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The Mechanism of the Formation and Transformation of Ettringite

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The Mechanism of the Formation and Transformation of Ettringiteof,OF,The,the,and,THE,AND

    The Mechanism of the Formation and

    Transformation of Ettringite

    Vo1.2lNo.3JournalofWuhanLnivm~ityofFechnologyMater.Sci.Edpt.2006

    TheMechanismoftheFormationandTransformationofEttringite

    PENGJiahuiZHANGJianxinQUJindong

    (DeparlmentofBuildingMaterials,ChongqingUniversity,Chongqing400(05,China) Abstract:Theformationandtransfornuttionofettringitewerestudiedmeasuringtheionconcentration

    inliquidphca~eandana1)zingthecompositioninsolidphase.The@ctsofC3A,g)psurn,limeandC-S-Hgel

    onettrit~giteformationandtransformationweren

    0investigated.Theexperimentalresultsslum,that,wheng)P

    SIDTtwagpresentedinsolidphase,thecompositionofliquidphasewasinf~1)orofettringiteformation.Ettringite

    formationcorsetedofthreereactionsincludingtheformationof[Al(DH)63

    octahe&al,formationofcI4f

    polyhedraprisminwhichCaandAlpolyhedraarrangedaltenvate1),aswell?

    entranceofSOd.intothechan

    fofpolyhedra.Al(OH6Jformation,u,hiehwastheslowestreaction,controlledettringiteformation.

    TheconcentrationofAlO2

    wasamainfactorttuuinflue~edettringiteformation.erg~psuminsolidphase wasconsumed,[sD4]decreasedqui~.k1)andlAIO!

    ]increased,anditwaspossiblethatettringitetrans

    fo.~dintohydratedmonsulfatecalciumalumimae(becalledMinshort)inthermodymanics.eoftral~

    formationwascontrolledbydiffusionofAIO2.

Keywords:ettringite;hydratedrno,uulfntecalciumaluminate;stabilit?;transform;control

    step

    1Intr0ducti0n

    Theformationofettringiteiscloselyrelatedtothe setting,strengthdevelopmentandexpansionofcem

    ent'14

    .Acloseattentionispaidtothestabilityof

    ettringiteinthehardenedpaste.Thereasonisthatwhen thegypsuminsolidphasespendsitselfettringiteislikely totransfonnintohydratedmonsulfatecalciumaluminate (becalledMinshort),resultinginthereductionofsolid volumeofhydratedpaste,theincreaseofporosity,and thedropofmechanicalproperties.Extensiveresearches havebeencarriedoutonitsformationandstabilityall overtheworld'".Thereisacommonviewthatettringite formationismainlyascribedtodissolution..precipitation.. crystallizati0nmechanism一一一,butitsc0ntrolstepand

    determinationfactorduringtheformationarenotvery clearyet.Usually,thestudyonettringitestabilitvisfo

    cusedontheeffectoflSO4jinliquidphase7,8,but when[S04]inliquidphaseisverylow,itsstabilityis quitedifferent,showingthatettringitestabilitydepends onthekineticconditionduringthetransformation. Inthispaper,inordertodeterminethecontrolstep andpredominatefactorsinfluencingtheformationand transformationofettringite,thethermodynamicsandki

    neticsoftheformationandtransformationofettringitewere studied.Thestudywasbasedonthefollowingthreeas

    pects:thecharacteristicsofthedissolutionandstructure propertyofettringiteandM;therelationshipbetweenthe

    variationofaqueousionconcentrationandtheformation (Received:Ju1.2,2005;Accepted:Dec.10,2005) PENGJiahui(彭家惠):Prof.;E-mail:pengjh@cqu.edu.cn andtransformationofettringite;theeffectoftheconcen

    trationofC3A,lime,andgypsumadditionontheforma

    tionandtransformationofettringite.

    2Experimental

    2.1Preparationofpureminerals

    ThepuremineralsofC3AandCAwerepreparedac

    cordingtotheirchemicalcomposition.T}1erawmaterials appliedwereARgradeCaCO3andA1(OH)3ndedbv agateball,thenpassedthrougha80msieve.Thesam

    plesweremolded.Afterdrying,theywerecalcinedat 1350respectivelyinanelectricalovenfor4hours. Then,theproductobtainedwasgrindedandpassed througha80Ixmsieve.Thisprocedurewasrepeateduntil thefc0=0,checkedbytheglycerinalcoholmethod.T}1e ettringiteandMweresynthesizedwiththeARgrade Ca(OH)2,A12(SO4)3'18H2OandCAbythereactionas follows:

    H,O

    A12(S04)3?18H2O+6Ca(OH)23CaO?A12O3?

    3CaSO4.32H2O

    A12(S04)3'18H2O+lOCa(OH)2+2(CaO?A12O3)3 (CaO'A12O3.CaSO4'12H20)

    2.2Method

    Fortheexperimentwithmonomineralsandcement

    system,ahighW/Cratioof10andasmallW/Cratioof 0.27wereapplied,respectively.

