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Ceramics International 40 (20

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pena process commonly used to produce ceramic foams. Geopolymer foams were prepared by stirring an activated blend of metakaolin and y ash

composition similar to that of zeolite, but with a mixed

of geopolymers, those based on potassium activators showsimproved mechanical and thermal properties due to the largersize of the potassium ion compared to sodium [2].

excellent re resistance, and are considered as replacement for

silicates, thereby limiting the energy required to produce acomponent, and can also be obtained from industrial waste.Several papers describe the production of porous compo-nents based on geopolymers. The typical approach is borrowedfrom the procedure used in the cement industry, which consistin the addition to aqueous geopolymer slurry of components

0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.http://dx.doi.org/10.1016/j.ceramint.2013.11.011

nCorresponding author. Tel.: 39 3493416679; fax: 39 0498275505.E-mail address: marcelocilla@ufscar.br (M. Strozi Cilla).microstructure (from amorphous to semi-crystalline). The silica(SiO2) and alumina (Al2O3) species present in the raw materialsreact in a highly alkaline medium, organizing themselves in acontinuous three dimensional structure by sharing oxygenatoms, forming bonds such as SiOAlO, SiOAlOSiOor SiOAlOSiOSiO. Geopolymers are also known aspolysialates, being based on large molecular chains consistingof silicon, oxygen and aluminum, where the term sialate is anabbreviation for siliconaluminate. Among the different types

conventional cement-based components as well as for ceramicparts that can be used up to medium-high temperature(typically below 1200 1C) [1]. Explored applications includebricks, thermal insulation, and encapsulation of radioactive andtoxic waste, foundry equipment and composites. In addition,these materials can be considered sustainable, because theyreach their nal properties at temperatures not exceeding100 1C during the geopolymerization reaction, even consider-ing the thermal energy employed for obtaining metakaolin andbe efciently adjusted by the control of some parameters such as solid content, surfactant type and content and mixing speed. The inuence ofeach parameter on the porosity and other characteristics of the geopolymer foams were investigated. The foams were evaluated only after heattreatment at 80 1C, which was conducted in order to complete the geopolymerization reactions. The produced components could be heat treatedup to 1200 1C in air without melting, if desired.The characteristics (morphology, strength, chemical and thermal resistance) of the geopolymerfoams suggest that they could be employed as low cost replacement for highly porous ceramics in applications such as catalysis supports,adsorption and separation, ltration of hot gases and refractory insulation of furnaces. In addition, these components could be consideredsustainable, because they reach their nal properties after processing at temperatures not exceeding 100 1C and part of the raw materialsemployed are industrial waste.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Geopolymers; Foams; Gelcasting; Sustainable

1. Introduction

The term geopolymer was created by Davidovits [1] in 1978to dene a class of materials of mineral nature with chemical

Geopolymers have found application in virtually all elds ofindustry, depending in particular on the SiO2/Al2O3 molarratio, which provides, among its properties, high mechanicalstrength, resistance to freeze-thaw, high chemical inertness andwith a mixture of potassium hydroxide and potassium silicate with Si/K1.66. The cell size and size distribution of the geopolymer foams couldGeopolymer foa

Marcelo Strozi Cillaa,b,n, Paolo ColaGraduate Program on Materials Science and Engineering, Federal Uni

bDipartimento di Ingegneria Industriale, UniversitcDepartment of Materials Science and Engineering, The Pe

Received 2 October 2013; received in revised fAvailable online

Abstract

Highly porous geopolymers, with homogeneous microstructure, oCERAMICSINTERNATIONAL

14) 57235730

s by gelcasting

bob,c, Mrcio Raymundo Morellia

ity of So Carlos, Via Washington Luiz, km 235, So Carlos, SP, Brazilgli Studi di Padova, via Marzolo, 9, Padova, Italyylvania State University, University Park, PA 16802, USA

