Study of the Effect of Siliceous Species in the Formation of a Geopolymer Binder: Understanding the Reaction Mechanisms among the Binder, Wood, and Earth Brick.

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Study of the Effect of Siliceous Species in the Formation of a Geopolymer Binder: Understanding the Reaction Mechanisms among the Binder, Wood, and Earth Brick.

Fabrice Gouny, Fazia Fouchal, Pascal Maillard, Sylvie Rossignol

To cite this version:

Fabrice Gouny, Fazia Fouchal, Pascal Maillard, Sylvie Rossignol. Study of the Effect of Siliceous

Species in the Formation of a Geopolymer Binder: Understanding the Reaction Mechanisms among the

Binder, Wood, and Earth Brick.. Industrial and engineering chemistry research, American Chemical

Society, 2014, 53 (9), pp.3559-3569. �10.1021/ie403670c�. �hal-01016480�

Study ofthe E ectofSiliceous Species in the Form ation ofa Geopolym erBinder:Understanding the Reaction M echanism s am ong the Binder,W ood,and Earth Brick

Fabrice Gouny, ^†,§ Fazia Fouchal, ^‡ PascalM aillard, ^§ and Sylvie Rossignol* ^,†

†

Grouped’EtudedesM ateriauxH eterogenes(GEM -M M GD ),EcoleN ationaleSuperieuredeCeram iqueIndustrielle,12 rueAtlantis, 87068 Lim oges,France

‡

Groupe d’Etude desM ateriaux H eterogenes(GEM H -GCD ),Boulevard JacquesD erche,19300 Egletons,France

§

Research and D evelopm entD epartm ent,Centre Technique de M ateriauxN aturelsde Construction (CTM N C),EsterTechnopole BP 26929,87069 Lim ogesCedex,France

*

Supporting Inform ation

ABSTRACT: In building construction,geopolym erbinderorm ortarcan interactwith the structuralm aterialsand thusm odify the binderform ation m echanism s.In a geopolym erbinder,the availability and am ountofsiliceous species isa preponderant param eterin uencing the nature ofnetworksform ed afterconsolidation.In thisstudy,the interactionsbetween the binderand structuralm aterials(wood and earth bricks) were investigated by

Sim agic angle spinning nuclearm agnetic resonance (M AS N M R) and Fouriertransform infrared spectroscopy (FTIR) during and afterthe consolidation.Then,the e ectofthe am ount and natureofthesiliceousspeciesavailablein thereaction m edium wereanalyzed.According to thesiliceousspeciesavailable,it ispossible to form di erenttypesofm aterials(hardening orsedim ented m aterials).By corroborating these resultswith M AS N M R and FTIR analyses,a form ation schem e ofthe binderin contactwith the m aterialswasproposed.

INTRODUCTION

Currently,thereduction ofCO

em issionshasbecom eaglobal concern, and the investigation of environm entally friendly m aterialshasbecom eincreasinglyim portant.

In thiscontext, the use of natural m aterials, such as the wood and earth m aterialsthathavebeen used forseveralm illennia,seem sto be relevant.

Earth m aterials(i.e.,un red clay bricksorram m ed earth) o er m any advantages, including a weak em bodied energy,theabilityto regulatetherelativehum idityofabuilding asa resultoftheirhygroscopic properties,theirabundance in m ost areas, and ease of recycling.

Indeed,by using local m aterials, the environm ental im pact of the construction is drastically reduced as wellas the price ofthe nalproduct.

M oreover,Allinson and H all

have shown thatearth wallscan bu er relative hum idity changes in a room by adsorbing m oisturein ahigh hum idity period and releasing itlater,which im provesthehygrotherm alcom fortofthebuilding.Thewood, from its m echanical characteristics, brings lightness to the structure. Tim ber fram e construction with earth brick in ll appears today as a sustainable design thatis prom ising in the building construction eld. H owever, cracks form at the interfaces of the bricks and fram e with tem perature and hum idity uctuations because of shrinkage and swelling phenom ena (with wood and earth being hygroscopic).

Research on new binders with the ability to adhere to wood, especially on earth bricks,hasthusbeen perform ed.

Am ong the potentialm aterialscurrently available,the geopolym ertype is a potentially good candidate.For exam ple,a recent study showed thattheaddition ofsilicafum eto ageopolym erm ixture leadsto theform ation ofafoam thathastheabilityto adhereto wood.

M oreover, previous works on construction designs

m ade from wood,earth bricks,and these geom aterialfoam s haveindicated theviabilityofthesestructuresbydem onstrating the ability ofthe geopolym erbinderto produce strong bonds between the wood and earth.

M echanicalshear tests on these m asonry have shown that their shear strength values range from 1.5 to 2 M Pa,depending on the type ofbrick.

Furtherm ore, from the results and observations of pull-out tests,the adhesion m echanism was principally explained by m echanical interlocking.

In the case of binder−brick adhesion,it has been shown that there is rst absorption of the binder by the pore of the brick and then chem ical interactionswith the creation ofan interphase.

In the case of binder−wood adhesion,anchorpointswerealso created bythe penetration ofthe binderinside the pore ofthe wood before the binderconsolidation.This observation was justi ed by an X-ray m ap ofthe elem ent potassium realized on the wood/

binderinterphase.

N ext,the e ectofthe penetration ofthe binderinsidethem aterialsand theconsequenceson thebinder form ation m ust be understood. Speci cally, if the binder penetrates the m aterials,itinduces chem icalinteractions that could m odify the reaction m echanism s during the consol- idation.Investigation ofthenatureofthenetworksform ed after consolidation and contactwith the m aterialsisthusnecessary.

