formation of ceramics from metakaolin-based geopolymers. part ii: k-based geopolymer

Formation of Ceramics from Metakaolin-Based Geopolymers. Part II:K-Based Geopolymer

Jonathan L. Bell,* Patrick E. Driemeyer,* and Waltraud M. Kriven*,w,**

Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

The structural evolution and crystallization of potassium-basedgeopolymer (K2O .Al2O3

. 4SiO2. 11H2O) on heating was stud-

ied by a variety of techniques. On heating from 850–11001C,potassium-geopolymer underwent significant shrinkage and sur-face area reduction due to viscous sintering. Small, 15–20 nmsized precipitates present in the unheated geopolymer coarsenedsubstantially in samples heated between 9001 and 10001C. How-ever, the microstructural surface texture was dependent on thecalcination conditions. Leucite crystallized as the major phaseafter being heated to410001C, although a minor amount ofkalsilite was also formed. Prolonged heating for 24 h at 10001Cled to the formation ofB80 wt% of leucite, along with 20 wt%of remnant glassy phase. The surface of geopolymers heated to10001C attained a smooth, glassy texture, although closed po-rosity persisted until 11001C. Thermal shrinkage was completedby 11001C, and the material reached 99.7% of the theoreticaldensity of tetragonal leucite.

I. Introduction

GEOPOLYMERS are aluminosilicate binders which harden atlow temperature (B251–801C) in a relatively short amount

of time (B2–48 h).1 They are typically made by mixing an al-uminosilicate source such as metakaolin or fly ash into alkalinesilicate solution and curing in a sealed environment. The neces-sary processing details required to fabricate high-strength, well-reacted geopolymers have been established in previous work.2–4

Geopolymers are being considered for a variety of applicationsincluding low CO2 producing cements,5 fiber-reinforced com-posites,6 refractories,1 and as precursors to ceramic formation.7

Geopolymers are generally X-ray amorphous if cured at stan-dard pressures and temperatures o801C,1,8 but convert intocrystalline ceramic phases upon heating.9,10 Potassium-basedgeopolymers are more refractory compared with sodium-basedsystems, and do not melt until around 14001C.10 Geopolymersof the composition K2O �Al2O3 � 4SiO2 � 11H2O have been shownto crystallize into leucite (K2O �Al2O3 � 4SiO2) upon heating.11

Leucite ceramics tolerate a high degree of ionic substitu-tion,12,13 and the thermal expansion of leucite is lowered bythe incorporation of cesium.14,15 Tetragonal leucite, the stablephase at low temperature, has a high thermal expansion (15.1–31� 10�61C�1),14 which makes it useful in a variety of metalbonding applications. For example, dental porcelains used inporcelain-fused-to-metal restorations often rely on tetragonalleucite to increase thermal expansion.16,17 Geopolymers havebeen shown to bond well with metals18,19 and can be applied as arefractory coating or adhesive in which the thermal expansioncan be tailored via alkali variation to match that of the metal. In

leucite-based dental porcelains, it is desirable to have a highvolume fraction of small leucite grains in order to balance bothstrength and aesthetics.20 The use of geopolymer precursors mayprovide a novel approach by which these properties can beobtained.

In part I of this work, the thermal evolution and crystalli-zation of pollucite from Cs2O �Al2O3 � 4SiO2 � 11H2O geopoly-mer was investigated using a variety of techniques. In thisinvestigation, the physical evolution and densification ofK2O �Al2O3 � 4SiO2 � 11H2O geopolymer on heating was studiedusing dilatometry, nitrogen absorption/desorption (by theBrunauer–Emmett–Teller (BET) method21), pycnometry, the-rmogravimetric analysis (TGA), and electron microscopy. Crys-tallization into leucite was examined using X-ray diffraction(XRD), differential scanning calorimetery and (DSC), and scan-ning electron microscopy/transmission electron microscopy(SEM/TEM).

II. Experimental Procedures

(1) Geopolymer Synthesis

A 10.1M potassium silicate solution (K2O � 2SiO2 � 11H2O) wasmade by dissolving potassium hydroxide pellets (� 85 wt% met-als basis, Fisher Scientific, Pittsburgh, PA) in deionized water,followed by addition of a proportional amount of amorphousfumed silica (Cabot EH-5, T. H. Hilson Company, Wheaton,IL). The solution was mixed overnight to ensure silica dissolu-tion. Geopolymer of composition K2O �Al2O3 � 4SiO2 � 11H2O(KGP) was then prepared by mixing metakaolin (Al2O3 � 2SiO2,BASF Metamax high reactivity metakaolin, Iselin, NJ) into thepotassium silicate solution using an IKA overhead mixer (ModelRW 20, Wilmington, NC) equipped with a dispersion blade. Themetakaolin used in this study had an average particle size of 1.2mm, a specific surface area of 13 m2/g, and was of 497% purity.