30 October 2013; accepted 5 November 2013ecember 2013

cells and porosity up to 80 vol%, were fabricated by gel-casting,

www.elsevier.com/locate/ceramint

Inte(such as silica fume or Al powder) capable of generating in situgaseous H2 because of the oxidation reaction occurring withmetallic Si or Al in a highly alkaline environment [24].Another approach that has been proposed is the addition ofperoxides, which decomposes into water due to the high pHgenerating gas [5].These approaches provide a suitable way of fabricating highly

porous components, but when these processing routes are usedthe cells are typically closed, i.e., no interconnecting pores arepresent on the cell walls, thereby greatly limiting properties suchas the permeability to liquids or gases of the component. Despiteall these studies, little work has been devoted to the productionof geopolymer foams in alternative ways.Ceramic foams typically possess high permeability, high

specic surface area, good insulating characteristics, highrefractoriness, chemical resistance and good mechanical prop-erties. These characteristics enable ceramic foams to be widelyused in several industrial applications such as hot gas lters,solid/liquid separation process, catalyst support and thermalinsulators [6]. There are several ways to produce porousceramics, such as the replica technique, the sacricial templatemethod, and the direct-foaming technique [7]. Belonging to thelatter category, the gelcasting process is used to obtain foamswith porosity levels up to 90% from ceramic suspensions. Thisprocess consists in vigorously stirring a slurry containingwater-soluble organic monomers and a surfactant [8]. Thepolymerization reaction occurring among the monomer mole-cules enables the rapid stabilization of the wet foam, followedby sintering of the ceramic particles [810]. Wet foams are, infact, thermodynamically unstable systems in which processessuch as drainage of the liquid phase and gas bubble coarseninglead to uncontrolled increase in cell size and ultimately foamcollapse by rupture of the liquid lm. Gas diffusion occursbetween bubbles of different size and consequently differentconcentrations of gas, due to the difference in Laplace pressurebetween them (Ostwald ripening), leading to the degradationof the foam structure governed by the reduction of the Gibbsfree energy of the system [8]. To avoid this, surfactants can beused as surface-active agents for the stabilization of wet foams,because they stabilize the liquidgas interface decreasing thesurface tension of the system. These long-chain amphiphilicmolecules adsorb at the gas bubble surface with their hydro-philic tail in contact with the aqueous phase. The foamingability of a surfactant is related to its effectiveness to lower theinterfacial energy or the surface tension at the gasliquidinterface. Surfactants are classied, according to the nature ofthe hydrophilic group, as anionic, cationic, non-ionic, andamphoteric [11].In the case of a slurry containing geopolymer precursors,

which has several ions in solution (K , Al3 , Fe3 , SiO42),

non-ionic surfactants have a more pronounced effect since theypossess hydrophilic groups without electric charges. The typeof surfactant can inuence the cell size, size distribution anddegree of interconnection among adjacent cells (open/closedcell ratio). The wet foam can be rapidly gelled simply by

M. Strozi Cilla et al. / Ceramics5724exploiting the geopolymerization reaction itself, with no needfor organic monomers or other stabilization/gelling additives.In this paper, we explore for the rst time a gel-castingapproach based on the presence of appropriate surfactants forthe production of highly porous open cell components fromgeopolymer precursors.

2. Materials and methods

Experiments were carried out using as geopolymer precur-sors metakaolin obtained from the calcination at 750 1C for 6 hin a mufe of kaolin (Minasolo Minerals and AbrasivesGrains (Brazil)), y ash class F (#200 mesh; Tractebel Energia(Brazil)), and as alkaline activators potassium hydroxide KOHpellets (85% purity, Dinmica Qumica Contempornea Ltda(Brazil)) and potassium silicate (Si/K1.66, density 1.39 g/l,viscosity 800 cP; Una Prosil Usina Nova Amrica (Brazil)).Considering the high content of iron oxide (Fe2O3) present inthe y ash used (10.2 wt%), a maximum addition of 30 wt% ofy ash with respect to metakaolin was used, because, as quotedby Lloyd et al. [12], during alkaline activation iron dissolvesfrom iron-rich y ash particles and forms either crystallinehydrates or colloidal hydrates. In order to decrease theviscosity of the suspension, polyacrylic acid (Dolapix CE-64,Zschimmer and Schwarz) was used. For the stabilization of thewet foams, two non-ionic surfactants were added in differentamounts: Tween 80, a Polyoxyethylene 20 sorbitan mono-oleate C64H124O26 (VWR BDH Prolabo) and Triton X-100,a polyethylene glycol tertoctylphenyl ether C14H22O(C2H4O)n, n910 (Sigma-Aldrich).The rst step in the preparation the geopolymer foams was