Geopolym ers are am orphous three-dim ensionalalum inosili- cate binder m aterials thatare synthesized atam bienttem per- ature by the alkaline activation ofalum inosilicate sources,such

Received: O ctober30,2013 Revised: January 27,2014 Accepted: February 10,2014 Published: February 10,2014

pubs.acs.org/IECR

as calcined clays, industrial waste, and m ore.

These m aterials show good resistance to high tem peratures and acid degradation, as well as good com pressive strength. Their form ation results from the dissolution and reorganization of raw m aterials. First, reactive alum inosilicate sources are dissolved to form free Si[O H ]

and Al[O H ]

species,which will then react by a polycondensation reaction to form an am orphous geopolym er network.To further understand and highlightthe di erentnetwork form ations,atom ic bond scale analyses(Si−O −M ,M = Si,K,Al) are necessary.M agic angle spinning nuclearm agneticresonance(M AS N M R)analysisisa usefulcharacterization technique for investigating the silicon environm ent in the binder. This technique allows the determ ination ofthe di erentnetworksthatcan be form ed in the binder after contact and consolidation with the natural m aterials.

SiM AS N M R analysis,conducted byAutefetal.on geopolym er m aterials,has highlighted di erent contributions that correspond to di erent silicon environm ents.

These results showed ve contributions centered around −80,−88,

−97,−106,and −113 ppm ,which are attributed to Q

(2Al), Q

(3Al), Q

(2Al), Q

(1Al), and Q

(0Al) from silicic acid bonds,respectively(m oredetailsin theResults).In situ Fourier transform infrared spectroscopy (FTIR)isanotherwidely used technique for the study of the form ation of the polym er network.

Itisparticularly e cientforfollowing the structural evolution ofm aterials in the polym erization process,notably, the substitution of Si−O −Si by Si−O −Al bonds, which is characteristic ofthe geopolym erization reaction.

This work thus com pletes previous works on the use ofa geopolym er binder in tim ber-fram ed construction using earth bricksasin ll.Thisstudy attem ptsto understand the reaction m echanism s intervening between the natural geopolym eric binder and natural m aterials (wood and earth brick) and proposes a form ation m odelofthe binder in the function of m aterialsin contact.First,the resultsofin situ FTIR and

Si M AS N M R analyses perform ed on the binder, which was consolidated in contactwith m aterials,are presented.N ext,30 geopolym er binder form ulations were investigated by in situ ATR (attenuated totalre ection) FTIR analysis,which should allow a better understanding of the e ect of the siliceous specieson the nature ofthe form ed networks.

M ATERIALS AND M ETHODS

Raw M aterials.In this study,severalprecursors and raw m aterials were used with distinguished precursors for the bindersynthesisand raw m aterialsused forthebuilding system (earth brick and wood). Concerning the m ineral polym er binder, various sam ples were synthesized using potassium hydroxide pellets (85.7% purity),potassium silicate solution (H

O = 76.07% ,SiO

= 16.37%,and K

O = 7.56%),m etakaolin M 1000 from AGS (Clerac,France),and silica fum e.Three types of silica were used: silica fum e supplied by Feropem (denoted FD S),silica fum e supplied by Cabot(denoted M 5), and crushed quartz(denoted Si400).D etailsofallsilicaused in thiswork are presented asSupporting Inform ation (Table S1).

Concerning m aterials applied for the building system ,two extrusion-m anufactured industrialearth brickswerechosen,the sam e bricks used in previous work.

The bricks di ered in m ineralcom position (denoted Br

and Br

),and theirdensities were 1.70 and 2.00 g cm

forBr

and Br

,respectively.A local com pany ofLim oges,France,supplied D ouglas rwood that was dried and planed.The details ofassem bly m anufacturing are presented in previouswork.

Sam ple Preparation. All binders were prepared by m agnetic stirring of the potassium hydroxide pellets in the solution ofpotassium silicate,followed by the addition ofthe silica fum e (or quartz) and m etakaolin. It is im portant to distinguish thatthereferenceFD S

binderisused forthe nal com posite system and the other binders were used to investigate the e ect ofsilica on the nature ofthe networks form ed during the consolidation of the m aterial. Figure 1 sum m arizes the synthesis protocoloffoam FD S

and other binders.

The type and am ountofsilica introduced into the m ixture directly in uence the characteristics of the nal binder. To assess theirin uence,two binderseries have been developed, which are noted assubstitution (FD S

M 5

;FD S

Si400

; M 5

Si400

) and variation (FD S

M 5

Si400

).For each series ofbinder,the am ountofm etakaolin,potassium silicate, and potassium hydroxide rem ain identical,asforthe reference foam FD S

;only the type and the am ountofsilica added to the m ixture have been m odi ed.N om enclature exam ples for the synthesized binders are presented as Supporting Inform ation (Table S2).The indexed num berbetween 0 and 100,which appears after the nam e of the used silica,is the weightpercentofthe silica introduced into the m ixture

The substitution sam pleshave the sam e weightpercentofsilica as the reference FD S

;only the type ofeach silica introduced changes. For variation sam ples, the weight percent of introduced silica varies from 0 to 100% in 20% increm ents.

Shown on Si−Al−K−O ternary,it is noted that synthesized binderscorrespond to sedim entarym aterialsand gelsaccording to the work ofGao etal.,

asshown in Figure 2.

