics typical of comparisons of specimensat all Si/Al ratios. The rate of dehydration decreases mar-ginally in the order of K > NaK > Na.

Fig. 10b indicates that the shrinkage of K-geopolymerduring dehydroxylation in Region III is marginally greaterthan that of the Na- and NaK-specimens, which exhibitsimilar values across the Si/Al ratios investigated in thecurrent work, especially at Si/Al ratio of 1.65. Fig. 10cshows the extent of thermal shrinkage occurring duringdensication and viscous sintering in Region IV. Theamount of shrinkage during densication can be observedto generally increase with Si/Al ratio in all alkali series,with the exception of the Na1.90 specimen that exhibits asmall degree of thermal expansion prior to 1000 C. At

high Si/Al ratio the extent of densication in NaK- and

K-specimens appears to reach a maximum ( Fig. 10c),whereas the Na2.15 specimen exhibits a large degree of shrinkage, characterized by a constant rate of shrinkageat high temperature (>900 C), which is typical of viscousow rather than densication. The NaK-specimens gener-ally exhibit the highest extents of densication for Si/Al 6 1.90, which correlates with the observations inFig. 8. K-geopolymers exhibit a reduced extent of densi-cation compared to the Na- and NaK-specimens(Fig. 10), despite the rate of densication in these speci-mens being greater than for the Na- and NaK-specimens(Fig. 9).

Fig. 12 shows the linear shrinkage of K1.65 andNaK1.65 for heating rates of 1, 2, 5, 10 and 20 C min 1

to allow comparison of previous results for the Na1.65

Fig. 9. Derivative thermal shrinkage of ( )Na-, ( )NaK- and ( )K-geopolymer with Si/Al ratios of (a) 1.15, (b) 1.40, (c) 1.65, (d) 1.90 and (e) 2.15.Data for Na-geopolymers taken from Duxson et al. [8].



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specimen from the previous study [8]. The extent of thermalshrinkage reduces with increasing constant heating rate inRegion I and II for both K- and NaK-geopolymer. How-ever, there is no readily observable correlation betweenthe nal extent of thermal shrinkage after heating to1000 C and heating rate. The thermal shrinkage of NaK-geopolymer is observed to be greater than for K-

geopolymer.

Fig. 13 shows the degree of linear shrinkage observed inthe four regions of thermal shrinkage for Na-, NaK- andK-geopolymer with Si/Al ratio of 1.65 subjected to differ-ent constant heating rates. The amount of shrinkage inRegions I and II decreases slightly with increased heatingrate for each alkali composition (ie. Na-, NaK- and K-geo-polymer) ( Fig. 13a). However, the extent of shrinkageobserved in Region II can be observed to followNa > NaK > K, as also observed readily in Fig. 10. Theextent of thermal shrinkage in Region III is similar forNa- and NaK-specimens and appears to be independentof heating rate ( Fig. 13b). The extent of thermal shrinkagefor the K1.65 specimen is comparatively higher in RegionIII, and displays some level of dependence on heating rate,though there is no clear trend.

Thermal shrinkage observed in Region IV during densi-cation and sintering increases considerably with increasedheating rate in all specimens ( Fig. 13c), though the trendand absolute extent of densication are different in eachspecimen. In general, the extent of shrinkage follows theorder NaK > Na > K. However, in Fig. 13c the extent of densication observed in Region IV for geopolymer speci-mens heated at 5 C min 1 is similar, but large differencesin the extent of shrinkage are observed between these spec-imens at both low and high heating rates. Therefore,

despite the similar appearance of the extent of densication

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01.15 1.40 1.65 1.90 2.15

Si/Al ratio

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a b

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Fig. 10. Thermal shrinkage of ( j ) Na-, (m ) NaK-, and ( ) K-geopolymers in (a) Region II, (b) Region III, and (c) Region IV. Data for Na-geopolymerstaken from Duxson et al. [8].