the preparation of a 15 M KOH solution, which should be usedafter 24 hours [13]. Then, a solution of potassium-basedactivators and distilled water was prepared in a mixer(500 rpm, 30 min, Ika-Werke Ost Basic, Staufen, Germany),according to the following weight ratio: 2 silicate: 1 KOH15 M:0.25H2O. To this solution, Dolapix CE-64 was added(0.32 wt% on the total weight). Metakaolin and y ash werethen added at room temperature to the activator solution,stirring at 1000 rpm for 30 min, producing suspensions witha solid content ranging from 61 to 71 wt%. Based on these rawmaterials, the porous geopolymers were prepared consideringthe three oxide molar ratios as follow: SiO2/Al2O33.78,K2O/SiO20.24 and H2O/K2O16.The geopolymer precursor suspension was placed in an oven

at 80 1C for 20 minutes to accelerate the geopolymerizationreaction, which is key to enabling the retention of porousmorphology of the wet foam subsequently produced. Thereafter,the suspension was removed from the oven and stirred againwhile adding dropwise one of the surfactants. Surfactantaddition ranged from 2 to 4 wt% with respect to total weight,and the suspension was stirred at different mixing velocities(800, 1500 and 2000 rpm for 5 min) in order to generate wetfoams by the entrapment and stabilization of air bubbles.Finally, the geopolymer foam was cast in a polystyrene moldand placed for 1 h at 80 1C into an oven after sealing it intoa plastic bag. The sample was then removed from the plastic bag

rnational 40 (2014) 57235730and left at 80 1C for further 4 h. It should be noted that thesamples were characterized as prepared, and were not subjected

Inteto any heat treatment at high temperature. If required, theproduced components could be heat treated up to 1200 1C in airwithout melting to improve the mechanical strength by sintering,as it occurs in ceramics. Fig. 1 shows the owchart of processused for fabricating geopolymer foams by gelcasting.The bulk density of the geopolymer foams was computed by

dividing the mass of foam cut into a parallelepiped divided by itsgeometric volume measured with a caliper. The total pore volume(XP) was obtained based on the relation XP100n(1/0) [14],where 0 is the true (skeleton) density of the pore-free solidmaterial, measured with an helium pycnometer (Accupyc 1330,Micromeritics, Norcross, GA), and the open porosity was quanti-ed by the Archimedes method using water as the inltra-ting uid.The morphology of the foams was investigated using an

optical stereoscope (Wild Heerbrugg, Type 376788, coupled witha digital camera) and a Scanning Electron Microscope (FEI

Fig. 1. Schematic diagram of the production of geopolymer foams using thegelcasting process.M. Strozi Cilla et al. / CeramicsInspect S 50). The pore size distribution was evaluated from theacquired images using the Axio Vision 4.8.2 LE image proces-sing software (Carl Zeiss, Oberkochen, Germany). Valuesobtained by image analysis were converted to 3D values usingthe stereological equation DsphereDcircle/0.785 [15], in order todetermine the effective cell-size.

