HydrationheatevolutionandkineticsofblendedcementcontainingsteelslagatdifferenttemperaturesFanghui Hana,b, Zengqi Zhanga, Dongmin Wangb, Peiyu Yana,*aKey Laboratory of Civil Engineering Safety and Durability of China Education Ministry, Department of Civil Engineering, Tsinghua University, Beijing, ChinabChina University of Mining and Technology (Beijing), Beijing, ChinaARTICLEINFOArticlehistory:Received12November2014Receivedinrevisedform19February2015Accepted21February2015Availableonline23February2015Keywords:SteelslagBlendedcementHydrationheatKineticsABSTRACTHydration heat characteristics of blended cement containing up to 50% steel slag were studied at 25

C,45

C and 60

C by isothermal calorimeter. Kinetics equations were used to explore kinetics of blendedcement. Kinetics parameters, n,K, and Ea,were calculated and analyzed. Results indicated that the activityof steel slag was very low.Steel slag accelerated the hydration of aluminates but decelerated thehydration of silicates in cement clinker. Small amount of steel slag affected little the hydration process ofcement. Elevated temperature obviously promoted the hydration of blended cement. The hydration ofblended cement containing no more than 35% steel slag was controlled by nucleation of hydrates inacceleration period and by diffusion of ions in decay period. The hydration of blended cement containing50% steel slag was mostly dominated by phase boundary reaction, then directly by diffusion. Eaincreasedwith increasing steel slag content.2015 Elsevier B.V. All rights reserved.1. IntroductionMineraladmixturesarewidelyusedinmodernconcreteeitherinblendedcementsoraddedseparatelyintheconcretemixer[1].Theuseofmineraladmixturessuchasgroundgranulatedblastfurnaceslag,yashandsilicafumewhichhavebothpozzolanicandlatenthydraulicpropertiescanimprovethepropertiesofconcreteandlimitenvironmentalimpactaswellasbringeconomicbenets.Withtheresourceshortageandrisingpriceofslag,yashandotherqualitymineraladmixtures,developingmorekindsofmineraladmixturesthatleadstosustainableconcretedesignandgreenerenvironmentisavaluablework.Currently,steelslagshowsagoodpotentialasmineraladmixtureinconcrete.Steelslagistheindustrialwastedischargedfromthesteel-reningprocessinaconversionfurnace.Itaccountsforapproximately1520%bymassofthesteeloutput[2,3].Morethan80milliontonsofsteelslagisdischargedinChinaeveryyear[4].However,thecurrentutilizationrateofsteelslaginChinaisonly22%,farbehindthedevelopedcountries[5].Thus,intensiveresearchworkisneededforhighutilizationrateofsteelslag.Amoreattractiveapproachisthereplacementofcementbysteelslag.ThechemicalcompositionsofsteelslagconsistofCaO4560%,SiO2 1015%,Al2O3 15%,Fe2O339%,FeO720%,MgO313%,andP2O514%[6].AnditsmainmineralsconsistofC3S,C2S,C4AF,C2F, C12A7,ROphase(CaO-FeO-MnO-MgOsolidsolution)andfree-CaO[68].DuetothemineralsofC3S,C2S,C4AF,C2F, andC12A7,steelslaghashydraulicproperties.ButtheactivityofthecementitiousmineralsinsteelslagismuchlowerthanthatinPortlandcement[9].Itisrelatedtothecrystallinestate,thecementitiousphasesofsteelslagcrystallizemuchbetterthanthatofPortlandcementclinkerowetothelowcoolingrateofsteelslag.Meanwhile,steelslaghaslargeamountofnon-activecomponents,suchasRO phaseandFe3O4.Theeffectofsteelslag ontheearlyhydrationofPortlandcementhas beenstudied byseveralresearchers.Kourounisetal.[6]investigatedthepropertiesandhydrationofblendedcementswithsteelslag andfoundthatblendedcementdevelopedlowerearly-agestrengthcomparedtoPortlandcementandits strengthdecreasewashighwhenthecontentofsteelslag washigh.Theaddition ofsteelslag sloweddownthehydration ofblendedcements,andtheblended cementsshowedlongersettingtimethanPortlandcement. Tsakiridis etal. [8]studied theutilizationofsteelslag for Portlandcement clinkerproductionandpointedoutthattheaddition ofsteelslag by10.5%intherawmealdidnotaffectthehydrationprocessduring Portlandcement production.Wangetal.[10]foundthatthedormantperiodofblendedcementcontainingsteelslagislongerthanthatofPortlandcement.Monshiand Asgarani[11]found thatblending10%extraironslagtoacementcomposedof49%ironslag,43%calcinedlimeand 8%steelslag keptthecompressivestrengthofconcretefortype I*Correspondingauthor.Tel.:+8613501215836;fax:+8601062785836.E-mailaddress:yanpy@tsinghua.edu.cn(P.Yan).http://dx.doi.org/10.1016/j.tca.2015.02.0180040-6031/ 2015ElsevierB.V.Allrightsreserved.ThermochimicaActa605(2015)4351ContentslistsavailableatScienceDirectThermochimicaActaj our nal homepage: www. el sevi er . com/ l ocat e/ t caPortland cement.Kriskova etal. [12] foundthatmechanicalactivationsignicantly increasedthereactivity ofsteelslag duetotheincrease insurfaceareaandamorphousphase,and theamountofheat releasedpersurfaceareaincreasedwithmillingtime.Wangetal. [13]also foundthatsuper-ne steelslagexhibitedamuchhigheractivityatearlyages,butits activitywasstill obviouslylowerthanthatofPortlandcement.Liu and Li[14]pointedthattheearlyhydrationsofcement andsteelslag tendedtohinder eachother,especiallyinthecase oflargereplacementratioofsteelslag andhighneness ofsteelslag.Lietal. [15]proposedanewmethodfor modifying thechemicalandmineralogicalcompositionsofsteelslag byadding industrialwastestomoltenbasicoxygenfurnace steelslag, themodicationreducedthefree-CaOcontentandremarkablyimprovedthecementitious propertyofsteelslag.Wang etal. [16] foundthattheinitialalkalinity ofhydrationconditioncouldpromotetheearlyhydrationofactivecomponentsinsteelslag, but thehydrationdegreeofnon-activecomponentsisverylowevenunderstrongalkaline conditionwithpHvalue of13.8.Basedonareviewofcurrentliteratures,itisobviousthatblendedcementhasalowearly-agestrengthandlonginitialsettingtimebyreplacingpartofcementwithsteelslag.Inordertoimprovetheactivityofsteelslag,methodsofmechanicalactivation,mineralmodicationandalkalinityactivationareused.However,thehydrationheatevolutionofblendedcementcontainingsteelslagwhichbelongstothemaincharacteristicsoftheearly-stagehydrationprocesswasinvestigatedveryrarelyinthepast.Meanwhile,thereislittleinformationregardingthehydrationkineticsofblendedcementcontainingsteelslag,whichisimportanttounderstandthehydrationprocessandmechanismofblendedcement.Moreover,mostinvestigationsarecarriedoutwithsmallamountorsinglecontentofsteelslagoratroomtemperature.Therefore,studyofdetailedheatevolutionbehaviorandkineticsofblendedcementcontainingsteelslagareneeded.Inthispaper,thehydrationheatevolutionrateandcumulativehydrationheatofblendedcementcontaining0,10%,20%,35%and50%steelslagweremeasuredwithin168hat25