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Fig. 11. Comparison of the weight loss of Na- (thin line), NaK- (dottedline), and K- (bold line). geopolymers with Si/Al ratios of 1.65.



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for Na1.65 and NaK1.65 specimens at a heating rate of 5 C min 1 , the use of different heating rates is able to iden-tify signicantly different densication characteristics of these specimens, which would not be observed by analysisat a single heating rate of 5 C min 1 .

4. Discussion

4.1. Physical evolution of K-geopolymer

The increase in the onset temperature of Region II(Fig. 1) with increasing Si/Al ratio has been linked withthe increase of the Youngs modulus of Na-geopolymers[8]. The Youngs modulus of K-geopolymers is similar toNa-geopolymers [9], suggesting the trend of increased onsettemperature of Region II with increasing Si/Al ratio inFig. 1 may also be related to structural rigidity. Despitelarge compositional differences, specimens with Si/Al P1.65 exhibit almost identical thermal shrinkage with respectto temperature in Region II (up to approximately 300 C).

Furthermore, at the upper temperature of Region II the

K1.40 specimen displays a similar extent of thermal shrink-age as the specimens with Si/Al P 1.65. In comparison toall other K-geopolymers, the K1.15 specimen clearly exhib-its a reduced extent of shrinkage in Region II despite hav-ing the lowest onset temperature ( Fig. 1ii).

The trends observed in the DTA thermograms of Fig. 2

have also been observed during the dehydration of Na-geo-polymer [8], suggesting the characteristics of geopolymerdehydration are largely independent of alkali. However,the upper bound temperature of dehydration in K-geopoly-mers with low Si/Al ratio is lower than for Na-geopolymer.This is likely to be a reection of the comparativelydecreased ordering of K-geopolymer with low Si/Al ratio[11]. For instance, the K1.15 specimen exhibits an amor-phous XRD diffractogram after 7-days ageing, while theNa1.15 specimens are partially crystalline after the sameperiod of ageing [9]. Therefore, the K1.15 specimen is unli-kely to be able to retard dehydration compared to Na1.15specimens, which contain intercrystalline water. Further-more, the temperature of the minima in the endotherm of Na-geopolymer appears at approximately 100 C, whichis signicantly higher than that observed in Fig. 2 for K-geopolymer (i.e. 70 C). The decrease in the temperatureof the minima is most likely a result of the decreased energyof hydration of K aq compared to Na

aq [16]. Therefore,

water is more easily liberated from the hydration shell of alkali cations associated with aluminum in K-geopolymerthan Na-geopolymer.

Similar magnitudes of shrinkage and weight lossobserved in the TGA thermograms in Fig. 3 for specimenswith Si/Al P 1.65 up to approximately 700 C imply that

the physical and chemical distribution of hydroxyl sites inthese specimens is similar. The specimens with Si/Al 6! 1.40 lose water over a greater temperature region, indicat-ing that some of the hydroxyl groups on the surface of these specimens are more tightly bound than those on cor-responding specimens at higher Si/Al ratio. The nominalSi/Al ratio of geopolymer specimens determine the propor-tion of silanol and aluminol groups on the surface of thegel, with higher Si/Al ratio specimens containing a greaterproportion of silanol groups. Condensation of silanol oraluminol groups on the surface of the geopolymeric gelproceeds according to the following generalized exothermicreaction:

T OH HO T ! T O T H 2O 1

where: T is Al or Si. The energy of condensation for tetra-hedral linkages follows the order SiOAl > SiOSi > Al OAl [17]. Therefore, it may be expected that compara-tively higher temperatures will be required to fully dehydr-oxylate geopolymeric gel containing a greater proportionof aluminol groups, than those with larger amounts of sil-anol groups (i.e. lower Si/Al ratio). A greater fraction of weight loss can be observed to occur during dehydroxyla-tion (i.e. >250 C) in the specimens with Si/Al 6 1.40, com-pared to higher Si/Al ratio specimens. The differences in

the proportion of water liberated during dehydroxylation

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Fig. 12. Linear shrinkage of (a) K- and (b) NaK-geopolymer with Si/Al

ratios of 1.65 measured at heating rates of 1 C min1

, 2 C min1

,5 C min 1 , 10 C min 1 , and 20 C min 1 . The arrow indicates increasingconstant heating rate.