3. Results and discussion

With the aim of producing components possessing a highamount of total porosity, we rst evaluated the inuence of thesolid content in the slurry on this feature, maintaining xed allother parameters to a specic value, which was set according topreliminary optimization experiments (surfactant content2 wt%; mixing speed1500 rpm). In Fig. 2 are presented the datawhich indicate that, as expected, with increasing the solidcontent in the suspension a reduction in total porosity occurredfor both types of surfactant, because of the increase in viscosityof the slurries. We can observe that Triton X-100 appeared to bemore effective in incorporating and maintaining a large amountof gas into the liquid, which led to a larger amount of totalporosity after gelling and drying. This behavior is certainlyrelated to the difference in the chemical structure of the twosurfactants, but further investigations are necessary to determinethe precise mechanism. Early stability theories assume that thefoam stability is determined by the adsorbed surfactant whichcontrols the mechanicaldynamical properties of the surfacelayer (surface tension gradients) [16]. Also, important para-meters to be taken into consideration are the surface viscosity,surface occupancy, gravity drainage and capillary suction.Considering these aspects, Gibbs and Marangoni proposedtwo theories of elasticity for surfactant solutions, both dealingwith the surface elasticity effect caused by different mechan-isms. In this sense, surfactants with different chemical structureand foaming properties can notably inuence the microstructureof porous scaffold prepared by the direct foaming method [16].The solid content of the slurry was set at 68 wt% to evaluate

Fig. 2. Effect of solid content in the slurry on total porosity. All samples wereproduced using 2 wt% of surfactant and 1500 rpm mixing speed.

rnational 40 (2014) 57235730 5725the effect of the mixing speed (800, 1500 and 2000 rpm) fortwo amounts of surfactant (2 and 4 wt%). The results arereported in Fig. 3a and b which shows that, again, samplesproduced with Triton X-100 surfactant possessed a higher totalporosity, in comparison to those produced using Tween 80.With increasing mixing speed, the total amount of porositydecreased, and no signicant difference between the two typesof surfactant used as well as their amount was observed. Theoverall change in porosity was about 8 vol% in all cases, whengoing from 800 to 2000 rpm and both for 2 or 4 wt% ofsurfactant, and the decrease in porosity was particularly limited(15 vol%) when increasing mixing speed from 800 to1500 rpm. The reason for this could be ascribed to an increaseof shear stress in the suspension at higher rotational speed,which resulted in a lower total volume of entrapped air into thesuspensions [1719].As far as the amount of surfactant was concerned, the data

indicate that, regardless of the mixing speed used, an averageincrease in total porosity was achieved when passing from 2 to4 wt% of surfactant addition, which was of 10% for samplesproduced using Tween 80 and of 15% for those made using

Triton X-100. Increasing the surfactant concentration favors itsadsorption at the gas/liquid interface and decreases the surfacetension, thereby promoting foaming. For a certain concentra-tion Cmax, the surface tension is minimal and foaming ismaximum. This concentration is close to the critical micelle

concentration (CMC), which represents the minimal surfactantquantity to reach the minimal surface tension. For higherconcentrations, the surface tension remains minimal, but theaddition of more surfactant will generate an increase in theviscosity of the system, which inhibits the foaming effect [20].

of

Fig. 3. Effect of rotation speed and surfactant content in the slurry (solid content set at 68 wt%); (a) 2 wt% surfactant and (b) 4 wt% surfactant.

M. Strozi Cilla et al. / Ceramics International 40 (2014) 572357305726Fig. 4. Effect of mixing speed on the average cell size and cell size distribution

Tween 80, 1500 rpm; (b) Tween 80, 2000 rpm; (c) Triton X-100, 1500 rpm; and (d)is reported.samples produced using 2 wt% of surfactant (solid content set at 68 wt%). (a)

Triton X-100, 2000 rpm. In the insets, the pore size distribution for each sample

When considering the effect of processing parameters(mixing speed, surfactant type and amount) on the morphologyof the geopolymer foams, we observed that the samplespossessed a different average cell size and size distributiondepending on how they were obtained. Figs. 45 show theoptical images, taken at low magnication, illustrating thegeneral morphology of the samples and report the cell sizedistribution computed by image analysis for each sample.Firstly, we can observe (see Fig. 4) that when increasing themixing speed from 1500 to 2000 rpm, the average cell size D50decreased by 20% (from 345734 to 272736 mm) forsamples produced using 2 wt% of Tween 80, and by 26%(from 463780 to 338753 mm) for samples produced using2 wt% of Triton X-100.A similar decrease in D50 value (25%, from 555790 to