C,45

Cand60

Cbyanisothermalcalorimeter.Byapplyingthekineticsequations[1719],thereactionofblendedcementwasevaluatedasafunctionofhydrationtime,takingintoaccountthesteelslagcontentandcuringtemperature.Thenthehydrationreactionmechanismofblendedcementcontainingsteelslagwasdetermined.2.Experimental2.1.MaterialsP.I42.5PortlandcementconformingtoChineseNationalStandardGB175-2007andbasicoxygenfurnacesteelslagwereused.ThechemicalcompositionsofcementandsteelslagareshowninTable1. Themineralogicalphasesofsteelslag,whichweredeterminedbyXRDanalysis,usingaTTRIIIX-raydiffractom-eter(CuKa,45kV,200mA),aregiveninFig.1. AccordingtothecalculationmethodofalkalinityproposedbyMason[20],thealkalinityofsteelslagis3.59,whichisindicatedthatsteelslagusedinthispaperhashighalkalinity.AsshowninTable1, CaO,SiO2,Fe2O3,MgO,Al2O3 arethemainoxidesofsteelslag,itschemicalcompositionsaresimilartothatofPortlandcement.ButtherearesignicantdifferencesinmineralogicalphasesbetweenPortlandcementandsteelslag.ThemaindifferenceinsteelslagisthehighcontentsofironoxideandRO phase,whichhavenocementitiouspropertiesanddonotcombinetoformhydraulicphases.ThespecicsurfaceareasofPortlandcementandsteelslagare350m2/Kgand458m2/Kg,respectively.Theparticlesizedistributionwhichwasmeasuredbyalaserparticlesizeanalyzer(MASTERSIZER2000)ispresentedinFig.2.Itisevidentthatsteelslagwasnerthancementintherangeofneparticles,upto6mm,andsteelslagwascoarserthancementwhentheparticlesizewasgreaterthan60mm,whiletheparticlesizeofsteelslagbetween660mmaremuchlessthancement.2.2.TestmethodThehydrationheatevolutionrateandcumulativehydrationheatofblendedcementcontainingsteelslagweremeasuredwithanisothermalcalorimeter(TAMAirfromTA instruments).Thetestswereperformedatthreeconstanttemperatures(25