Analysisofliquidcomponents:[SO4jwasdeter

    minedbyBaSO4massmethod,[C]and[A102]by

    anatomicabsorptionspectmmeterandpHvaluebyanu

    mericalpHmeter.

PENGJiahuietaf:TheMechanismoftheFormationandTrmlsformationof159

    Analysisofsolidcomposition:thehydrationprocess formono--mineralsandcementhydratedforacertainperi-- o({oftimewasstoppedbyalcoho1.Theyweredriedin vacuumandanalyzedbyXRDandDTA.

    3ResultsandDiscussion

    3.1CharacteristicsofettringiteandM

    ThedissolutioncharacteristicsofettringiteandMin deionizedwaterandIimesolutionwithvariousconcentra

    tionsat20?aregiveninTable1.

    Table1DissolutioncharacteristicsofettringiteandM EttringiteandMarebothsaltswhicharedifficultto bedissolved,andtheirsolubilityproductconstantsare 3.9x10~and1.35x10,respectively.Thesolubil

    ityofettringiteandMindeionizedwaterandlimesolution isuneven,andthedissolutionabilityofvariousionsis quitedifferent.Asforettringite,theSO4i'seasiestto

    bedissolved,andtheA102isthemostdifficulttobe

    dissolved,n(SO)/n(A10)being7.9(1.5for

    uniformsolubility),whiletheabilityofionstobedis- solvedinMisreversetothatofettringite,n(S04)/n

    (A102)beingonly0.09(0.5foruniformsolubility). Limecaninhibitthedissolutionofettringite,andespe- ciallythatofA102strongly.Withtheadditionof0.3g/

Llime,theequilibriumconcentrationof?dO2isno

    morethan0.4x10_.'Mandn(SO4)/n(A10,):

    37.9.

    600

    200

    0

    ×

    

    0

    Theunevennessofettringitedissolutionshowsthat itsdissolutionproceedsbysteps.Theliquidcomposition ishighlSO4]andlow[A102jwhenettringitestartsto

    crystallize,whilelowlSO4jandhighlA101jwhenit

    startstotransfoFITIinto!M.

    3.2Relationshipbetweenionconcentrationinliquid phaseandettringiteformation

    Thevariationoftheionconcentrationinliquidphase withhydrationtimeforCAandCAgypsuin(2:1bv

    wel'ght)systemsisgiveninFig.1.

    Gypsumandaluminatesmineraldissolvedinwater quickly.Inafewminutesaftertheadditionofwater,a supersaturatedsolutionofettringitewasformedand ettringitecouldbedetectedinsample.Afterhalfanhour ofhydration,theionconcentrationinliquidphaserema. inednearlyconstant,indicatinggypsumandC3Adis

    solvedwitharatesimilartothatofettringiteformation. After24hou~sh~drgtion,lhe1gypsuminsolidphase spentitself,lSOu'jandlCjdecreaseddramatical

    ly,and[A102]increasingrapidly.[SO4]reduced

from12O0x10Mto50x10~M.andCa2lfrom

    580×10Mtobelow100x10...M,whileA10,lin

    creasedfrom15x10Mto80×10~M.Atthisstage.

    theliquidcompositionwasunfavorableforthestabilityof ettringite,soitstartedtotransformundertheactionof A10,.After48hour'shydration.1otsofMphasetrans

    formedfromettringitecouldbefound.Thepreviousfacts haveshownthatatahighW/Cratio,thestabilitvof ettringitedependsontheionconcentrationinliquid phase,ie,thermodmamicconditioninthesystem. 3.3Eff&tofgypsunladditiononthestabilityof ettringite

    BvadditionofvariousamountofCsH,(PA:2.0%

    SO,PB:4.0%S03)intocementclinkerwith15%CA content,theeffectofgypsuinadditiononthestabilityof ettringiteWasinvestigated.

    m(c.A):m(Cs-I~):2:1lOl5

    Fig.1Compositionsofliquidphaseof

    C3A,CgH2atdifferentperiods

    InthesampleofPA.gypsumwasuseduponlyafter 1dayofhydration,andthentheettringitetransformation immediatelyoccused.7daylater,allettringitehad transformedintoM.InthesampleofPB,gypsuminsolid phasewasdepletedafter7day,andthenettringitestarted 2O253O351百—下—?—'

    20/.20/.

    (8)PA(b)PB

    Fig.2XRDpatternsofhydratedsampleofgroupP totransformintoM.TheMcontentinsolidphasein- creasedwithhydrationtime.28daylater,thetransforma-

    tionbecameslow,andafterthenettringitecoexistedwith Mphase.However,asrespecttothesampleinvolving 6.7%C3Aand3.5%SO3,ittook28daytoconsumeall ?

    ?,

    &h(3

    _

    

    _【×,0?r.

    160JournalofWuhanUniversityofTechnologyMater.Sei.EdSept.2006 thegypsum,soettringitewasverystable,andtherewas noMdiffractionpeakinXRDpatternsofhydratedsample for90day'shydration.

