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The fundamentals behind the visco-elastic properties ofconcentrated solid suspensions hmany years.1 Beyond scientic cdriven by the prevalence of concindustrial formulations. Amongused to cast concrete in the buildthe most used today in volumematter properties of fresh Ordpastes have been the subject oflate 80's.3 It so happens thatconcrete, despite being commontion materials, actually hide a clogical behaviour that has only bknowledge of three disciplineschemistry and uid mechanics.4

Indeed, signicant practicalment of self-compacting concreteect of gravity, high-mechanicow while having a solid volum

7,8

limited range of shear rates by using a simple linear Bingham

Soft Matter

PAPERbre-reinforced concretes able to ow while containing

elongated rigid steel bres wefundamental questioning of gran

aIFSTTAR, Universite Paris Est, FrancebInstitute for Construction and InfrastructurcSIMM, UMR CNRS 7615, ESPCI ParisTech,dInstitute for Building Materials, ETH Zuric

1134 | Soft Matter, 2014, 10, 11341141ave been a subject of study foruriosity, this interest has beenentrated suspensions in manythem, the cementitious pasteing industry is arguably by far.2 As a consequence, the soinary Portland Cement (OPC)particular attention since theOPC pastes and OPC-basedand seemingly dull construc-

omplex and challenging rheo-een grasped by combining the: colloidal physics, inorganic

advances such as the develop-es5 able to ow under the soleal strength concretes6 able toe fraction higher than 90% or

model.3,6,10

s s0 hpgc (1)where s is the shear stress and _g is the shear rate. From apractical point of view, this approximation of the fresh concretepaste behaviour with a Bingham model considerably simpliesthe work as only two empirical parameters need to be deter-mined to fully describe the rheological behaviour in the usualrange of industrial shear rates: the plastic yield stress s0 and theplastic viscosity hp. These two parameters are shown to becontrolled by the competition between three major types ofinteractions:11,12 colloidal interactions due to electrostatic andvan der Waals forces between particles, viscous forces in theinterstitial liquid squeezed between cement particles, anddirect contact forces.

In cement pastes, van der Waals colloidal forces dominatehydrodynamic forces (both viscous and inertial) in the lowstrain rate regime (i.e. less than several tens of s1) and giverise to a shear thinning macroscopic behaviour. In the mostFlow properties ocomparative studpastes

Aurelie Favier,ab Julie Hot,a

and Jean-Baptiste d'Espino

Geopolymers are presented in ma

have focused on their chemical

applications, but very few have

fundamental dierences in the o

and Ordinary Portland Cement (O

are negligible and that hydrody

geopolymers can then be describ

viscosity of the suspending alkal

between metakaolin grains. This

developments made in cement te

be ecient for geopolymers and t

1. Introduction

Cite this: Soft Matter, 2014, 10, 1134

Received 10th July 2013Accepted 11th November 2013

DOI: 10.1039/c3sm51889b

www.rsc.org/softmatterre only possible because ofular paste rheology.9

e Management, ETH Zurich, Switzerland

France

h, Switzerlandf MK-based geopolymer pastes. Ay with standard Portland cement

uillaume Habert,b Nicolas Roussela

e de Lacailleriecd

y studies as alternatives to ordinary Portland cement. Previous studies

nd mechanical properties, their microstructures and their potential

ocussed on their rheological behaviour. Our work highlights the

properties, which exist between geopolymers made from metakaolin

C). We show that colloidal interactions between metakaolin particles

amic eects control the rheological behaviour. Metakaolin-based

as Newtonian uids with the viscosity controlled mainly by the high

e silicate solution and not by the contribution of direct contacts

ndamental dierence between geopolymers and OPC implies that

nology to improve rheological behaviour such as plasticizers will not

at new research directions need to be explored.