C,45

Cand60

C)within168h.TAMAirhaseightparalleltwin-chambermeasuringchannels:onechambercontainingthesample,anothercontainingthereference.Inordertoavoidconsiderabletempera-turedifferencesbetweenthepasteandtheisothermalenvironment,thebinderandwaterwerekeptatatemperatureclosetothemeasurementtemperaturebeforemixing.Aftermanuallymixinghomogeneously,thepasteswereimmediatelyplacedintothechambermaintainedataconstanttemperature.ThehydrationheatevolutionrateandcumulativehydrationheatofFig.1.XRDpatternofsteelslag.Fig.2.Particlesizedistributionofcementandsteelslag.Table1Chemicalcompositionsofcementandsteelslag(w/%).CompositionSiO2Al2O3Fe2O3CaOMgOSO3TiO2P2O5Na2Oeqf-CaOLOICement20.554.593.2762.502.612.930.530.832.08Steelslag12.772.1223.4949.173.540.231.020.910.451.86Na2Oeq=Na2O+0.658K2O;w:massfraction.44F.Hanetal./ThermochimicaActa605(2015)4351blendedcementcontainingsteelslagcanbecontinuouslymonitoredasafunctionoftime.Inordertoinvestigatethehydrationpropertiesofsteelslag,thesteelslagpasteswerepreparedbymixingsteelslagwithwaterorsodiumhydroxidesolutionswithpHvalueof13.0and13.6.Thewatertobinderratioandsodiumhydroxidesolutiontosteelslagratio(W/B)were0.4forallsamples.ThemixproportionsofpastesareshowninTable2.3.Resultsanddiscussion3.1.HydrationheatevolutionofsteelslagThehydrationheatevolutionrateandcumulativehydrationheatofsteelslagwithwaterorNaOHsolutiontosteelslagratioof0.4at25

CareshowninFig.3.ItcanbeseenfromFig.3(a)thatthehydrationprocessofsteelslagcanbedividedintove stages,whichissimilartothatofcement.Therststagewasassociatedtotherstpeakofthecurve,itcorrespondedtotherstminutesofreactionanditwasrelatedtothepartialdissolutionofsteelslag.Forpastesmixingsteelslagwithwater,NaOHsolutionsofpH13.0andpH13.6,thevaluesoftherstpeakwere56.2J/gh,66.5J/ghand89.5J/gh,respectively.Thenthehydrationheatevolutionratedecreasedrapidly,andreactionenteredthesecondstagecalleddormantperiodinwhichtheconcentrationofCa2+neededtoreachsaturationstatebeforefurtherhydrationofsteelslag[21].Thethirdandfourthstagesareassociatedtothesecondpeak.ThehydrationofactivecomponentsinsteelslagsuchasC2S,C3Smadereactionproductsprecipitate,andthenwiththereductionofactivephases,thehydrationheatevolutionratedecreased.Finally,thehydrationprocessenteredalowreactivityperiodandtheexothermicratedecreasedtoaverylowlevel.AsshowninFig.3(a),thedormantperiodsofpastesmixingsteelslagwithwaterandNaOHsolutionofpH13.0were43.3hand36.4h,respectively.ForpastemixingsteelslagwithNaOHsolutionofpH13.6,thehydrationprocessalmostdidnotexperiencedormantperiod.Theoccurringtimeofthesecondexothermicpeakwasabout80h,40hand20hforabovementionedthreepastes,respectively.ThepeakvalueofpastemixingsteelslagwithNaOHsolutionofpH13.0isalittlelargerthanthatofpastemixingsteelslagwithwater.ButincreasingalkalinityofsolutiontopH13.6increasedthevalueofsecondexothermicpeaknearlybythreetimes.ItisindicatedthatsteelslagwasstimulatedbyNaOHsolution.ItisduetothefactthatOHishelpfultodisintegratevitreousandthusacceleratethehydrationofC2SandC3S[22]. ThepromotingeffectwasnotobviouswhenpH13.0(Fig.3(a)and(b)),butthecumulativehydrationheatstillincreasedafter168hbyincreasingthealkalinityofsolutiontopH13.6.ThehydrationofcementcouldmakethepHvaluealittlehigherthan13.0inashorttime[23], evenforconcretewithlargeamountofmineraladmixture,thepHvalueofitsporesolutionwashigherthan12.0[24].AsshowninFig.3,theactivityofsteelslagwasstilllowwhenpH13.0.Therefore,thehydrationheatevolutionrateandcumulativehydrationheatweremuchlowerthanthatofcement.3.2.HydrationheatevolutionofblendedcementcontainingsteelslagThehydrationheatevolutionrateandcumulativehydrationheatofblendedcementcontainingsteelslagat25