    Thepreviousexperimentsindicatethatthestability ofettringiteiscloselyrelatedtothegypsumaddition.rhis canbeexplainedthat,inagivencementsystem.the gypsumadditiondeterminesitsdepletiontimeandaccord

    inglytheconditionforettringitestartingtotransforminto Mphase.Themorecompactthestructureofthehardened paste,thegreaterthedynamicassistanceduringtransfor

    mation,andthemorestabletheettringite.

    3.4EffectofCAcontentontheformationandsta. bilityofettringite

    BymeansofaddingC3Aintocement.astudywas madeontheeffectofC1Acontentontheformationand stabilityofettringite.rr}1eresultsareshowninFig.3. FromFig.3,itcouldbeseenthattheC1Acontent ?

    U

?

    C

    0

    0

    ?_

    .

    

    cA/%

    Fig.3Strengthvariationofmainettringite characteristicpeak(d=9.73A)with

    C3Aadditionandhydrationtime

    hadagreateffectontherateofettringitef0rmation.,,ih

    theincreaseofC1Acontent,theintensitvofmaincharac

    teristicpeaksofettringiteincreasedtoo,meaningtherate ofettringiteformationaccelerated.InthecaseofCA contentexceeding10%.theamountofettringitein 28day'ssamplereducedduetotherapidconsumptionof gypsuminthepresenceofhighC3Acontentandthe transformationofettringiteintoMaftera1lgypsumcon

    sumed.TheeffectofC3Aonthestabilityofettringite couldbeascribedtoitsdeterminationoftheconsumption rateofgypsumandthekineticconditionaftergypsumcon

    sl1med

    3.5EffectofC-S-HgelonthestabilityofeUrin~te TheeffectofC-S-Hgelonthestabilityofettringite wasstudiedbymeansofsamplesofgroupG.

    cA

    3d

    7d

    28d

3d

    7d

    14d

    28d

    10152O_253035]————厂20/.''

    201o

    (a)Gl(C3A:CgH2=2:1)(b)G2(portlandcement,C3A5.6%,SO33.6%) Fig.4XRDpatternsofsamplesofgroupGatdifferenthydrationtime FromFig.4.itcouldbeseenthatoncea1lthegYP

    SU1TIinsolidphaseofsampleGlwasusedup,thetrans

    formationofettringiteimmediatelyoccurred,because ettringitewasnotstablewithouttheprotectionofCSH

    ge1.AsrespecttosampleG2,thestabilityofettringite wasgreatlyimprovedbecauseofthepresenceofC?S-Hgel

    inhydrationproduction.Incementpastetheformation rateofettringitewasmuchfasterthanthatofCSHge1.

    Furthermore,theirpotentialwasreverse,CSHgel

    negativeandettringitepositive.Withtheprogressofhy

    dration,CSHgelthalargespecificsurfacearea

    formedagelprotectionlayeronthesurfaceofettringite gradually.rr}1islayerresistedthediffuseofions.which increasedtheresistanceofettringitetransformation,there

    byenhancingthestabilityofettringite.Ifgypsumwas usedupattheearlystageofhydration,ettringitewaseasy totransformbecausetheprotectionlayerwasnotstillper

    fect

    4Analyses

    4.1Mechanismofettringiteformation

    Aftertheadditionofwaterincement,clinkerminer

    alandgypsumdissolvedquickly,generatingionssuchas

[Ca2],[OH],[Al()2],[S04],whichformeda

    togetherbyconcentrationdifference,andformedettringite asthefollowingthreesteps:

    Al02+20H+2H20:lA1(OH)6J(1)

    2[A1(OH)6]+6Cd+24H20={Ca6[A1(OH)6]2?

    24H,0}(2)

    {Ca6[A1(OH)62?24H20}+3S04+2H20=;Ca6

    [AI(OH)2?24H20}[3S04?2H0](3)

    Accordingtotheeffectsofdifferentionsonthefor- mationrateofettringite,itcanberefenedthatreaction (1)isthedominantstep.TheformationofAl

    (oH)6oetahedronisofmuchimportancetoestablish thebasicstructureofettringite.anditsformationratede

    pendedontheaqueous0Hconcentration.Bvanalvses

    oftheionconcentrationinliquidphase,itcanbejudged that0Hismuchmorestable.1OH1isabout170x

    10~M,whichismuchhigherthanlA102]atpH=12,

    whilelAl02]isgenerallybelow10x10~M,especial

    lyinthesaturatedlimesolution.Therefore,thereaction (1)rateismainlyinfluencedbylA10,.

    4.2Mechanismofettringitetransformation

    Aftergypsuminsolidphasewasusedup,ettringite transformedintoMasfollowing:

    C3A?3CsH32+4AlO2+6Ca2+80H=3C3A?CHl2

    Inthisreaction,?Gwas5.12kCal/mol,which

    supersaturatedsolutionforettringite.Theseionsdiffusedindicatesthatthereactionwasther

    modynamicallyfeasi

Vo1.21No.3PENGJiabuieta:TheMechanismoftheFormationandTransformationof16l

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