To evaluate the e ect of the contact of the structural m aterials (earth bricks and wood) on the consolidation and network form ation ofthe reference binderFD S

,the binder in contact with these m aterials was analyzed. For this experim ent, a hole was drilled in the m aterials, and the reference binder (foam FD S

) was poured inside. In situ FTIR analysis was perform ed during consolidation and M AS N M R analysisafterconsolidation.

Characterization Techniques. FTIR spectra were ob-

tained using a Therm o Scienti c N icolet 380 infrared

Figure 1. Synthesis protocol of geom aterial foam (A) and other

sam plessynthesized based on foam form ulation (B).

spectrom eter using the ATR m ethod. The IR spectra were collected between 500 and 4000 cm

,with a resolution of4 cm

.To m onitor sam ple form ation,a m acro was used that allowed spectra to be recorded every 10 m in for 10 h.The acquisition wasinitiated afterplacing a drop ofsam ple,orthe m aterialwith binderinside,onto a diam ond substrate.Finally, overlaid raw spectrawereobtained.To rem ovethecontribution from atm ospheric CO

, the spectra were corrected with a straightline between 2.280 and 2.400 cm

.The spectra were corrected using a baseline and then norm alized before being com pared.

H igh-resolution M AS N M R experim entswere perform ed at room tem perature in a Bruker AVAN CE-400 spectrom eter operated at79.49 M H z (

Sisignal).The

Si(I=

/

) M AS N M R spectra were recorded after /2-pulse irradiation (4 ls) using a500-kH z lterto im prove the signal/noise ratio.In the M AS N M R experim ents,powdersam pleswerespun at10 kH z.

The num ber ofscans was 400 for silicon.The tim e between accum ulations was setat10 s to m inim ize saturation e ects.

Spectral deconvolution was perform ed using the W in t (Bruker) software package. The deconvoluted spectra are only used qualitatively,and no quantitative studies have been perform ed.

Figure 2.Blue squaresshow the position ofm ixturessynthesized on the (Si−Al−K−O ) ternary (existence dom ains of several m ixtures proposed byGao etal.:1,sedim ented m aterials;2,gel;3,geopolym er;

and 4,hardening m aterials).

Figure 3.

SiM AS N M R results:(A) deconvoluted spectra in the case ofFD S

(foam );(B) exprerim entaldata forFD S

W ood,FD S

Br

,

FD S

Br

,and FD S

;and (C)percentageoftheareaofeach contribution determ nided from deconvulated spectrafor(white)FD S

W ood,(light

gray) FD S

Br

,(dark gray) FD S

Br

,and (black) FD S

.

W ettability testswere perform ed by placing a drop offoam FD S

on the wood and bricksatroom tem perature.Contact angle,absorption,and spreading characteristicswere observed.

The BET speci c surface area ofthe silicaswasdeterm ined byN

adsorption at−195.85 °C usingaM icrom etricsTristarII 3020 volum etric adsorption/desorption apparatus. Prior to m easurem ent, the sam ples were degassed at 200 °C under vacuum for4 h (Supporting Inform ation,Table S1).

RESULTS

Interaction betw een M aterials. To determ ine if the adhesion between the m aterials is possible,itis im portantto obtain data on the wetting propertiesand m aterialssurface.

W ettability Test. W ettability test results are gathered as Supporting Inform ation (Table S3).W ettability testson bricks and wood provide inform ation aboutthe drop spreading and underline the di erentabsorption properties ofthe m aterials.

The wettability testsshow thatthe contactangle att= 0 was closed to 90°forwood and Br

,whileforBr

itisbelow 90°.At t= 7 m in allanglesarebelow 90°.Thisresultthereforejusti es an adhesive com patibility between these m aterials. The absorption is signi cantly higher for brick Br

than for brick Br

.After less than 5 m in,an aureole was visible around the drop deposited on Br

,justifying the absorption ofpartofthe binderby the brick.ForBr

,no aureole was observed,which can be explained by a di erence in term softotalporosity and pore size distribution of the bricks. The porosity values determ ined by m ercury intrusion ofeach brick are 35% and 21% forBr

and Br

,respectively.

M oreover,them edian pore diam eter of the Br

brick is approxim ately 3.7 m ,and the poresare m ostly between 1 and 10 m .Forthe Br

brick,the pores are sm aller,with a pore size distribution ranging from 0.005 to 0.8 m and the m edian diam eteratapproxim ately 0.7

m .Penetration ofthe binder into the brick depends on the pore size.In the case ofthe Br

brick,the binder can easily penetrate into the large pores,allowing itto penetrate deeper and fastercom pared to the Br

brick.Forwood,absorption is interm ediate and di ers according to its nature,as itis m ore im portantforthespringwood than thesum m erwood.

These observationswarrantapenetration ofbinderm aterials.Thelow contact angle also re ects an easy adhesion between these m aterials.

The wettability tests have shown that there was binder penetration through theporesofthem aterial.Thisobservation is corroborated by the m echanical shear and pull-out tests perform ed in apreviousstudy.

Thepenetration ofthe binder induces necessary chem icalinteractions,with m aterials being able to m odify the nature ofthe networks form ed inside the once consolidated binder.

Analysis of Interactions betw een the Binder and StructuralM aterials.To highlightand better understand the various interactions occurring at the local scale, nuclear m agnetic resonance and FTIR spectroscopic analyses were perform ed to determ ine the environm ents of the silicated speciesofthe binder.

M AS NM R Analysis.The silicated speciesare described with the usual notation, Q

, where n ranging between 0 and 4 indicatesthe degree ofconnectivity ofsilicon,i.e.,the num ber of bridging oxygens.