The resultant slurry was cast into 50 mL centrifuge tubes(Corning Inc., Corning, NY), sealed, and cured at 501C for 24 h.The hardened geopolymer was then removed from the tubes,fractured into B10 g specimens, and stored in 25 mm� 25mm� 25 mm snap action polystyrene containers. Samples wereprepared for SEM, BET, XRD, and pycnometry analysis byheating the fracture specimens in a Radatherm high temperaturefurnace (Model HT 05/18, Wetherill Park, NSW, Australia) be-tween 8501 and 11001C, to a variety of temperatures in a plat-inum crucible. These samples were heated and cooled at 101C/min,with no isothermal soak. A few additional samples were preparedby heating to 9001C for a 10 h soak (101C/min heating and coolingrate), 10251C for 5 h (51C/min heating and cooling rate), and11001C, for 24 h soak (101C/min heating and cooling rate).

In order to estimate the weight fraction of leucite formed onheating, KGP fracture samples were heated in a Blue M furnace(Model 2010C-3, Blue M Electric Co., Blue Island, IL) at 201C/min to 10001, 10251, 10501, and 10751C. A total of six sampleswere placed in the furnace for each temperature, and were sub-sequently air quenched after being heated isothermally for a spec-ified period of time. The heated samples were then ground andsieved tor44 mm, mixed with a 10 wt% Si standard (NIST SRM640B, Gaithersburg, MD), and prepared for X-ray analysis.

C. Jantzen—contributing editor

This work was supported by Air Force Office of Scientific Research (AFOSR), USAF,under Nanoinitiative Grant No. FA9550-06-1-0221, through Dr. Joan Fuller.

*Member, The American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: [email protected]**Fellow, The American Ceramic Society.

Manuscript No. 25058. Received July 31, 2008; approved December 3, 2008.

Journal

J. Am. Ceram. Soc., 92 [3] 607–615 (2009)

DOI: 10.1111/j.1551-2916.2008.02922.x

r 2009 The University of Illinois at Urbana-Champaign

607

i:/BWUS/JACE/02922/[email protected]

(2) Analytical Techniques

SEM analysis was carried out on a Hitachi S-4700 high resolu-tion SEM (Hitachi High Technologies, Schaumburg, IL) onfracture surfaces. Some of the samples were etched in 3 wt%HFat 451C for 3 s to assist in crystalline phase identification. AllSEM samples were mounted on aluminum stubs and sputter-coated with B6 nm of a Au/Pd alloy to facilitate imaging.Additionally, samples were prepared for energy dispersive X-rayanalysis (EDS) by carbon coating fracture samples. Flat sampleregions were examined using a JEOL 6060 LV SEM (JEOLUSA Inc., Peabody, MA) operated at 20 kV and a 10 mmworking distance. Copper was used to calibrate the energy and aminimum of six acquisitions were taken on each sample.

BET analysis was performed on fracture samples using aQuantachrome Instruments Surface Area and Pore Analyzer(Model Nova 2200e, Boynton Beach, FL). Samples were de-gassed and dried under vacuum at 1501C for 24 h before beinganalyzed using nitrogen absorption/desorption. Surface areaswere calculated using the BET method.21 The density of samplepowders was determined using a helium-based pycnometer(Model 1330, Micromeritics, Norcross, GA). After being filledwith powder, the sample chamber was purged 50 times beforeanalysis to ensure removal of atmospheric gases. A total of 10measurements were acquired for each sample.

XRD patterns were collected using a Rigaku X-ray powderdiffractometer (Model D-max II, Danvers, MA) equipped witha CuKa source (l5 0.1540598 nm). A single crystal mono-chromator in the diffracted beam path was used to acquire X-ray data in Bragg-Brentano geometry, over a 2y range of 51–701with a step size of 0.021. Before X-ray analysis, fracture speci-mens were ground to powders and sieved to less than 325 mesh(r44 mm). Jade 7 software (Minerals Data Inc., Livermore, CA)was used to determine the phases present and the weight fractionof leucite via whole pattern fitting of X-ray data.22

Simultaneous TGA and DSC studies were conducted onr44mm powders up to 12501C at 101C/min in a Netzsch DSC/TGA(Model STA409 CDt, Export, PA) instrument. An aluminapan fitted with a lid was used to hold the specimen and as areference. During the analysis, the sample chamber was purgedwith He (25 mL/min) and air (50 mL/min). Specimens wereprepared for dilatometry by casting KGP into a 3.97-mm-di-ameter Tygon tube. Samples were subsequently cut to B15 mmlength using a Beuhlert low speed diamond saw (Isomet series,Lake Bluff, IL) to ensure their ends were flat, and were thenanalyzed in a Netzsch dilatometer (Model DIL 402 E) up to14001C at 101C/min in air.