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The specimens with homogeneous microstructures exhibitlarge extents of shrinkage, implying that the distributionof porosity in these specimens and microstructural evolu-tion with temperature may be related to the extent of den-sication observed.

The pore volume of Na-geopolymer is known to

increase upon heating to 300 C [8], and correlates withthe more open and porous appearance of the microstruc-ture in Fig. 6a. This suggests that dehydration causes con-traction of the gel, resulting in aggregation of pores, whichappears as pores that can be observed by SEM, and thedevelopment of cracks where capillary strain forces exceedthe tensile strength of the gel. These observations are con-sistent with capillary strain causing the shrinkage observedin Region II and the localized microcracking of the gel torelease stress. The evolution of K-geopolymer microstruc-ture in Fig. 6 is consistent with the theoretical model forthermal shrinkage of geopolymers proposed previouslybased on nitrogen porosimetry and dilatometric results[8]. The observation of gel softening in the K1.65 specimenbetween 600 C and 1000 C provides evidence to explainthe large extent of shrinkage observed only where thehomogeneous gel microstructure is observed ( Fig. 4ce).In the low Si/Al ratio specimens the skeletal density of the gel is higher [13] with larger pores observed ( Fig. 4aand b) [13,18]. The skeletal density of the gel in the highSi/Al ratio specimens is lower, with porosity dispersedwithin the gel itself. Therefore, in the low Si/Al ratio spec-imens the gel is unable to undergo the same extent of cap-illary strain, structural relaxation and sintering uponheating as the high Si/Al ratio specimens, and less shrink-

age is observed (Fig. 1).

4.2. The effect of alkali cation on physical evolution

From analysis of K-geopolymer above, it is clear thatthe main characteristics of geopolymer evolution duringheating are present regardless of alkali cation (sodium of potassium). However, the structure and mechanical prop-erties of geopolymers are known to be subtly affected byboth the presence of different alkali cations, and someextent of mixed-alkali interactions [9,11]. Furthermore,the effect of alkali cation is known to change with Si/Alratio in geopolymers [9].

It is thought that the increase in the onset temperatureof Region II is linked to the improved mechanical proper-ties of geopolymer with increasing Si/Al ratio. However,the similarity of the mechanical properties of Na- and K-geopolymer [9] does not correlate with the difference inthe onset temperature of Region II in Figs. 8 and 9. Despitethe fact that mechanical properties are likely to play themajor role in determining the onset temperature of RegionII with respect to Si/Al ratio, it is clear from Fig. 9 that thenature of the alkali cation also affects the onset tempera-ture (independent of Si/Al ratio). Therefore, the behavior

of the NaK-geopolymer specimens should provide an

insight into the way that alkali affects the thermalshrinkage.

The behavior of the NaK-specimens in Region II(Fig. 9) indicates that the role of the alkali cation in ther-mal shrinkage changes with Si/Al ratio. At low Si/Alratios, NaK-geopolymer appears to behave as the average

of the pure Na- and K-specimens, while at higher Si/Alratios the behavior appears more heavily inuenced bysodium than potassium cation. The rates of thermal shrink-age during Region III (dehydroxylation) are similar forNa-, NaK- and K-geopolymer specimens at each Si/Alratio. Therefore, it appears that the alkali cation does notplay a signicant role in the extent of shrinkage of thegel during dehydroxylation. As only a small proportionof weight is lost as a result of dehydroxylation, it wouldnot be expected that any small differences in shrinkage dur-ing the condensation of TOT linkages (T is Al or Si) withdifferent alkali cations would greatly inuence the overallextent of thermal shrinkage.