The di erentiation of the num erous species is obtained by the study ofthe chem icalshift,which strongly depends on the coordination num berofsilicon.In a geopolym eric binder,the silicon is principally in a tetrahedral coordination and is bonded with alum inum . Thus, for

geopolym erm aterials,theQ

(mAl)notation isused to describe the environm entofsilicon,where m ranges from 0 to 4 and represents the num ber of connected alum inum atom s. As before,n ranging between 0 and 4 indicates the num ber of bridging oxygens.

For exam ple, Autef et al. have put in evidence for a dense geopolym er for which bands centered around −90, −95, −100, −105, and −110 ppm can be attributed to Q

(4Al), Q

(3Al), Q

(2Al), Q

(1Al), and Q

(0Al),respectively.

M oreover,ithas to be noted thatin function ofthe chem icalcom position the position ofthe band can be slightly di erent.

Figure3A showsthespectrum ofthereferencebinder(foam FD S

)used forthe construction ofthe building system .This spectrum was deconvoluted into ve contributions, in accordance with the phases highlighted by the work of Prud’hom m eetal.

on thism aterial:threem ajorcontributions centered at−88,−97,and −106 ppm ,denoted ascontribution 2,3,and 4,respectively,and two m inorcontributionscentered at −80 and −113 ppm , denoted as contribution 1 and 5, respectively.

M inority phases m ay be attributed to the contribution of depolym erized species[speciesQ

(1Al),Tognonvietal.

]for band 1 (−80 ppm )and silicicacid forband 5 (−113 ppm ).

Band 2 (−88 ppm ),presentin the binder,can be attributed to the m ain contribution of the geopolym er phase.

In com parison to the literature on geopolym er m aterials, the position ofthe observed chem icalshift(band 2)variesslightly.

Thiscan beexplained bythedi erenceofthereaction m edium asa resultofthe release ofdihydrogen and di erentlevelsof silica.

Contribution 4,centered around −106 ppm ,can be attributed to quartz

present in the m etakaolin or silicate speciesofsilicagel.

Finally,contribution 3,thelargestband located at−97 ppm ,m ay be attributed to (i) a zeolite phase, (ii)aK

Si

O

phase,or(iii)thepresenceofsiliceousspeciesin Q

(2Al)correspondingto an environm entofan alum inosilicate m aterial.

This contribution willbe used for reference laterin thestudyastheprim aryalum inosilicatecom pound,but itshould be noted thatthe elem entpotassium isalso present.

Figure 3B presents the

Si M AS N M R spectrum of the reference binder(foam FD S

,Si/K = 2.6,Si/Al= 3.93) and the spectrum ofthe sam e binderconsolidated in contactwith each of the three m aterials (FD S

Br

, FD S

Br

, and FD S

W ood).

In general,whicheverm aterialisin contactwith the binder, there is always a prevalence ofband 3 centered at−97 ppm (Figure 3B).O ther contributions located at −88,−106,and

−113 ppm show variable intensities.It should be noted that contribution 1 (−80 ppm )isnotdetected on thedeconvoluted spectrum ofthe FD S

Br

sam ple.

To understand the phenom ena responsible for these intensity di erences,the integrated area ofeach contribution for FD S

W ood,FD S

Br

,FD S

Br

,and FD S

(foam ) sam ples was plotted in Figure 3C.This representation allows fortheevaluation ofthecontributionsofthedi erentnetworks present after consolidation.As seen above,contribution 3 is alwayspresentand wastherefore considered asthe m ain band.

This shows thatthe species within the porous alum inosilicate

geopolym er binder are notchanged.It should be noted that

contribution 2,attributed to the geopolym erphase,decreased

forallthe sam ples.Thissuggestsa decrease ofalum inosilicate

species inside the binder necessary for the form ation of the

geopolym er network.This fact is m ore pronounced for the

FD S

Br

sam ple.In return,thisphenom enon iscom pensated

forbyan increasein thecontributionsofenvironm ents4 and 5 forallthe sam ples,butin di erentproportions,depending on the substrate in contactwith the binder.

(i) In the case ofwood support(FD S

W ood),the slight decrease ofthe geopolym erband (−88 ppm ) associated with an increase ofthe characteristic band ofsilica gel(−106 ppm ) and silicicacid (−113 ppm )isaresultofthepenetration ofthe binder into the wood bers. Indeed, this very hygroscopic supporthasahigh a nityforalkalineaqueoussolutions.

This is in agreem ent with the transfer of the potassium elem ent observed in a previousstudy.

The speciesin interaction with the wood are silicated speciesenriched with potassium .

(ii)In thecase ofthe FD S

Br

sam ple,thelow intensity of thecontribution ofsilicicacid (−113 ppm )can beexplained by an in-depth lim ited penetration ofthebinder.Indeed,thisbrick com posed of sm allpores (from 0.01 to 0.80 m ) does not allow the speciespresentin the reaction m edium to penetrate deeply(<100 m )into thebrick.Thisfactwilllocallyinducean alteration ofthe brick by a partialdissolution ofclays in the alkaline m edium and release species thatwillbe able to react and supply the geopolym er phase whose contribution varies slightly.

(iii)In thecaseoftheFD S

Br

sam ple,thebinderwillalso enterthe brick butm ore deeply (>500 m ) as a resultofits higherpore size (1−10 m ).In thiscase,the speciesreleased by the deterioration of brick will tend to react locally, consum ing the species necessary for the form ation of the geopolym erphasein thebinder.Thiswillresultin thecreation ofdi erentspeciation equilibrium sand thuscreate a chem ical interaction in the liquid precursor,thuschanging thepH value.