TEM work was conducted on both powder and thin, ion-milled samples, using a JEOL 2010F scanning transmission elec-tron microscope (S)TEM. This TEM was operated at 200 kVand was equipped with an Oxford INCA 30 mm ATW EDSdetector (Oxford Instruments Concord NanoAnalysis, Concord,MA). Powdered samples were prepared by grinding KGP using amortar and pestle, followed by dispersion in ethanol, and depo-sition on holey, carbon-coated copper grids. Ion milled sampleswere prepared by slicing 300 mm thick disks from 25.4-mm-length� 6.35 mm KGP cylinders, using a Buehler low speed di-amond saw. These disks were then heated to 11001C for 24 h in aRadatherm high temperature furnace, and subsequently cut into3-mm-diameter disks using a Gatan ultrasonic disk cutter (Model601, Warrendale, PA). The 3-mm-disks were thinned to 100 mmusing a Buehler Minimet disk polisher (Model Ecomnet III), fol-lowed by dimple grinding to 20 mm with a Gatan dimple grinder(Model 656). Finally, samples were ion-milled at low tempera-ture, using a Fischione ion mill (Model 2, Export, PA).

III. Results and Discussion

(1) X-ray Analysis

X-ray patterns for KGP after being heated to a variety of tem-peratures (101C/min heating and cooling rate, no isothermalsoak) are shown in Fig. 1. Unheated KGP was X-ray amor-

phous and had a large diffuse peak centered at 281 2y, as hasbeen observed in previous work.1 However, a small peak at25.51 2y for KGP was due to TiO2 impurity (1.7 wt%) present inthe metakaolin used to fabricate the geopolymer. On heating,the X-ray pattern for KGP remained predominately amorphousuntil about 10001C. The major reflections for kalsilite(K2O �Al2O3 � 2SiO2) and leucite (K2O �Al2O3 � 4SiO2) first ap-peared at 27.41 and 28.81 2y respectively, after being heated to9751C. In samples heated to higher temperatures, leucite was themajor phase formed (space group I41/a), while the major reflec-tion for kalsilite grew only slightly. The leucite phase formedwas tetragonal at room temperature after cooling, but is ex-pected to be cubic above 6301C.23

In order to estimate the weight fraction of leucite formed,KGP was calcined isothermally at 10001, 10251, 10501, and10751C (Fig. 2). When heated at 10001C for 24 h, B80 wt%leucite was formed. In samples calcined at 10501 and 10751C, ittook only B1 h to form470 wt% leucite. The remaining B20–30 wt% of heated samples was believed be an amorphous glassyphase, as has been suggested in previous work.11 Compositionalheterogeneities, along with the presence of free alkali in the geo-polymer structure, favored the formation of a glassy phase onheating.

The formation of glassy phases and kalsilite are common im-purities in chemically derived leucite.24 Moreover, there is asubstantial amount of glass formation in the K2O–Al2O3–SiO2

system near the leucite composition field as shown in Fig. 3. Thisis largely due to the difficulty of crystallizing in the highly vis-cous glasses produced in this system.25,26 Any deviation fromleucite composition or inhomogeneities in the KGP can facilitatethis glass formation. The formation of mullite or alumina wasnot observed upon heating. Therefore, liquids formed duringheating of KGP were expected to be on the K2O rich side of thephase diagram (Fig. 3). The glassy phase composition can varysuch that the overall mass balance is maintained.

In potassium-based geopolymers made using metakaolin asthe aluminosilicate source, it is well-known that not all of themetakaolin is dissolved before setting.2,3,27,28 This is especiallytrue in systems having an 4SiO2/Al2O3 � 4, such as the geo-

Fig. 1. X-ray diffractograms for unheated K-geopolymer, and afterbeing heated to the specified temperature at 101C/min heating and cool-ing rates, with no isothermal soak. After heating to � 10001C and cool-ing, KGP crystallized into tetragonal leucite (K2O�Al2O3�4SiO2) and aminor amount of kalsilite (K2O�Al2O3�2SiO2). A best fit to the crystal-line data was determined using Jade 8.0 software. The major leucite re-flections (D) and major reflection for kalsilite (.) are labeled above.

608 Journal of the American Ceramic Society—Bell et al. Vol. 92, No. 3

polymer examined here. Metakaolin is the only KGP sourcematerial that contains Al. Because it is not completely dissolved,some Al will be present that is not bound within the geopolymerstructure. Charge balance requires that K1 ions balance thenegative charge on Al31. However, because not all of the Al wasdissolved and incorporated into the geopolymer matrix, therewill also be excess alkali that is not bound within the geopolymermatrix. The presence of alkali has been found to persist in thepore water of some geopolymers.27,29 Additionally, alkali hasbeen shown to leach out when geopolymers are placed in deion-ized water.30

On heating, pore water will be evaporated leaving behind al-kali on surfaces which can subsequently react with silica and

alumina to form various crystalline or amorphous K-based sil-icates or K-based aluminosilicates.31 Most alkali silicates crys-tallize with difficulty, so that they tend to form glassy phases.31

This glass forming tendency increases with silica content anddecreasing atomic weight of the alkali metal.31 In a part I of thiswork,32 glass formation was also observed in Cs-based geopoly-mer (CsGP).