Fresh behaviour of cementitious suspensions such asconcrete, cement pastes and grouts may be approximated over aextreme cases, a viscosity inversely proportional to the shearrate can be measured. At an intermediate strain rate (i.e.around 100 s1), hydrodynamic viscous forces oen dominateand can be at the origin of a plateau in viscosity. Finally, in thecase of highly concentrated systems (i.e. for shear rates of theorder of a couple hundreds s1 or higher), particle inertiadominates the high strain rate response and may lead to shear

This journal is The Royal Society of Chemistry 2014

this research is Argical M-1000 from AGS Mineraux (France). Its

Paper Soft Matterthickening. In the case of deocculated highly concentratedsystems, a viscosity almost proportional to the shear rate caneven be measured. The transitions between these regimes aregoverned by critical strain rates that depend on the cementparticle average size, the uid viscosity which can be modiedby polymer admixtures, the cement density, and the intensityof the van der Waals forces, which can be controlled byadsorbing polymers.13 However, the industrial success ofconcrete raises new issues beyond its engineering properties.Indeed, its environmental impacts, albeit low per unit of mass,are leveraged by its massive consumption. For example,concrete production is responsible for 5 to 10% of all anthro-pogenic CO2 emissions.1416 As a consequence, since the lastdecade, a growing interest in alternative binders has grown.Examples are sulfo-aluminate and magnesia cement.17 Geo-polymers, which are based on the chemistry of alkali-activatedinorganic binders, are also considered as a highly potentialsolution to reduce CO2 emissions in the eld of constructionmaterials.18,19 These binders are obtained by reaction of anactivated silica-alumina with an alkali silicate solution. Inorder to develop a building material that could be used as acement replacement for specic applications, the under-standing of the fundamentals at stake behind its fresh rheo-logical behaviour is, as it was the case in cementitious systems,primordial. Some studies on geopolymers based on industrialwastes such as y ashes from the coal industry or blast furnaceslags from the steel industry20,21 showed that similarly tocement pastes, these materials display visco-plastic behaviour.Criado et al.22 and Palacios et al.23,24 studied the eect ofcommercial rheology modifying polymers used in cementapplications on geopolymers obtained by alkali-activation of yashes and slags. They showed that these plasticizers have a loweciency in an alkali-activated system due to the fact thatsuperplasticizers such as polycarboxylates are altered aer longexposure in a highly alkaline environment.25 However, recentstudies have identied PCE resistant in mild alkali activationsystems (0.5% NaOH),26 which could then be also more stablein a highly alkaline environment.

In this study, we focus on a simpler model system where apure aluminosilicate phase is used and dissolved in a sodiumsilicate solution in order to highlight the fundamentals of therheological behaviour of a fresh geopolymer. This model systempresents the interest of having a relatively simple chemistrywhere Si, Al and Na are the predominant ions. Finally, in orderto address the potential use of these geopolymers as replace-ments for Portland cement, we conduct a comparative studybetween these geopolymer mixtures and standard OPC pastes.

As the rst approximation,27,28 we assume that fresh geo-polymer pastes can be viewed as suspensions of particles(metakaolin) in a continuous uid (sodium silicate solution).In that sense, the physics which govern their rheologicalbehaviour is similar to that of a cement paste and grasped bythe physics of concentrated suspensions of rigid particles.29

Below, we will focus on two main parameters, the yield stressand the viscosity, and evaluate through detailed rheologicalmeasurements the main types of interactions that arecontrolling them.This journal is The Royal Society of Chemistry 2014BET specic surface area is equal to 17 m2 g1 and the averagediameter in mass is approximately 10 mm. The alkaline solu-tions were prepared from a commercial sodium silicate solutionwith a SiO2/Na2O molar ratio of 3.2 and 65 wt% of water (VWRInternational, France), sodium hydroxide pellets (Merck KGaA,Germany) and distilled water. They were prepared by mixing anappropriate amount of NaOH, commercial sodium silicatesolution and distilled water in a plastic bottle, which wassubsequently closed to avoid evaporation and carbonation. Thesolutions were then allowed to cool for 24 h. Numerous studieshave shown that Si/Al close to 2 and Na2O/Al2O3 between 1 and1.2 are the best molar ratios for structural applications30,31 anddevelopment of mechanical properties.