Then,therewillbeareduction in thatvaluewhich willprom ote the appearance ofsilica geland silicic acid.

The low contribution at −80 ppm for FD S

W ood and FD S

Br

characterizes the presence of alum inosilicate depolym erized diluted species.

These m aterials willnot be ableto condenseand participatein thevariousnetworks.In the case ofthe FD S

Br

sam ple,with thereaction m ixture having di used into brick, there is little or no form ation of these isolated depolym erized species.

The N M R spectroscopy results have evidenced various networksform ed afterinteractionswith m aterials.

In Situ FTIR Analysis. FTIR spectroscopy results on the binder with support tests (FD S

Br

, FD S

Br

and FD S

W ood) provide inform ation on possible interactions during the consolidation of the binder in contact with the m aterials.In situ m onitoring of sam ples over a 10-h period givesinform ation aboutthe rearrangem entofthe network.In particular,the band centered atapproxim ately 980 cm

shifts to alowervalueduring theform ation,which ischaracteristicof thegeopolym erization reaction and correspondsto the Si−O − M (M = Si or Al) vibration band.

By m onitoring this displacem ent, it is possible to evaluate the kinetics of the geopolym erization (slope of the curve) and the structural evolution ofthe network (totaldisplacem entofthe band).In situ FTIR m onitoring testsperform ed on the reference binder, FD S

W ood, FD S

Br

, and FD S

Br

show a strong di erence in term s ofreaction kinetics and form ed networks.

This resultcon rm s the interactions between the binder and m aterials during the consolidation. Figure 4 shows the evolution of the position of the band corresponding to the Si−O −M bondsversustim e forFD S

,FD S

Br

,FD S

Br

, and FD S

W ood sam ples.The variationsm ay be divided into two regim es depending on tim e. At the beginning, for all

sam ples,there is an increase ofthe displacem entvalue ofthe band Si−O −M . After approxim ately 100 m in, there is an inversion ofvariation.

(i)From the rstm inutesofthe reaction (t< 100 m in),the increase of the position of the Si−O −M band translates to di erentspeciation equilibrium s,likely resulting from absorp- tion phenom ena by the various supports.This suggests that thereisSi−O −Sientitiesform ation assilicicacid isobserved by N M R.

(ii)After100 m in,two typesofbehaviorsare observed,one characteristic ofgeopolym erization reactions with a signi cant decrease in the Si−O −M band position (FD S

Br

) and the othercharacteristic ofa m etastable equilibrium with very little variation ofdisplacem ent(FD S

Br

and FD S

W ood).In the case of the FD S

Br

sam ple, the existence of interactions between thealtered clay particlesand thereaction m edium can lead to variousalum inosilicatenetworksobserved byN M R.For FD S

Br

and FD S

W ood sam ples, as a result of greater binderpenetration into thesupport,thespeciation equilibrium s are m odi ed. This leads to a m etastable state that is characterized by few transfers of species. Interactions are lim ited,and thus,in situ geopolym erization reaction detection isdi cult.

These results are in agreem ent with the wettability tests presented previously,which show thattheabsorption tim e ofa drop ofbinderism ore im portantforBr

brick than forwood and the Br

brick.

E ectofSiliceous Species on the BinderForm ulation M echanism .D i erences were observed between the binder aloneand thebinderin contact.Thedi erenceswereexplained by avariation ofthe availability ofthe siliceousspecies.Itthen appears judicious to understand the role of certain siliceous speciesin speciation equilibrium sby m odifying the am ountof silica introduced.N ew form ulations denoted “variation” and

“substitution”weretested,wherethenatureand theam ountof silicaintroduced weretheparam etersofstudy.In thiscase,each binder was consolidated under the environm entalconditions withoutsupportcontact.Sixdi erentsetsofform ulationswere synthesized and analyzed by in situ FTIR.

Description ofSubstitution and Variation Tests.Foreach

form ulation ofeach set,the m onitoring ofthe Si−O −M band

was perform ed and the totaldisplacem ent and slope of the

curve were m easured and com pared.PartsA and B ofFigure 5

Figure 4.W avenum berevolution ofthe Si−O −M band asafunction

of tim e for (blue circle) FD S

Br

, (red star) FD S

Br

, (green

triangle) FD S

W ood,and (black square) FD S

(foam ).

present the evolution of the Si−O −M band position as a function oftim e forthe series substitution Si400

M 5

and variation Si400

,respectively.M oreover,apictureofallsam ples synthesized was taken after 1 week of drying at am bient tem perature and are gathered as Supporting Inform ation (Table S4). It is observed that as a function of type and am ountofsilica added,the m orphology ofthe sam ples di er.

Subsequently,both the features ofthe sam ples synthesized as wellasthe m onitoring ofthe Si−O −M band willbe discussed.

FDS

Si400

. The FD S

Si400

, FD S

Si400

, and FD S

Si400

sam ples show two distinct phases after consolidation: one having the appearance of a dense geo- polym er withoutcracking (lower phase) and the other being glassy,sim ilar to a silica gel (top).The other form ulations (FD S

Si400

and FD S

Si400

) have a uniform appearance but with shrinkage on the bottom and an upper portion resem bling a gel.Both typesofbehaviorare prim arily because of the FD S/Si400 ratio that im poses the rate of silicon available, which goes into solution and participates in the reaction.Regardlessofthe sam ple type,there isa decrease of the displacem entvalue,which characterizesthe polycondensa- tion reactions between the di erent alum inosilicate species.