Despite these complications, B80 wt% of leucite was formedin heated KGP. This amount of leucite formation was higherthan that observed in previous investigations of heated K-basedgeopolymers. Duxson et al.33,34 examined the thermal behaviorof a variety of K-based geopolymers as a function of varying Si/Al atomic ratio (Si/Al5 1.15, 1.40, 1.65, 1.90, and 2.15). Leuciteand kaliophilite (K2O �Al2O3 � 2SiO2) formation was observed insamples with a Si/Al5 1.40, 1.65, and 1.90, while only leuciteformation was observed in samples with a Si/Al5 2.15. Samplesanalyzed for crystalline content were heated to a maximum of10001C for 2 h, ando50 wt% of leucite formation was observedfor Si/Al5 1.9 and 2.15. The leucite content for the sample pre-pared with a Si/Al5 2.15, decreased on heating above 10001C.A similar trend has been observed in the formation of leucitefrom potash feldspar (K2O � SiO2 � 6SiO2). Due to the higheramount of SiO2 in feldspar, it melts incongruently to leuciteplus liquid above 11501C (Fig. 3).35 Upon multiple firings, theleucite content decreases along with the formation of sanidine(KAlSi3O8).

17

Fig. 2. Weight fraction of leucite crystallized as a function of time forKGP soaked at 10001C (& ), 10251C (J), 10501C (m), and 10751C (H).Fracture samples were heated in a platinum crucible at 201C/min for thespecified amount of time, and were subsequently air quenched.Quenched sample were then ground and mixed with 10 wt% siliconstandard (NIST SRM 640b) and were analyzed over a 2y range of 5–701with a Rigaku D-Max II XRD (CuKa source, l5 0.1540598 nm, stepsize of 0.021, 11/min acquisition rate). The weight fraction of leucite wasdetermined by whole pattern fitting of X-ray data using Jade 7 software(Minerals Data Inc., Livermore, CA).

Fig. 3. Isothermal section of the K2O–SiO2–Al2O3 phase diagram at12001C (from Kingery et al.25). The composition of KGP examined inthis study corresponds to leucite (KAS4).

Fig. 4. (a) DSC/TG, (b) dilatometry (c) the derivative thermal shrink-age and (d) dilatometry results on subsequent heatings showing a re-versible phase transition at B6501C. Samples were heated at 101C/minup to 14001C in air. The characteristic regions are numbered at thebottom of (b). The derivative of thermal shrinkage shown in (c) moreclearly shows changes in axial shrinkage. DSC, differential scanningcalorimetery.

March 2009 Formation of Ceramics from Metakaolin-Based Geopolymers 609

KGP analyzed in this work was found to have an averageatomic Si/Al ratio of 1.9870.03 as determined from SEM–EDS.This was close to the ideal composition for leucite. In sol–gel-based systems, having a precursor composition equivalent tothat of leucite has been found to favor leucite formation onheating. For example, Zhang et al.36 produced high purity leu-cite powders by heating sol–gel precursors with starting atomicratios Si/Al/K of 2:1:1.

(2) Thermal Analysis

As shown in Fig. 4(a), the DSC pattern for KGP heated at 101C/min had an exotherm centered at B11301C due to leucite crys-tallization over the range of 10801–11501C. This was in agree-ment with the expected crystallization temperature observedfrom X-ray results. Zhang et al.36 observed an exotherm dueto leucite crystallization from a sol–gel-based precursor over therange of approximately 10001–12001C, with the peak centered at11701C. Duxson et al.34 did not observe a substantial leucitecrystallization exotherm for K-geopolymers with a Si/Al5 1.9or 2.15. This was consistent with the small amount of leuciteformed.

A large endotherm was present over the range of 251–4001Cand had a minimum value at B901C (Fig. 4(a)). TG data indi-cated that a dramatic weight loss occurred over this temperaturerange and was attributed to evaporation of free water. In part Iof this work, the endotherm minimum value was centered atB1001C for CsGP. The slightly lower value for KGP may berelated to the structure and degree of order within the geopoly-mer. The X-ray pattern for CsGP contained diffuse peaksmatching those of pollucite32 which suggested a higher degreeof atomic order than that observed for KGP. As shown inFig. 1, the X-ray pattern of unheated KGP was devoid of anystructural features other than a diffuse peak centered at 28.01 2y.In the less ordered KGP, smaller K1 will be less tightly packedin the tetrahedral framework compared with CsGP, leavingmore space and mobility for water molecules. This will allowfor water to be more easily removed from KGP.

According to dilatometry results (Fig. 4(b–d)), the thermalshrinkage of KGP could be divided into four regions. Similarresults have been observed in a variety of geopolymers, regard-less of alkali choice (Cs, K, Na) or Si/Al ratio.32–34,37 A summaryof the shrinkage, densification, weight loss, and surface areaevolution on heating is given in Table I for both K- and CsGP.