The onset temperature and degree of thermal shrinkagein Region IV (viscous sintering) in Figs. 8 and 9 exhibit thegreatest variation with alkali type. The specimens with Si/Al ratio of 1.15 are observed to undergo only nominal den-sication above 700 C (Fig. 8a), especially when comparedto higher Si/Al ratio specimens ( Fig. 8be). Clear onsettemperatures of viscous sintering can be observed inFig. 9be, indicated by the rapid decrease in the value of the derivative beyond a temperature of approximately600800 C. As observed in the previous section, the onsettemperature of densication for K-geopolymer occursapproximately at 730 C, 780 C, 880 C and 930 C for

specimens with Si/Al ratios of 2.15, 1.90, 1.65 and 1.40,respectively. The onset temperature of sintering for K-geo-polymer is clearly higher than the Na- and NaK-geopoly-mer specimens at all Si/Al P 1.40 (Fig. 9). The onsettemperature of densication of Region IV is observed todecrease with increasing Si/Al ratio for all specimens.The reduction in the onset temperature to densicationwith increasing Si/Al ratio is thought to relate to reductionin the softening temperature of the specimens at high tem-perature with increasing Si/Al ratio [8].

While the strength of AlO bonds is weaker than SiObonds in the presence of sodium, they are expected to bestronger when potassium is present as the charge-balancingcation [19]. Therefore, the onset temperature of densica-tion of K-geopolymer should increase with decreasing Si/Al ratio, which is observed in Fig. 8. The same trend of reducing densication temperature with increasing Si/Alratio is observed in Na-geopolymer [8], which should exhi-bit an increase in the onset temperature of densicationwith increasing Si/Al ratio if AlO bond strength was todominate. Furthermore, the effect of unreacted materialin Na-geopolymer is thought to be important, leading toamounts of sodium not associated with aluminum in thegel (i.e. Al/Na < 1) [8]. The reduction in Al/M ratio iswidely known to reduce the softening temperature of alkali

aluminosilicates [1921] and would explain a reduction in



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the densication temperature with increasing Si/Al ratio inNa-geopolymer. Therefore, it may be implied that the effectof unreacted material on the gel composition has a largereffect than the changes in strength of AlO bonds in thepresence of different alkali cations.

Taking into account the possible effects of unreacted

material and bond strengths in K-geopolymer, the onsettemperature to densication should reduce with increasingSi/Al ratio, though it may be expected that the K-geopoly-mer specimens exhibit additional thermal stability from theincreased strength of AlO bonds compared to Na-geo-polymer, which is observed in Fig. 8. Furthermore, theamount of unreacted material in K-geopolymers is lessthan Na-geopolymer [10], so the effect of the free potassiumon softening may be reduced, which would further increasethe softening temperature. The analysis of the geopolymersin the current work is unable to distinguish between theeffects of AlO bond character and unreacted material onthe basis of the data presented in the current work, andis worthy of further investigation.

The onset temperature of Region IV is similar for allNaK-geopolymers and Na-geopolymers ( Fig. 9). Indeed,the onset temperature of Region IV in NaK-geopolymerswith Si/Al P 1.40 (Fig. 9b) is observed to be lower thanthat of the Na-specimens. For example, the NaK1.40 spec-imen begins to densify at a lower temperature (700 C) thanthe Na-specimen (740 C), while the K-specimen exhibitsrapid shrinkage only at 800 C (Fig. 9b). The similar orreduced onset temperature to densication of NaK-speci-mens is reasonable, given that the mechanism of densica-tion thought to be responsible for thermal shrinkage in

Region IV is viscous sintering [8], which is preceded bysoftening of the gel. Therefore, the reduction in the onsettemperature of densication for NaK-geopolymer belowthat of either Na- or K-geopolymers is likely to be theresult of a reduction in the viscosity of NaK-specimensnear the eutectic in the quaternary Na 2 OK 2 OAl 2 O3 SiO2 system [2023]. A reduction in the viscosity and soft-ening temperature of specimens is likely to have a greatereffect on densication temperature than changes in thebond strength due to the presence of potassium. It is alsoapparent from Fig. 8b that the NaK1.40 specimen under-goes a greater degree of densication than the Na1.40 spec-imen, indicating that the reduction in the barrier todensication is reduced greatly in the presence of mixed-alkali, correlating with a reduction in viscosity.