H owever,the m ore silica Si400 increases,the m ore the overall value ofthe displacem entquickly increases.Thisresultcan be explained by the following param eters.

(i) W hen X = 0,a reagentm ixture between the m etakaolin and the alkaline solution is created, which leads to the geopolym er network form ation. Excess species that do not react contribute to the developm ent ofa gelon the surface.

Si400 particlesare encapsulated in the geopolym erbinder.

(ii) W hen 0 < X < 60, com petition between di erent networks (geopolym er and gel) exists by the interaction of silica fum e FD S, which generates dihydrogen and releases siliceousspeciesthatcan participate in geopolym ericnetworks.

(iii) W hen X 60,a very high reactivity ofthe silica fum e FD S isobserved,which controlsthe reactivity ofthe m edium by releasing siliceousspecies.

FDS

M 5

.Allsam ples in this series exhibit one phase,

except for the biphasic FD S

M 5

.A di erence of volum e

expansion is observed in these sam ples, which is m ore

im portantthan the high am ountofFD S silica.

The shiftof

theSi−O −M band decreaseswhen theproportion ofM 5 silica

fum e increases.The di erence in reactivity ofthe two silicasis

directly related to theirspeci c surface area,40 and 202 m

/g

forFD S and M 5,respectively.In the presence ofa signi cant

am ount of M 5,after several m inutes,the m edium becom es

saturated with siliceous species. Then, there is com petition

between the form ation of a Si-rich network and geo-

polym erization reactions, which are characterized by a very

low shift of the Si− O − M band. This observation is

Figure 5.Evolution ofSi−O −M band position in function oftim e for(A) Si400

M 5

and (B) Si400

series.

corroborated by the slope ofthe curve near zero for M 5

, characteristic of a saturated siliceous m edium and by its increase with the augm entation ofthe am ountofFD S silica.

Si400

M 5

Series.Two distincttypesofbehaviorand an interm ediate behavior are observed.For X < 40,the m ixture appears hom ogeneous, but evolves with tim e. N ext, there appears a centralcollapse,suggesting that som e species have

precipitated.

For X 40, two phases appear; the lower

portion resem blesadensegeopolym er,whiletheupperportion

exhibits a glassy aspect.M oreover,the volum e of the upper

phase increaseswith the increase ofSi400 silica.Regardlessof

thesam ple,them oretheM 5 quantityincreases,thegreaterthe

shiftofthe Si−O −M band and the slope decreases.Asforthe

FD S

Si400

substitution, the low-reactive silica Si400

Figure 6.FTIR totaldisplacem entvariation value according to the (nSiintroduced)/(nSitotal) ratio for (A) Si400

,(B) FD S

Si400

,(C)

Si400

M 5

,(D ) FD S

M 5

,(E) FD S

,and (F) M 5

.

(crushed quartz) has little in uence on the form ation ofthe network and the viscosity. This di erence in reactivity is directlyrelated to theirspeci csurfacearea,1 and 202 m

/gfor Si400 and M 5,respectively.

FDS

series.Two typesofconsolidation areobserved.ForX

< 60,there are two separate phases;the lower portion has a dense geopolym er appearance without cracking, while the upperphase showsaglassy featurewith aporousinterface.For X 60, sam ples after consolidation show heterogeneities because ofthe release ofH

gas and the am ountofsiliceous speciesin the reactive m edium .These speciesare the origin of theform ation ofvariousphases.Thevariation in theam ountof FD S silica fum e certainly causes a di erent distribution of observed phase within the geopolym er foam , i.e., silica gel, K

Si

O

com pound,zeolite,and the geopolym er network.

The valueofthe shiftofthe Si−O −M band decreaseswith the augm entation ofthe am ountofFD S silica fum e.FD S

/FD S

and FD S

/FD S

show nearly identical nal displacem ent values ofapproxim ately 32 and 26 cm

,respectively.These variationsarein agreem entwith thezonesidenti ed previously (speci cally forX < 60,ahighervalue ofdisplacem ent,and for X 60,a lowervalue) and with the work ofPrud’hom m e et al.

Indeed,the reduction ofFD S silicafum e willprom ote the form ation ofa geopolym ernetwork,and itsform ation willbe m ore signi cant than the high displacem ent value.For low levelsofsilica,thecom position approachesasinglegeopolym er network.In contrast,when the am ount of FD S silica fum e increases, there is com petition between the form ation of di erentcom pounds,such assilicagel,K

Si

O

com pound,the zeolite phase,and the geopolym ernetwork.

M 5

Series.As before,the M 5

series shows two types of behavior.ForX 60,asingle phase isobtained,which hasthe appearance ofa gelthatwilleventually collapse.For X < 60, two phases appear; the lower portion resem bles a dense geopolym er,while the upperphase exhibitsa glassy aspect.As previously described,the increase in available silica causes a decreasein displacem entasaresultofthecom petition between thedi erentalum inosilicate species.W hen the am ountofsilica is su cient to form a network geopolym er, it form s very quickly (0 < X < 40).In the opposite case (X 40),there is again a com petition between a geopolym er network and a saturated siliceous species liquid,which leads to a gel.These phenom ena are also observable by the variation in slopes.