In region I, little shrinkage was observed for KGP on heatingfrom room temperature to 1001C (Fig. 4(b)). However, therewas substantial weight loss due to water evaporation from largepores and surfaces. In this region, KGP had a lower extent ofdensification and weight loss compared with CsGP. This differ-ence in behavior can be attributed to surface area and hydrationenergy considerations. The specific surface area after being de-

gassed at 1501C for 24 h was measured at 66.6 and 116.8 m2/gfor KGP and CsGP, respectively (Fig. 5). Since KGP had alower starting surface area, there was expected to be less freelyevaporable water from surfaces. Additionally, the K1 ion has alarger hydration energy compared with Cs1 and is therefore lesslikely to dehydrate at lower temperature. A similar trend wasobserved when comparing Na- and K-based geopolymers.33 TheNa-geopolymer had a lower rate of dehydration in region I dueto its higher hydration energy.

Because pycnometry was used to measure densification, thelower extent of densification in region I for KGP compared withCsGP was related to the extent of water loss. As water was re-moved, the He gas could more easily permeate the structureleading to a lower estimate of the sample volume and a higherapparent density. Because less water was removed from KGP, ithad a lower extent of densification in region I.

Over the range of 1001–3001C, the shrinkage and weight lossin region II was due to capillary contraction created as waterwas desorbed from small pores. As shown in Fig. 4(c), the max-imum rate of shrinkage occurred at B1751C. In this region,there was a larger relative mass loss, shrinkage, and surface areareduction for KGP compared with CsGP. Above 1001C, theintrinsic geopolymer gel structure is expected to play a largerrole than cation hydration. Because KGP had a lower initialsurface area and extent of water loss in region I, more water wasexpected to be bound within the gel pores, rather than on sur-faces, as was the case for CsGP.

Table I. Summary of Thermal Analysis Results for KGP and CsGP

GP Type Physical parameter Region I Region II Region III Region IV

KGP Temperature range (1C) 25–100 100–300 300–850 4850Shrinkage (% change over 251–11001C) 1.48 22.65 20.31 55.55Overall shrinkage (dL/L) 3.2� 10�2 5.3� 10�2 9.7� 10�2 2.2� 10�1

Apparent density (% change over 251–11001C) 3.84 41.02 41.21 13.92Surface area (% change over 251–11001C) – 21.32 17.65 61.02TG (% mass change over 251–11001C) 34.45 55.68 9.74 0.11

CsGPw Temperature range (1C) 25–100 100–300 300–1200 41200Shrinkage (% change over 251–14501C) 0.07 8.78 8.3 82.85Overall shrinkage (dL/L) 3.4� 10�5 9.5� 10�3 1.9� 10�2 1.0� 10�1

Apparent density (% change over 251–16001C) 19.17 33.64 40.59 6.60Surface area (% change over 1001–15001C) – 14.10 65.50 20.39TG (% mass change 251–14001C) 48.38 43.61 7.48 0.52

wTaken from part I of this work32 for CsGP samples heated at 101C/min. Dilatometry results were collected in situ, while pycnometry and BET were collected after samples

had been cooled at 101C/min to room temperature. The values given for shrinkage, apparent density, and specific surface area represent the percentage change within each

region relative to the total change over the measurement temperature range. A value for the specific surface area change in region I could not be given as samples were

degassed at 1501C before BET analysis. BET, Brunauer–Emmett–Teller; CsGP, Cs-based geopolymer.

Fig. 5. Apparent density measured on r44 mm sieved sample powdersusing pycnometry. The specific surface area was determined using theBET method21 on fracture specimens. Results were collected after sam-ples were heated to the specified temperature and cooled to room tem-perature at 101C/min. BET, Brunauer–Emmett–Teller.


The higher thermal energy in region II was sufficient toremove this water from the pores of KGP leading to higherweight loss, shrinkage, and surface area reduction comparedwith CsGP. The pores size and distribution will also be a factorin region II. Cs1 is a larger ion than K1 and is expected to forma geopolymer which has less framework pore space. Forexample, in natural pollucite, there is insufficient space forwater to be located in cavities containing Cs1.38 KGP maytherefore be able to accommodate water more easily withinits disordered tetrahedral structure. Similar behavior has beenobserved in crystalline zeolites. Less thermal energy is re-quired to dehydrate crystalline zeolites with larger frameworkchannels.39

Region III for KGP occurred over a temperature rangeof approximately 3001–8501C. CsGP was found to be farmore refractory as region III extended from 3001 to 12001C.In pollucite, the larger Cs1 has been shown to be moreeffective at ‘‘propping’’ up the framework leading to lower ther-mal expansion and a higher melting point compared with leu-cite.40 A similar argument can be extended to the geopolymerstructure. In fact, the short to medium range order ofK- and CsGP was found to resemble that of pollucite and leu-cite, respectively.41,42

In region III, water loss and shrinkage occurred concomi-tantly due to removal of water by polycondensation of T–OHgroups (T5Si, Al), also known as dehydroxylation. Both KGP

Fig. 6. SEM micrographs of unheated KGP fracture surfaces showing (a) partially reacted metakaolin (sheet-like particles) surrounded by the geo-polymer binder phase, and a (b) a high magnification view of the geopolymer binder phase showing the presence of 10–20 nm sized precipitates. Samples(a) and (b) were cured at 501C for 24 h. SEM, scanning electron microscopy.