Thus, geopolymer pastes were prepared by adding to analkaline solution with a H2O/Na2O molar ratio of 15 and aconstant SiO2/Na2O molar ratio of 1.15 the amount of MKnecessary to reach a Si/Al ratio of 1.8. The volume fraction of MKis 0.3. The suspension was then mechanically mixed with astirrer for 5 minutes before immediate testing. The mass of MKwas adjusted relatively to the alkaline solution in order toprovide the ratio given in Table 1.

Furthermore, to better dene the rheological behaviour ofthe interstitial phase, additional sodium silicate solutions ofvarious molar ratios (0.6 # SiO2/Na2O # 2 and 10 # H2O/Na2O# 20) were studied independently of the geopolymer pastepreparation. They were obtained by dissolving silica (silica gel60, 0.0630.2 mm from Merck KGaA, Germany) in an aqueoussolution of NaOH prepared by dissolving NaOH pellets indistilled water.

Portland cement pastes were prepared by mechanicallymixing cement powder (CEM I type cement from Lafarge LeHavre) with water (the water/cement ratio (W/C) between 0.35and 0.5 and f between 0.39 and 0.47) for 2 minutes. This rangeof solid concentrations was selected because it allows for theformation of a paste that remains stable under the eect ofgravity.32 In contrast to the industrial practice, most of thesecementitious systems were prepared without any rheologymodifying polymers in order to keep the system simple enoughto be compared with the geopolymer paste. However, in order toinclude this aspect in the present study, some cement pasteswere also prepared with an addition of a commercial poly-carboxylic ether13 (Tempo 12, Sika technology) typical of theorganic polymers used in the construction industry. The poly-mer dosage was in the typical range of industrial dosages (i.e.0.5% of the mass of cement).

2.2. Rheological measurements

The rheological measurements were carried out with a C-VORBohlin rheometer equipped with a vane geometry for thegeopolymer and Portland cement mixtures and parallel plate2. Experimental procedure2.1. Material preparation

Geopolymer pastes were prepared by mixing a sodium silicatesolution with metakaolin (MK) particles. The chosen MK forSoft Matter, 2014, 10, 11341141 | 1135

geometry for the solutions. When pastes were tested, an initialoscillatory pre-shear with a rate of 100 s1 was applied for 60 sprior to each test to ensure that all samples were in the samereference state of stress and strain.

3. Results and discussion

dominated over the contributions of both the colloidal inter-actions and particle inertia over a larger range of shear ratesthan in the case of cement pastes.

3.2. Evaluation of the colloidal contribution to the yieldstress

We then extrapolated the yield stress s0 from shear stressmeasurements in the low shear rate range. In Fig. 3, theevolution of shear stress for cement paste, geopolymer andgeopolymer constitutive silicate solutions is shown as a func-tion of shear rate. Table 2 gives the estimated values of shearstresses. Obviously, compared to the cement and MK suspen-sions, the silicate solution could be considered as a purelyviscous uid as its yield stress is between one and two orders ofmagnitude lower than that of the cement paste (Fig. 3). Theyield stress could be fully neglected compared to the contribu-tion of viscous dissipation to the shear stress. The viscosity ofthe silicate solution being almost constant, it behaved similarlyto the interstitial water in the cement paste also shown in Fig. 3and can be described as a Newtonian uid.