Sintering in specimens with Si/Al 6 1.65 exhibits a two-step densication process, as indicated by the two minimain Fig. 9ce. The maximum rate of densication for thesespecimens occurs in the order K > NaK > Na, implyingthat the addition of potassium hinders the onset of densi-cation, but also increases the rate of densication once ini-tiated. Furthermore, the difference in the densicationtemperature of the Na- and NaK-specimens compared tothe K-geopolymers is observed to decrease with increasingSi/Al ratio in specimens with Si/Al 6 1.65 from 200 C, to

100 C and


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the extent of thermal shrinkage observed in Region IV of geopolymers resulting from different alkali cations mustarise from differences in the response of the gel to thermaltreatment. The effect of alkali cation on the thermal shrink-age can be more clearly analysed by determining the effectof different heating rates of specimens with different alkali

cations.4.3. Effect of heating rate on physical evolution

It is well known that the gel densication process iskinetically limited [25], and that the rate of constant heat-ing in dilatometric experiments has a large effect on thedensication process of Na-geopolymer [8]. The indepen-dence of thermal shrinkage in Region III from heating ratein Fig. 12 implies that shrinkage in this region is not kinet-ically limited and is a function of temperature and cationalone, and is consistent with previous ndings that dehydr-oxylation is thermodynamically driven by Si/Al ratio.

The variation in the extent of densication at differentheating rates in Fig. 12 may be accounted for by consider-ation of the underlying processes that are likely to controlthe extent of densication. For instance, it has been previ-ously suggested that water may become entrapped in thespecimen at increased heating rates (either as molecularwater in small pores or as hydroxyl groups), which canreduce viscosity during sintering and increase densication[8]. Indeed, the increase in heating rate alone may be suffi-cient to reduce the viscosity of the gel and result inincreases in densication with heating rate [25]. The extentto which densication mechanisms are able to affect densi-

cation will be discussed in greater detail with respect toexperimental evidence of structural evolution in Part 2 of the work [15]. Nonetheless, at high constant heating rates,the extent of densication will ultimately be limited byeradication of porosity, which explains the limit of densi-cation observed in all specimens in Fig. 13. Therefore, it isconceivable that changes in material structure resultingfrom Si/Al ratio and alkali cation will directly affect theextent to which and rates where geopolymer densicationin Region IV will vary with constant heating rate.

In Fig. 13 the Na1.65 specimen exhibits a signicantincrease in the extent of shrinkage with increase of the heat-ing rate from 1 C min 1 to 5 C min 1 , while similarshrinkage is observed for heating rates greater than5 C min 1 (Fig. 13c). The densication of the K1.65 spec-imen increases for heating rates up to 10 C min 1 , beforeremaining constant between 10 C min 1 and 20 C min 1 .A higher value of heating rate above which little or nochange in the amount of densication of the K1.65 speci-men is observed (compared to Na1.65) may relate to a fas-ter limiting rate of water diffusion from polycondensationwithin the gel of the K-specimen compared to the Na-spec-imen. Therefore, the difference between the rate of diffu-sion/polycondensation and the rate of constant heating isreduced, which reduces the increase in driving force for

densication. The NaK1.65 specimen also exhibits an

increase in densication with increasing heating rate,although the extent of shrinkage is constant for heatingrates 6 5 C min 1 , and increases for 5 C min 1 6 heatingrate 6 20 C min 1 . The similarity in the extents of densi-cation for heating rates 6 5 C min 1 , implies that the den-sication processes in the NaK1.65 specimen are faster

than for the K-specimen, though little difference isobserved at high temperature in Fig. 11. However, higherrates of densication in mixed-alkali aluminosilicates arefeasible, with transport processes in mixed-alkali specimensbeing faster than in pure alkali specimens [26]. Furtherstudies of the structural evolution of geopolymer as a resultof exposure to elevated temperature, including the effect of heating rate and annealing, are explored in greater detail inPart 2 of the work [15].