Fig. 7. High-resolution SEMmicrographs of KGP fracture surfaces after being heated to (a) 9251C, (b) 9501C, (c) 9751C, and (d) 10001C. Samples wereheated and cooled at 101C/min, with no isothermal soak. Coarsening of the precipitate structure was due primarily to viscous sintering. After beingheated to 10001C, the surface of KGP had a smooth, glassy texture. SEM, scanning electron microscopy.


and CsGP underwent extensive densification and a nominalamount of weight loss. However, the extent of surface area re-duction was much less for KGP. This is likely related to thelower initial surface area of KGP and the more narrow temper-ature range observed for region III. KGP had a larger amountof shrinkage, which was partially due to its higher amount ofwater loss. In addition, KGP did not crystallize into leucite untilB10001C, which was above the temperature range for region III.However, the development of long-range order due to polluciteformation from CsGP was evident by 10001C, which was withinits region III temperature range. The presence of inclusions orcrystalites before the onset of sintering will slow the sinteringrate by raising the viscosity of the matrix.43–45 Therefore, thesecrystallites may have postponed the onset of significant shrink-age for CsGP.

Leucite is known to undergo a reversible tetragonal to cubicinversion on heating at 6201–6901C, leading to a large volumedecrease and ferroelastic spontaneous deformation of the cubiccell.46,47 Dilatometry results for KGP reheated for a second andthird time up to 14001C clearly show this transformation(Fig. 4(d)). The inversion temperature on heating and coolingat 101C/min was observed at B6501C. Below 6501C, the mate-rial had a high degree of thermal expansion in the tetragonalphase. Above 6501C, the expansion was minimal as a function oftemperature for cubic leucite.

Structural changes in region IV (above 8501C) were attrib-uted primarily to viscous sintering. As shown in Fig. 4(b) andFig. 5, substantial changes in shrinkage rate, surface area, andapparent density were observed in region IV. Approximately

55% of the overall shrinkage occurred in this region for KGPdue to both skeletal densification and elimination of large pores.The significant shrinkage above 8501C was consistent with thelarge reduction in specific surface area and increase in densityabove 9001C. In fact, after being heated between 9001 and9251C, the surface area reduced from 51.5 to 21.7 m2/g andthe apparent density decreased from 2.44 to 2.35 g/cm3. Thisdecrease in density was attributed to the creation of closedpores. CsGP also exhibited a decrease in measured densityover 11001–14001C, which increased on heating to 16001C. Inregion IV, viscous sintering leads to the closure of previouslyaccessible pores and pore channels. This in turn means the Hegas used in pycnometry is unable to reach the closed pores cre-ated during sintering leading to a larger estimate of the samplevolume and lower calculated density.

Shrinkage caused by viscous sintering reached a maximum at9441C. On heating to 11001C the shrinkage was fully completed.The maximum rate of change for density and specific surfacearea was measured at a lower temperature (i.e., between 9001and 9251C). This was because dilatometer samples were exam-ined in situ, while pycnometry, BET, and SEM samples wereanalyzed after being cooled at 101C/min to room temperature.Therefore, the samples analyzed after being cooled to roomtemperature were exposed to high temperature for an extendedperiod of time.

KGP examined in this study was found to be more refractorythan K-based geopolymers analyzed by Duxson et al.33 The re-gion I–III onset temperatures for the KGP in this study weregenerally consistent with those determined by Duxson. How-ever, Duxson observed a region IV onset temperature of 7801and 7301C for KGP with a Si/Al5 1.9 and 2.15, respectively.The higher onset observed for KGP in this study (B8501C) wasexpected to be primarily related to the composition. As shown inFig. 3, precursors with a Si/Al5 1.9 and 2.15, will form leuciteplus liquid and were therefore less refractory. Other importantfactors to consider are the purity and amount of unreactedmetakaolin present in the geopolymer. Geopolymers containingmore unreacted metakaolin are expected to have additional freealkali. The metakaolin used in this work was BASF Metamaxhigh reactivity metakaolin, which has an average particle sizeand specific surface area of 1.2 mm and 13 m2/g, respectively.The metakaolin used by Duxson et al.33,34 had similar physicalcharacteristics with an average particle size and specific surfacearea of 1.58 mm and 12.7 m2/g respectively.