453786 mm for samples made using Tween 80 and 13%,from 5307113 to 459790 mm for samples made usingTriton X-100) occurred also for samples produced using asurfactant amount of 4 wt% (see Fig. 5). This effect was also

observed when producing liquid foams, and considering thestresses applied to the slurry by the mechanical shearingprocesses the Taylor model can be used to explain therheological behavior and respective cell size obtained [2123].This model illustrates that, when an isolated, spherical

droplet of radius R0 with a relatively low viscosity d isdispersed in a uid of higher viscosity c, the droplets willdeform into an ellipsoid or elongated cylinder. Ordinarily, therupture of these elongated cylinders in smaller droplets isachieved when reaching the so-called Rayleigh instability,which reduces the high interfacial energy possessed by theelongated droplets. Deformation of the dispersed phase onlytakes place when the shear stress c exceeds the interfacialstress s/R0, where is the shear rate and s is the interfacialtension. The ratio between these two stresses is dened by thecapillary number (Ca). When the capillary number overcomesa critical value Cacrit, the elongated droplet will rupture intosmaller droplets of average radius R according to Eq. (1). Inthis sense, Cacrit depends on the viscosity ratio between the

ion

M. Strozi Cilla et al. / Ceramics International 40 (2014) 57235730 5727Fig. 5. Effect of mixing speed on the average cell size and cell size distribut

(a) Tween 80, 1500 rpm; (b) Tween 80, 2000 rpm; (c) Triton X-100, 1500 rpm; ansample is reported.of samples produced using 4 wt% of surfactant (solid content set at 68 wt%).

d (d) Triton X-100, 2000 rpm. In the insets, the pore size distribution for each

dispersed and continuous phase (d/c) and the type of ow[23,24].

R pCacritsc

1

This model has also been applied for the prediction ofbubble size as a function of suspension composition for both,particle-stabilized emulsions and surfactant stabilized emul-sions [24].Secondly, it is possible to observe an inverse relation

between the average cell size and the relative density, asshown by Fig. 6 for a denite set of processing conditions.

Specically, at the same mixing speed of 1500 rpm, increasingthe surfactant amount from 2 to 4 wt% increased the D50nearly by 61% for samples produced using Tween 80, andabout by 15% for samples produced using Triton X-100, whileat the same time the relative density decreased by 20% and28%, respectively. Also, the number of macropores increasedfurther and the mean pore diameter also increased, causing themajority of pores to share pore edges and the interconnectivityto increase. This behavior is related to the increased foamvolume obtained after stirring a slurry with 4 wt% of surfactantwith respect to a slurry with 2 wt% of surfactant, processed atthe same mixing speed. In this work, an increase in the volumeof the wet foam on the order of 2 and 3 times with respectively2 wt% and 4 wt% surfactant content was observed for bothsurfactants, regardless of the mixing speed. The volume of thewet foam is the rst indication that the greater the volumeobtained in the foam is, the greater the average pore sizebecomes [23].Fig. 7 shows SEM images with different magnications of

the microstructure of geopolymer foam obtained from slurrywith 68 wt% solids, 2 wt% Tween 80 surfactant and stirred at2000 rpm, where it is possible to observe the presence ofspherical and interconnected cells and dense struts.Fig. 8 reports the total and open porosity values for the

samples investigated in this study, showing that, for asurfactant content of 2 wt%, an open porosity in the range of52 vol% for Tween 80 and of 46 vol% for Triton X-100 was

Fig. 6. Effect of surfactant content on the average cell size and its relation withthe relative density of samples (solid content set at 68 wt% and mixing speed at1500 rpm).