5. Conclusions

It can be observed in the current work that the thermalshrinkage of geopolymeric materials derived from metaka-olin is greatly inuenced by the composition of the alkaliactivating solution, both in terms of Si/Al ratio and alkalicontent (sodium and potassium). These elements appear todetermine largely the characteristics of thermal shrinkageof geopolymers. The extent of thermal shrinkage observedin geopolymers based on different alkali compositionsincreases rapidly for 1.15 6 Si/Al 6 1.65, and is relativelyconstant at higher Si/Al ratios. Thermal shrinkage of allspecimens can be categorized by four characteristic regionsof structural resilience, dehydration, dehydroxylation andsintering. Microstructural analysis of K-geopolymer shows

that the increase in thermal shrinkage with increasing Si/Alratio relates to densication by reduction in porosity dur-ing dehydroxylation and sintering. Evolution of the micro-structure is not readily observed for low Si/Al ratiospecimens, which exhibit low thermal shrinkage. However,the microstructure of a high Si/Al ratio K-geopolymer wasobserved to exhibit signicant effects of dehydration, dehy-droxylation and viscous sintering.

The extent of thermal shrinkage observed in the geopoly-mers in the current work decreases in the order Na >NaK > K for specimens with Si/Al 6 1.65. In contrast, sim-ilar extents of thermal shrinkage have been observed for allalkali series at high Si/Al ratio (ie. Si/Al P 1.65). The alkalication type has a large effect on the onset temperature of vis-cous sintering observed in dilatometric data at all Si/Alratios, with K > NaK Na. The increase in resilience of K-geopolymer to densication is thought to be due toincreased strength of AlO bonds in the presence of potas-sium compared to sodium, which manifests itself as anincrease in the softening temperature of the gel. NaK-speci-mens are observed to behave more similarly to K-geopoly-mers at low Si/Al ratio, while they behave similarly to Na-geopolymer at high Si/Al ratio. The onset temperature of densication for geopolymers with different alkali cation isobserved to reduce with increasing Si/Al ratio. The extent

of densication observed during viscous sintering was



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observed to decrease in the order NaK > Na > K, alsothought to be a result of the increase in the viscosity of thesespecimens in the same order. The extent of thermal shrink-age of geopolymers has been observed to increase withincreasing constant heating rate for Na-, NaK- and K-geo-polymer specimens with Si/Al ratio of 1.65. The increase in

densication withheating rate was observed to be dependenton alkali cation, implying that the rates of important pro-cessesoccurringduringdensication areaffected by thepres-ence of different alkali cations.

The use of different cations in geopolymers intended forhigh temperature applications is only signicant when usinglow Si/Al ratio compositions, and demonstrates that thealkali cation and Si/Al ratio play reduced roles in thermalshrinkage of geopolymer as the Si/Al ratio increases. Thereduction in the effect of alkali and the dominance of nom-inal Si/Al ratio and microstructure on the thermal shrink-age properties of geopolymer, particularly at low Si/Alratios, correlates with and validates the structural modelof geopolymers which has been introduced recently [10,11].

Acknowledgements

The authors gratefully acknowledge the nancial sup-port of the Particulate Fluids Processing Centre (PFPC),a Special Research Centre of the Australian ResearchCouncil (ARC). Dr Tim Sercombe in the Division of Mate-rials of the School of Engineering at the University of Queensland is thanked for help in obtaining some of thedilatometric data.

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[5] V.F.F. Barbosa, K.J.D. MacKenzie, Mater. Res. Bull. 38 (2) (2003)319.

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