(3) Microstructure Analysis

SEM micrographs for unheated KGP fracture surfaces areshown in Figs. 6(a) and (b). A low magnification fracture sur-face (Fig. 6(a)) revealed that KGP consisted of a geopolymerbinder phase, but also contained partially reacted metakaolin

Fig. 8. SEMmicrograph for KGP fracture surface after being heated to9001C for 10 h soak. Samples were heated and cooled at 101C/min.SEM, scanning electron microscopy.

Fig. 9. SEM micrographs of KGP fracture surfaces after being heated to (a) 10401C showing the presence of partially reacted metakaolin and(b) 11001C showing that all large voids have healed. Samples were heated and cooled at 101C/min, with no isothermal soak. SEM, scanning electronmicroscopy.


and large voids left behind from remnant metakaolin. Thebinder portion of the KGP contained small precipitates ashas been found in previous work (Fig. 6(b)).3 As shown inFigs. 7(a–d), these precipitates coarsened substantially afterKGP was heated from 9251 to 10001C. This coarsening wasconsistent with the large amount of densification, surface areareduction, and shrinkage observed over the same temperaturerange. After being heated to 10001C, the KGP developed asmooth, glassy surface.

The nature of precipitate coarsening and resultant micro-structure for KGP was dependent on the calcination conditions.For example, by heating KGP at 101C/min to 9001C with noisothermal soak, and cooling at 101C/min, no structural changescould be observed and the sample surface looked similar to theunheated KGP shown in Fig. 6(a). When KGP was instead heldat 9001C for 10 h soak time, the KGP precipitates coarsenedsubstantially and adopted a surface texture which was differentfrom that obtained on heating the samples to a variety of tem-peratures with no isothermal soak (Fig. 8).

Although the binder phase of KGP had a glassy texture afterbeing heated to 10001C, voids left behind from partially reactedmetakaolin were still present until B11001C, as shown in Figs.9(a) and (b). This was consistent with the densification trendsobserved in Fig. 5 via pycnometry analysis. Above 9001C, theonset of significant microstructural coarsening and viscous sinte-ring led to the creation of closed pores. It was not until KGPwas heated to 11001C and the closed pores were removed thatthe sample reached 99.72% of the theoretical density for tetrag-onal leucite based on lattice parameters from Mazzi et al.23 Be-cause closed pores were observed prior to crystallization, this

supports the notion that closed porosity was related to the in-trinsic gel pores, entrapped gas, or voids from unreacted meta-kaolin rather than being due to pore entrapment in growinggrains.

The formation of small leucite crystallites (B1–2 mm) was ob-served in KGP fracture surfaces after being heated to 10001Cand etched (3 wt% HF, 3 s, 451C) as shown in Fig. 10(a). How-ever, the majority of the sample was glassy and only a minorfraction of leucite crystallization was expected from X-ray anal-ysis at 10001C. Additionally, the formation of KF needles wasfound on many regions of the KGP sample surface heated to11001C and etched (Fig. 10(b)). This suggested that K was pres-ent in a glassy or noncrystalline state which could easily be re-moved. Because not all of the metakaolin was dissolved in KGP,there is expected to be free alkali present which can react withsilica and alumina to form glassy phases.

Consistent with the X-ray results presented in Fig. 2, heatingKGP at 10251C for 5 h produced a substantial amount of leucitecrystallites. The unetched fracture surface, shown in Fig. 11(a),contained polydisperse leucite crystallites which were sur-rounded by a smooth, glassy phase. After the glassy phasewas etched away (Fig. 11(b)), the presence of 1–5 mm leucitecrystals could be observed. The presence of twinning was alsoobserved in the fracture surfaces of KGP heated to 11001C for24 h (Fig. 12) and was due to the phase change on cooling fromcubic to tetragonal symmetry.48

As shown in Fig. 13(a), TEM results of unheated KGP re-vealed that it was amorphous (i.e., no nanocrystals could beobserved), and contained small 10–20 nm sized precipitates. In-side of these precipitates, a finer texture could be observed,

Fig. 10. SEMmicrographs of KGP fracture surfaces after being heated and cooled at 101C/min to (a) 10001C and (b) 11001C with no isothermal soak.Samples were etched for 3 s in 3 wt%HF at 451C. Small leucite crystals are shown in (a) and the formation of KF needles in (b). SEM, scanning electronmicroscopy.