M. Strozi Cilla et al. / Ceramics International 40 (2014) 572357305728Fig. 7. Morphology of geopolymer foam obtained from slurry with 68 wt% solid(b) and (c) intermediate magnication in different regions; and (d) highest magnis, 2 wt% surfactant Tween and stirred at 2000 rpm. (a) lowest magnication;cation.

foaming ability can inuence the microstructure of porousgeopolymers fabricated by gelcasting.

M.I. Ahmad, Fly ash porous material using geopolymerization process forhigh temperature exposure, Int. J.Mol. Sci. 13 (2012) 53885395.

he A

InteAdditionally, the data indicate that the Tween 80 surfactantwas a more effective surfactant, as in all cases its use led tosmaller cell sizes indicating the more effective reduction insurface energy of the liquid/gas interfaces in the geopolymerfoams, enabling to stabilize a larger amount of bubble surfaces.It is also important to observe that the processing parameters

(slip rheology, foam volume, type of surfactant, idle time prior topolymerization, and other features) affect also the ratio betweenopen and closed cells in the foams [25,26]. Moreover, foams withdifferent characteristics (total porosity, average cell size and sizedistribution) possess a different average strut thickness, whichresults in different mechanical properties. The assessment of theinuence of the processing parameters on the strength ofgeopolymer foams produced from gelcasting, also in dependenceon the heat treatment temperature, will be reported in a differentpaper. Preliminary investigations indicate that values in the rangeof 0.53.0 MPa for the compression strength are obtained,depending on the process parameters and the amount of porositypresent in the samples.

4. Conclusions

Geopolymer foams were produced by gelcasting, using thegeopolymerization reaction to stabilize the gas bubbles intro-generated. Similarly, for a surfactant addition of 4 wt%,the open porosity was in the range of 56 and 54 vol% forTween 80 and Triton X-100, respectively. These resultsconrm that surfactants with different chemical structure and

Fig. 8. Relation between total porosity and open porosity estimated by t

M. Strozi Cilla et al. / Ceramicsduced in the liquid slurry by rotational mixing. It was shownthat the processing parameters affect the characteristics of thefoams. In particular, with increasing mixing speed the averagecell size and relative density decreased, while with increasingthe amount of surfactant the average cell size and total porosityincreased. The type of non-ionic surfactant affected the overallmorphology of the foams, with Tween 80 developing smallercells with respect to Triton X-100, and Triton X-100 producinga higher total porosity with respect to Tween 80. With respectto the generation of open porosity, Tween 80 had a betterbehavior when compared with Triton X-100 in the sameprocessing conditions. By using this approach, it was possibleto produce foams with a total pore volume as high as 80 vol%, with an amount of open porosity as high as 60 vol%.[5] J. Davidovits, Geopolymers Chemistry and Applications, Third ed.,Institute Gopolymre, Saint-Quentin, 2011.

[6] H. Kima, S. Leeb, Y. Hanc, J. Parkc, Control of pore size in ceramicfoams: inuence of surfactant concentration, Mater. Chem. Phys. 113(2009) 441444.

[7] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J. Gauckler, Processingroutes to macroporous ceramics: a review, J. Am. Ceram. Soc. 89 (2006)17711789.

[8] F.S. Ortega, F.A.O. Valenzuela, C.H. Scuracchio, V.C. Pandolfelli,Alternative gelling agents for the gelcasting of ceramic foams, J. Eur.Ceram. Soc. 23 (2003) 7580.Acknowledgments

MSC gratefully acknowledges the nancial support of theSo Paulo Research Foundation (FAPESP), the GraduateProgram on Materials Science and Engineering (PPGCEM)of Federal University of So Carlos (UFSCar) and theDepartment of Industrial Engineering of the University ofPadova.

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M. Strozi Cilla et al. / Ceramics International 40 (2014) 572357305730

Geopolymer foams by gelcastingIntroductionMaterials and methodsResults and discussionConclusionsAcknowledgmentsReferences