Si400

Series.Finally,forthe Si400

series,two phasescan be distinguished for allthe sam ples.The bottom phase has featuresofadensegeopolym er,whereastheupperphaseshows a vitreous aspect and presents a great dealofshrinkage,and their interface shows cracking. The augm entation of the am ount of Si400 silica has no in uence on the Si−O −M band shiftvalue,which rem ained approxim ately 40 cm

forall sam ples.Thishighlightsthe very low reactivity ofSi400 in the reaction m edium .In thiscase,the Si400 silica sim ply playsthe role ofreinforcem ent.

N evertheless,the observed dem ixing (geopolym erphase and gelupon the surface)suggeststhatthe consolidated sam ple is com posed ofatleasttwo networks,in accordance with the work ofAutefetal.

Sum m ary of the Reactivity and the Nature of the Networks Form ed. To com pare the di erent form ulations, Figure 6 shows the displacem entvalue ofthe Si−O −M band determ ined by FTIR spectroscopy asa function ofthe ratio of the num berofm oles ofsilica (Si400,M 5,orFD S) added by the totalnum berofsilica in the form ulation.

Si400 In uence.The low shiftofthe band from 38 to 46 cm

forsam plesSi400

to Si400

(Figure 6A)asafunction of the nSi400/nSi total ratio is characteristic of both the form ation of a geopolym er network and the coating of the quartz grain by an alum inosilicate binder.

In the presence ofreactive silica,increasing the nSi400/nSi totalratio causesan increasein thedisplacem entvaluefrom 29 to 46 cm

forsam plesFD S

Si400

to FD S

Si400

(Figure 6B) and from 12 to 46 cm

for sam ples Si400

M 5

to Si400

M 5

(Figure 6C).This characterizes the nonreactivity ofSi400 silicawithin thereaction m edium becausethereislittle or no release ofsiliceous species.N evertheless,the variations are not identicalwith those of the FD S or M 5 silicas.The di erencesobserved re ecttheirreactivity,which isa function of their ability to release siliceous species in the reaction m edium ,thusm odifying the speciation equilibrium s.

FDS and M 5 Silica.Thisreactivitywascon rm ed duringthe form ation of the FD S

M 5

series. The change of the nSiM 5/nSi total ratio results in a slight change in the displacem ent value from 26 to 19 cm

for sam ples FD S

M 5

to FD S

M 5

(Figure 6D ).The substitution of FD S by M 5 silica leads to m odi cations in the speciation equilibrium s,thusprom otingtheform ation ofasilicagelto the detrim entofthe othernetworks.

These features are again highlighted forthe variation series FD S

(Figure 6E) and M 5

(Figure 6F).A slightdecrease in the shiftfrom 32 to 28 cm

forFD S

to FD S

and a sharp decrease ofthe displacem entfrom 39 to 6 cm

for sam ples M 5

to M 5

are noted.Consequently,unlike the FD S silica, with the m ore reactive M 5 silica, the siliceous species are available m orequickly to form ageland therefore decrease the value ofdisplacem ent.

Global Reactivity. To exacerbate the reactivity of the siliceous species,the displacem entofthe Si−O −M band as a function oftheslopeforallthesam pleswasplotted in Figure7.

Three areas can be de ned with values ofthe slope around

−0.18 and −0.10 cm

s

. Above −0.10 cm

s

, super- saturated siliceousspecieswillform aSi-rich alum inosilicategel, whereas below this value,there willbe form ation ofdi erent

Figure 7.Evolution ofthe totaldisplacem entvalue versus the FTIR displacem ent slope value of Si−O −M band position for ( lled triangle)FD S

M 5

,(open circle)Si400

M 5

,(large lled circle) Si400

FD S

,(open triangle) M 5

,(green box) FD S

,and (sm all

lled circle) Si400

series.

alum inosilicate networks. This corresponds to a value of displacem entrangingbetween 0 and 22 cm

.Thisobservation iscorroborated by the work ofGao etal.,

which showsthat for a value ofdisplacem ent ranging between 0 and 22 cm

, there is preferential bridging between the siliceous species, whereas forvalueshigherthan 22 cm

,there isform ation of various networks. M oreover, the position of synthesized com pounds on the ternary Si−Al−K−O (Figure 2) shows that the di erent considered form ulations are located in the areas of m aterials leading to gels or sedim ented m aterials presenting severalphases.Thisisin perfectagreem entwith the

resultsobtained.M oreover,itcan de neporousm aterialsasthe sam plespresenting avalueofsloperanging between −0.18 and

−0.10 cm

s

.

From thesedata,itisthuspossibleto delim ittheexistenceof

varioustypesofm aterialsaccording to the displacem entofthe

Si−O −M band and the slope ofthe curve.Fora given silicate

solution,three areashave been identi ed:an area correspond-

ing to a Si-rich alum inosilicate gel, an area com posed of

di erentporous networks,and an area com posed ofdi erent

dense networks.M oreover,allpreviousanalyses,perform ed at

Figure 8.Form ation schem eofFD S

(foam )in interaction with wood and brick Br

(A)and Br

(B)atthebeginning (A1,B1)and attheend of

the reaction (A2,B2).

constant water content,suggest that it would be possible to m ovefrom one areato anotherby changing the wateram ount.

DISCUSSION

Thepreviousstudyhelped highlightthedi erentenvironm ents presentin the binderaftercontactwith the structuralm aterials (earth brick and wood),aswellasan existence rangeofseveral typesofm aterials by FTIR spectroscopic analysis.In fact,the existence of di erent networks, especially during the FD S

M 5

substitution series,has been shown.From allof these data,the binder form ation schem e is proposed for the two types of assem bly in Figure 8.Figure 8 puts forth the various hypotheses on the interaction between m aterials explained below.For the two types ofassem blies,the wood and the brick are represented.The evolution ofspecieswithin the binderisgiven fordi erenttim esduring the consolidation.