Moreover, we measured the onset of ow as a function of the

Table 1 Chemical composition given in molar ratios of the geopolymer

SiO2/Na2O (solution) H2O/Na2O (solut

Geopolymer paste 1.15 15

Soft Matter Paper3.1. Macroscopic ow curves

As already stated, depending on their formulation, cementsuspensions can display in steady state ow a large variety ofbehaviours, going fromNewtonian (constant apparent viscosity)to shear thinning (decreasing apparent viscosity with the shearrate) or shear thickening (increasing apparent viscosity with theshear rate).12,33

Fig. 1 shows the typical rheological behaviour which can beobtained by modifying the solid volume fraction of a cementpaste as well as by adding a rheology-modifying poly-carboxylicether polymer. Here, a typical cement paste is meant to corre-spond to a water cement ratio of 0.35 related to a solid volumefraction of f 0.48. It illustrates the rheological behaviour ofpastes used for standard concrete application32 whereas thepaste with W/C 0.4 (f 0.44) illustrates the behaviour of veryuid products such as grouts for soil injections.

Fig. 2 shows the viscosity of a typical MK geopolymer incomparison with the viscosity of a typical cement paste withW/C 0.35. In the loglog scale, such as the one in Fig. 2, thecolloidal contribution can be represented by a slope-1 whereasthe viscous contribution can be represented by a horizontalline. These respective contributions are plotted as dotted linesin Fig. 2 for cement pastes.

When comparing the ow curve for the typical cement pastewith the one for a MK-based geopolymer, we observed quitesignicant dierences. The viscosity of the geopolymer pastewas lower than that of the cement paste at low shear rates andhigher at high shear rates. Furthermore, it was almost shear rateindependent. This meant that the viscous contributionFig. 1 Apparent viscosity as a function of shear rate for cement pastes(SP superplasticizer) relative to dierent applications.

1136 | Soft Matter, 2014, 10, 11341141paste

ion) Si/Al (MK + solution) Na2O/Al2O3 (MK + solution)

1.8 1

Fig. 2 Apparent viscosity as a function of shear rate for geopolymerpaste (f 0.3 and f/fmax 0.7) and cement paste (f 0.47 and f/fmax 0.7). Dotted curves describe the theoretical colloidal contributionchange according to the inverse of the shear rate and theoreticalviscous contribution independent of the shear rate.12shear strain for both the standard OPC paste and MK pasteusing a vane test procedure.34 Let us recall that in this test, thesample is sheared at a very low shear rate and the evolution ofstress with shear strain is recorded. Fig. 4 shows the result ofthis vane test for cement and geopolymer pastes.

Fig. 4a shows results typical of a cement suspension. Itdisplays two characteristic critical strains. The rst one is

This journal is The Royal Society of Chemistry 2014

Fig. 3 Shear stress as a function of shear rate for geopolymer (H2O/Napolymer suspending uid (silicate solution) and the cement suspending

Table 2 Estimated yield stress values for the geopolymer, cementpaste, geopolymer suspending uid (silicate solution) and cementsuspending uid

Yield stress

Geopolymer paste 2 PaStandard cement paste 20 PaSodium silicate solution 0.01 PaCement interstitial uid 0.001 Pa

Fig. 4 Vane test experiments (a) for the standard cement paste plottedfrom (N. Roussel et al. 2012) (W/C 0.4 and f 0.44) after 20minutesand (b) for the geopolymer paste (H2O/Na2O 15, f 0.3) after 10minutes at room temperature.

This journal is The Royal Society of Chemistry 2014

Paper Soft Matteraround 103 whereas the second one is between 102 and 101.Recent studies have shown that the peak associated with thesmallest critical strain nds its origin in the breaking ofthe calcium silicate hydrates (CSH) percolating between thecement grains. The second peak nds its origin in the collapseof the colloidal interaction network.35 Above this second criticalstrain, the system is considered to be owing. In the geopolymersystem though, we observed only one peak (Fig. 4b). It waslocated between 102 and 101. This suggests that only one typeof interaction between particles is involved, and that once theparticles have moved suciently far from their initial positions,this interaction vanishes and the ow starts. Recently, Favieret al.36 have shown that the elastic modulus observed in the MK-based geopolymer is due to the formation of an inter-grain gel ata very early stage and is not due to colloidal interactionsbetween grains. They observed that the critical strain of a geo-polymer (as shown in Fig. 4b) is controlled by the critical strainof an inter-grain aluminosilicate gel. As a consequence, geo-polymer pastes seem to have their yield stress controlled solelyby the percolation of an aluminosilicate gel between MK parti-cles, and no colloidal interactions between MK particles areinvolved.