Fig. 11. SEMmicrographs of KGP after being heated and cooled at 51C/min to 10251C for 5 h soak time showing (a) leucite grains and (b) after etchingfor 15 s in 3 wt%HF at 451C. Before firing, samples were prepared by grinding KGP to a powder (r44 mm) and pressing to a disk shape with a uniaxialpress, followed by cold isostatic pressing at 200 MPa. SEM, scanning electron microscopy.


which appeared to be fractal in nature. Blackford et al.49 noticeda similar structure for Na-based geopolymer. The sample shownin Fig. 13(a) was prepared by grinding to a powder and depo-sition on a holey, carbon-coated, copper grid. Therefore, thethickness of the sample region was not uniform and thicker sec-tions appeared darker. The openly porous structure of KGP wassimilar to that seen in the TEM micrographs of sol–gel materi-als50,51 and zeolite gel precursors before crystallization.52

In previous work,3 the high resolution structure of ion-thinned KGP was found to have regions of alternating lightand dark texture in the TEM. This texturing can be related to gelporosity and the possible formation of small nuclei within thegeopolymer gel. In zeolite formation, it has been shown that in-corporation of gel onto growing nuclei leaves behind gaps be-tween the nuclei and surrounding gel.53 It has also beensuggested that zeolite nucleation can occur directly on or inthe precursor gel.54 The rapid setting at low temperature as wellas concentrated nature of geopolymer gels generally prohibitszeolite crystallization in geopolymers, especially when the Si/AlD2.55 However, the conditions utilized in geopolymer forma-tion have been found to be sufficient to form a disordered gelwith local structure similar to that of compositionally equivalentcrystalline zeolites for both K- and CsGP.41,42 Therefore, theformation of small precursor nuclei may explain the texturingseen in the TEM.

After being heated for 11001C for 24 h, KGP converted intolarge leucite grains. A TEM micrograph (Fig. 13(b)) revealedleucite grains and a smooth, glassy edge. In comparison, themicrostructure of CsGP observed in TEM contained smallpollucite crystallites and substantial porosity after being heatedfor 11001C for 24 h.

IV. Conclusions

Geopolymers are increasingly being considered in a variety ofrefractory applications and as precursors to ceramic formation.Despite this, little work has been done to systematically char-acterize their physical and microstructural evolution on heating.In this study, K-based geopolymer (K2O �Al2O3 � 4SiO2 � 11H2O),deemed KGP, was investigated due to its attractive refractoryproperties and ability to convert to leucite (KAlSi2O6) on heating.

The thermal behavior of KGP was characterized using a va-riety of techniques in order to explore the details of dehydration,sintering, and crystallization. On heating above 10001C, KGPcrystallized into leucite and a minor amount of kalsilite. Fullpattern fitting Rietveld analysis using a Si internal standard re-vealed that B80 wt% of leucite was formed along with B20wt% glassy phase. The presence of a glassy phase was believedto be primarily due to compositional inhomogeneities created bypartially reacted metakaolin as well as the presence of free alkali.

Significant changes in the physical structure were observed insamples heated above 9001C. The geopolymer gel precipitatestructure coarsened substantially between 9001 and 9751C, andattained a glassy appearance by 10001C. However, voids locatednear partially reacted metakaolin persisted until KGP was heatedto 11001C. Pycnometry results revealed that closed pores werepresent from 9251 to 10501C, and that KGP reached 99.7% ofthe theoretical density of leucite by 11001C. The surface area re-duced substantially from 51.5 to 21.7 m2/g in samples that wereheated to 9001 and 9251C respectively. The thermal shrinkagehad a maximum value at 9441C and was completed by 11001C.

KGP was found to be more refractory than K-based geo-polymer having an atomic ratio of Si/Al5 1.9 or 2.15, and wasmost likely due its close proximity to the leucite composition.These details could be clearly understood in terms of the K2O–SiO2–Al2O3 phase diagram. In terms of leucite formation, KGPwas found to offer many of the benefits of sol–gel precursorssuch as lower sintering temperatures and compositional controlwithout the high costs associated with expensive precursors.

Fig. 12. SEM micrograph of KGP fracture surfaces after being heatedand cooled at 101C/min to 11001C for 24 h soak time showing the pres-ence of twins formed due to the cubic to tetragonal phase transformationon cooling below 6501C. SEM, scanning electron microscopy.

Fig. 13. TEM micrographs of (a) unheated KGP and (b) after being heated to 11001C for 24 h. The unheated KGP was prepared by grinding with amortar and pestle, dispersing in ethanol, and evaporating the ethanol onto a holey, carbon-coated copper grid. The heated KGP was prepared bypolishing and ion milling. TEM, transmission electron microscopy.


The ability to achieve499% theoretical density after only heat-ing to 11001C (no soak) with little kalsilite formation is proofthat KGP precursors have unique potential to form leucite atmuch lower temperatures and costs compared with conventionalmelting of mixed oxides

Acknowledgments

The authors acknowledge the use of facilities at the Center forMicroanalysis ofMaterials, in the Frederick Seitz Research Laboratory at the University of Illinoisat Urbana-Champaign, which is partially supported by the U.S. Department ofEnergy under grant No. DEFG02-91-ER45439. The authors would also like tothank Dr. Pankaj Sarin for his valuable guidance and help in Rietveld analysis.

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formation of ceramics from metakaolin-based geopolymers. part ii: k-based geopolymer

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