It should be noted that there is a scale factor between the representation ofwood,brick,and species.Foreach tim e,the di erent contributions and the di erent networks described above were reported.

Interaction ofW ood/Binder.M orphological(wettability) and structural (N M R and FTIR) analyses have shown the absorption ofthe binder by wood porosity.M oreover,ithas been shown thatafterconsolidation,the Si-rich alum inosilicate network and silicagelwerestillpresent.Thegeopolym erphase is also present but in sm aller quantities. The decrease in geopolym er phase suggests thatthere has been a decrease in siliceousspeciesable to reactwith the alum inousspecies.This defectofconcentration in siliceous species m ay be character- istic ofan exchange with the wood.

Consequently, at the tim e of the contact with wood, as shown in Figure 8 (A1 and B1),the exchange highlighted in previous work

is thus ensured by the potassium silicate solution.Thisgivesthe localform ation ofsilicic acid (band at

−113 ppm ).Thistype ofacid can be form ed with a reduction ofpH value,which can occurduring thetransferofthem ixture within the wood bers,wood being acidic in nature.

This phenom enon will result in the creation of siliceous depoly- m erized speciesricherin alum inum (band at−80 ppm ).These data corroborate the observations by FTIR spectroscopy, initiated by an exchange ofsiliceousspeciesin supersaturation (form ation ofsilicicacid justi ed bytheincreaseoftheSi−O − M band position),followed bythedepolym erized species(light shiftofthe band position).

Interaction of Br

/Binder. In the sam e m anner, the m orphological and structural analyses showed a slow and di use absorption of the binder by the Br

, which is characterized by a sm all pore size.The sam e contributions, i.e.,a geopolym er network,a Si-rich alum inosilicate environ- m ent,and silicagel,arenoted.Thesiliceousspeciesin solution, in the presence ofthe surface ofbrick,willinteractand form sim ultaneously (i) the silicic acid from the penetration ofthe binderand (ii) a geopolym erphase because ofthe localattack ofthe clay particles.The form ation ofdepolym erized species also suggests thatan excess ofsiliceous species (in very sm all am ount) does notparticipate in the network form ation (low intensity at−80 ppm ).In addition,FTIR spectroscopy re ects thesolventabsorption with thecreation ofsilicicacid asbefore, as shown in Figure 8B1. Then, there is the form ation of di erentnetworksjusti ed bythelargedisplacem entoftheSi−

O −M band,asshow in Figure 8B2.

The presence ofthese depolym erized species and ofsilicic acid can certainly prom ote interactionsand enhance adhesion between m aterials.

Interaction ofBr

/Binder.In thiscase,the m orphological (visualand wettability tests) and structural(N M R and FTIR) analyses showed very strong absorption ofthe binder by the brick Br

because ofits large pore sizes.The m ajoridenti ed networks are the geopolym er, the Si-rich alum inosilicate com pound, and the silica gel with the largest contribution.

Again,upon contactwith thebricksurface,thereisform ation of silicic acid,followed by a quick di usion through the m aterial.

Thiscreateslocally,in sm allam ounts,thegeopolym ernetworks and thus increases the form ation ofsilica gel.The absence of theband at−80 ppm (depolym erized species)can beexplained by the reactionsin the brick thatconsum e allavailable species.

FTIR analysiscon rm sthe variousinteractions leading to the di erentnetworks.

CONCLUSION

Thisstudy wasfocused both to describeand to understand the phenom ena that occur between a geopolym er binder and structuralm aterials,such asm ud brick and wood,and also to identify the e ect of siliceous species on the binder consolidation.

Variousform ulationscontaining di erentsilicaswereused to establish the areas of com positions,which result in either a hom ogeneousm aterialordi erenttypesofnetworks,depend- ing on the reactivity ofsilica introduced.Allofthese results, associated with N M R (

Si)and FTIR analyses,allowed forthe com pleteunderstandingoftransfersthatoccurattheinterfaces.

The di erentnetwork contributionshighlighted in the porous geopolym er binder (FD S

) were m odi ed during the consolidation and depend on the interactions with the brick and the wood. In particular, di erent proportions of the geopolym erphasewereobserved asaresultofthecontactwith the bricks and the form ation of silicic acid in the m ajority resulting from thecontactwith wood.M oreover,thesedataare closely related to the am ount and nature ofsiliceous species available in solution thatgovern the nature ofthe m onom ers and the form ation kinetics.

Additionally,to com plem ent previous studies and validate the feasibility ofsuch building system ,a fullscale (2 × 2 m

) wallwas built,and various tests in a clim ate cham ber were perform ed to investigatethehygrotherm alregulation e ciency.

ASSOCIATED CONTENT

*

Supporting Inform ation

Tables S1−S4,as noted in the text.This m aterialis available free ofcharge via the Internetathttp://pubs.acs.org.

AUTHOR INFORM ATION Corresponding Author

*E-m ail:sylvie.rossignol@ unilim .fr.Tel.:+33 5 87 50 25 64.

Notes

The authorsdeclare no com peting nancialinterest.

ACKNOW LEDGM ENTS

W e thank Isabelle Sobrado and Jesus Sanz from Instituto de

Ciencia de M ateriales de M adrid, Consejo Superior de

InvestigacionesCienti cas(CSIC),M adrid,Spain.

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