Furthermore, as already observed above, geopolymers have ayield stress far lower (10 Pa). This suggests that the attraction between CSH ismuch stronger than the cohesion of the early aluminosilicategel produced in the early stages of the geopolymeric reaction.From these dierences, an important practical consequencecan be anticipated. As the role of cement rheology modifyingindustrial admixtures is to reduce the yield stress by mini-mizing colloidal interactions between particles, such admix-tures would be of no eect for geopolymer pastes since theiryield stress is not related to colloidal interactions but to thepercolation of the early reaction products.

3.3. Evaluation and comparison of the viscosities of freshcement and geopolymer pastes

From a theoretical point of view, geopolymer pastes can beconsidered as suspensions of rigid MK grains in a suspendingsodium silicate Newtonian solution. Their viscosities can be

Soft Matter, 2014, 10, 11341141 | 1137

predicted using empirical expressions such as the KriegerDougherty relationship.37 The general form of these relation-ships can be written as:

h h0 1 f

fmax

n(2)

with

n h fmaxwhere h0 is the solution viscosity, f is the solid volume fraction,[h] is the intrinsic viscosity, and fmax is the dense packingfraction. The viscosity of the paste is therefore controlled by twoparameters depending respectively on the suspending intersti-tial liquid and on packing properties of the grains. Below, thesetwo contributions are studied separately.

3.3.1. Evaluation of the interstitial liquid contribution. In aMK-based geopolymer, the sodium silicate solution, which canbe considered as the liquid phase, has a composition that isdetermined through two molar ratios: H2O/Na2O and SiO2/Na2O. It is known from studies on water glass that these ratiosstrongly inuence the viscosity of the solution.27,38 In Fig. 5, theviscosities of dierent sodium silicate solutions are presented.

This gure conrms that the sodium silicate solutions usedfor geopolymer formulations are Newtonian uids39 but with aviscosity which is 10 to 100 times higher than the viscosity of

water. As a consequence, for a similarf

(i.e. for a similar

3.3.2. Evaluation of the solid fraction contribution. Toisolate the contribution of the solid fraction to the viscosity, weconsider here the relative viscosity, which is the ratio betweenthe eective viscosity of the geopolymer and the viscosity of thesodium silicate solution. In this way, one can focus exclusivelyon the solid fraction contribution as shown in eqn (2). Thisrelative viscosity is shown in Fig. 6 for cement and geopolymerpastes with varying solid volume fractions.

Fig. 6 shows that the relative viscosity diverges as the solidvolume fraction f approaches the one corresponding to themaximum packing fraction. The exact value of this maximumpacking fraction was dicult to measure as, to the best of ourknowledge, no appropriate procedure has been developed forthat purpose.29 The results of Fig. 6, however, suggest that theMK powder has a lower fmax in the range of 0.50.6 (i.e. lowerpacking properties) in comparison with deocculated cementpowder (in the range of 0.60.7). This could be rationalized bythe fact that MK grains have a plate shape, which is very dele-terious for ow and for packing properties.40 As a consequence,

Soft Matter Paperfmaxjamming of the system), the viscous dissipation in geopolymerpastes is expected to be 10 to 100 times higher than that

measured in a standard cement paste of similarf

fmaxratio.

Fig. 5 Viscosity as a function of shear rate at 20 C (a) for dierentH2O/Na2O silicate solutions (SiO2/Na2O 1.06) (b) for dierent SiO2/Na2O silicate solutions (H2O/Na2O 16).1138 | Soft Matter, 2014, 10, 11341141for the same solid volume fraction, the grain contribution to theviscosity is expected to bemuch higher for geopolymers than forcement paste. However, it is interesting to note that whenchemical ratios of Al2O3/SiO2, Na2O/Al2O3 and H2O/Na2O areselected to provide optimal strength properties to the hardenedgeopolymer, the volume fraction of MK introduced in thesodium silicate solution remains limited (around 0.25)compared to the one used in typical cement pastes (>0.4). As aconsequence, even if MK's fmax is small, the limited volume ofsolid grains in the suspension will reduce the grain contribu-tion to the macroscopic viscosity of the mixture.

Another aspect of rigid grain contribution to the macro-scopic rheological behaviour of a suspension lies in the directfrictional contacts between particles. In Fig. 7 this point ishighlighted by plotting for both cement and geopolymer pastesthe ratio between normal and shear stress when both materialsare sheared at 100 s1. This comparison is made at identicalf/fmax, which in terms of contact contribution is a far morerelevant parameter than f. The normal to shear stress ratio is

Fig. 6 The relative viscosity as a function of the solid fraction for ageopolymer paste (H2O/Na2O 15) and a deocculated cement pastewith 0.5% superplasticizer (SP).This journal is The Royal Society of Chemistry 2014

within the system. As a consequence, from the point of view of

Geopolymers can therefore be seen as a semi-dilutesuspension of non-Brownian, non-colloidal non-deformableparticles in a highly viscous Newtonian uid.

The summary of the comparison of this system with stan-dard cement pastes is presented in Table 3.

The technical options to alter the rheology of the mixture willthen be completely dierent from those used for cement.

(1) Grains and interfaces: In cement paste, the mostcommonly used technique to control the rheology is the use of

Paper Soft Matterrheology, a geopolymer paste, in strong contrast to a cementpaste, does not behave as a granular suspension since itsviscosity is nearly exclusively controlled by the viscosity of theinterstitial uid h0 and not by the details of the direct frictionalcontacts between inclusions.

4. Conclusions and perspectives

In this study, it is shown that:- Colloidal interactions between MK grains suspended in an

alkaline silicate solution are negligible and only low energyinteractions between grains in the presence of an interstitial gelare at the origin of the small yield stress of the suspension.

- Hydrodynamic viscous dissipation in the Newtonian highlyoen considered in the literature as a good indicator of thecontribution of frictional direct contacts between non-deform-able grains. In geopolymer pastes, this ratio is one hundredtimes lower than that commonly measured in the case ofcement pastes, which conrms the fact that very low to no graincontribution can be measured. This, moreover, suggests thateven if some temporary contacts do occur between MK grains,there does not exist any percolated sustained contact network

Fig. 7 Ratio between normal stress and shear stress as a function oftime for geopolymer paste (H2O/Na2O 15, f 0.3 f/fmax 0.7) andfor a standard cement paste (f 0.47 and f/fmax 0.7) at roomtemperature at 100 s1.viscous sodium silicate solution is the dominating mechanismat the origin of the macroscopic viscosity of the suspension.

- Direct frictional contact contribution can be neglected asdespite the fact that the maximum packing fraction of a MKpowder is low, in a geopolymer the standard solid volumefraction needed to achieve adequate hardened properties

maintains thef

fmaxratio in the semi-dilute regime.

Table 3 Summary of dierent contributions

Yield stress So

MK-based geopolymer paste Near 0 Pa weak particle interactions 10

Typical cement paste Near 1020 Pa van der Waalsattractive interactions

W

This journal is The Royal Society of Chemistry 2014deocculating agents.4144 They actually reduce the magnitudeof the attractive colloidal interactions between cement particlesby being selectively adsorbed on the surface of the cementgrains in order to create a steric eect.11 For ourmodel system ofpure MK-based geopolymers, this action is of no interest as nocolloidal interaction between grains of MK exists. The only weakinteraction, responsible for the small elastic modulus at theearly stage, is the dissolution and precipitation reaction ofalumina silicate gel between grains.36 Reducing the precipita-tion of this gel would prevent the polymerisation reaction,which in turn would delay setting.

Polymer admixtures can also impact the rheology thoughother physical means. First, the polymers adsorbed on thegrains can create bridging forces that are attractive forces, butthis eect increases the viscosity and the yield stress of thesystem. Also, polymers that are not adsorbed remain in the poresolution and can cause an increase of its viscosity. They thencreate lubrication forces between the grains, but this is notnecessary in model geopolymer systems as direct contact eectsare very limited. Polymers in solution can also induce depletionforces. Depletion forces are mainly attractive, but the occur-rence of repulsive depletion forces has also been reported.45

However, the high signicance of these repulsive depletionforces in cement pastes is still under discussion and, further-more, are limited to the case of large polymers which also havethe eect of increasing the viscosity of the interstitial uid.

Consequently, we do not expect the use of traditionaladmixtures to reduce the viscosity of model MK basedgeopolymers.

As a word of caution, it must be understood nevertheless thatthese conclusions cannot be extrapolated to y-ash and slagbased geopolymers in which some colloidal interactions canexist due to the presence of calcium.

(2) Liquid viscosity: As discussed above, the only parameterwhich controls the properties of the geopolymer is the viscosityof the alkaline solution. This viscosity is controlled by thechemical proportions of species in the solution, which areimposed in order to achieve adequate hardened properties. As aconsequence, ratios are constrained to 10 to 20 for H2O/Na2O

lution viscosity Solid fraction

to 100 times higher than water f 0.20.3, fmax 0.50.6 for usualgeopolymers

ater 0.001 Pa s f 0.40.5, fmax 0.60.7Soft Matter, 2014, 10, 11341141 | 1139

Soft Matter PaperFig. 8 Viscosity of silicate solution depending on the cation for aformulation H2O/Na2O 20 and SiO2/M+2O 1.6 at 20 C (M+ K+ orNa+).

Fig. 9 Range of evolution of shear stress with the shear rate for thegeopolymer and OPC formulation range.and 1 to 2 for SiO2/Na2O.4648However, one option which has notbeen studied here is to change the nature of the alkali ion.27

Actually, changing from sodium to potassium silicate reducesthe viscosity by 5 for an equivalent formulation (Fig. 8).

It would then be possible to produce geopolymers with alower viscosity when potassium ions are used instead ofsodium.46 However, replacing sodium with potassium increasessignicantly the price of the mixture, thus limiting its applica-tions to niche applications such as re or acid resistantceramics48 or nuclear waste containment.46

If no changes of alkali are made, in Fig. 9, we show therheological behaviour limit we can meet when formulating ageopolymer binder with adequate hardened properties andcompare it with the range of OPC formulations. Due to itsrheological behaviour, the MK-based geopolymer does not seemsuitable as a binder in mortar or adhesive applications where ahigh yield stress is needed49 or in applications requiring fastows such as pumping or spraying because of its high viscosity.Applications where it could be substituted for cement are self-compacting or self-levelling applications such as oor screedwhere a low yield stress allows the material to ow and spreadunder its own weight while the high viscosity improves thestability of the coarse inclusions during casting.

Once again, as a word of caution before generalizing theresults of the present study, it is important to note that themechanisms governing the formation of an alumina-silicategel at an early stage are strongly dependent on the chemicalcomposition of the mixture and therefore the precursor

1140 | Soft Matter, 2014, 10, 11341141selected. The presence of Ca (expected for y-ash or slag basedgeopolymers) could dramatically change the chemistry of thesystem and the interaction forces between particles. Thepresence of divalent cations can result in attractive ion corre-lation forces such as the one present between CSH parti-cles50,51 whereas monovalent cations only induce repulsiveforces.52,53 We might then expect to enhance the colloidalinteractions between grains as well. In that case, the use ofplasticizers would be of certain interest as long as the chemicalstability of the organic plasticizer can be ensured in alkalinesolutions.25

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