CareshowninFigs.4and5, respectively.AsshowninFig.4(a),intheinitialhydrationtimeupto1h,thehydrationheatevolutionrateincreasedwithanincreaseinsteelslagcontentinblendedcement.Forblendedcement,steelslagalmostactedasinertmaterialintheinitialperiodduetoitslowactivity(Fig.3),anincreaseinthereplacementratioofsteelslagincreasedtheeffectivewatertocementratio,andtherewasmorespaceforthehydrationproductsofcement.Therstmaximumpeakwasformedinashorttimeduetothereleaseofsurfaceenergyandthefastreactionofaluminateswhenblendedcementcontactedwithwater[25]. Atthesametime,thecumulativehydrationheatincreasedwiththeincreasingamountofsteelslag(Fig.5(a)).ItcanbeseenfromFig.4(b)thattheendingtimeofdormantperiodincreasedwithincreasingdosageofsteelslag.Thiswasinagoodagreementwiththeinvestigationsthatthesettingtimeofcementandconcretebecamelongerbyreplacingpartofcementwithsteelslag,andanincreaseinsteelslagcontentincreasedthesettingtime[6,10,26,27]. ThereplacementofPortlandcementbysteelslagresultedinadecreasedhydrationheatevolutionrateandadecreasedpeakvalueofthesecondmaximumaswellasincreasedoccurringtimeofthesecondexothermicpeakcomparedtoPortlandcement(Fig.4(b)).Moreover,areductioninthetotalreleasedheatwiththeincreasingdosageofsteelslagwasobserved(Fig.5(b)).Thepromotingeffectofsteelslagonthehydrationofblendedcementisshownonlyintheinitialhydrationtime,thecumulativehydrationheatofPortlandcementexceedthatofblendedcementafterabout5.5h(Fig.5(a)).Thereactionofsteelslagwasmuchslowerthanthatofclinkerphases,andtheamountofcementdecreasedwithincreasingdosageofsteelslag,meanwhile,thespecicsurfaceareaofsteelslagwasonlyalittleTable2Mixproportionsofpastes.SampleW/B Massfraction(%)CementSteelslagCem0.41000SS109010SS208020SS356535SS505050Fig.3.Hydrationheatevolutionofsteelslagat25

C(a)hydrationheatevolutionrateand(b)cumulativehydrationheat.F. Hanetal./ThermochimicaActa605(2015)435145largerthanthatofPortlandcementandlargeparticlesexistedinsteelslag(Fig.2),sotheparticlesofsteelslagcannotactasnucleationsiteforthehydrationproductstodepositandgrowthduringtheprocessofnucleationandcrystalgrowth.Therefore,thehydrationheatevolutionrateandcumulativehydrationheatdecreasedwithanincreaseinsteelslagcontent.Itisnotedthatthecumulativehydrationheatofblendedcementcontaining10%steelslagwasalmostasmuchasthatofPortlandcementwithin168h.ItwasconsistentwiththeTsakiridissresultthatadditionof10.5%steelslaghadnoinuenceonthehydrationprocessofPortlandcement[8].Figs.6and7presentthehydrationheatevolutionandcumulativehydrationheatofblendedcementcontainingsteelslagat45

C,respectively.AsshowninFig.6(a),atthetemperatureof45

C,thepeakvalueoftherstheatevolutionwaslowerthanat25

Cexceptblendedcementcontaining35%steelslag.ThisphenomenonwasrelatedtotheincreasingrateoftheC3Ahydrationinthepresenceofgypsumwithincreasingtemperature.Thisreactionleadedtotheformationofprimaryettringiteonthesurfaceofthegrains,whichpreventedthefurtherhydrationforacertainperiodoftime[25]. Forblendedcementcontaining35%steelslag,thehydrationheatevolutionrateandcumulativehydrationheatwerehigherthanothersamplesintheinitialhydrationtimewithincreasingtemperaturefrom25

Cto45

C(Fig.6(a)andFig.7(a)).Itmightbeduetothecombinedeffectofelevatedtemperature,increasedwatertocementratioandsufcientquantityofcement,whichpromotedsignicantlytheinitialhydrationofsampleSS35.ItisapparentthatthecumulativehydrationheatofPortlandcementwasmorethanthatofblendedcementatabout4.0h,whichwasobviouslyshortened(Fig.6(a)andFig.7(a)).ItcanbeseenfromFig.6(b)thatthereactiontemperatureaffectedthehydrationprocessofblendedcementcontainingsteelslag.Anincreaseintemperaturefrom25

Cto45

Creducedtheendingtimeofdormantperiod,shortenedtheoccurringtimeofthesecondhydrationheatevolutionpeakbyabouttwotimesandincreasedthevalueofthesecondexothermicpeakbyaboutthreetimes.Theintensereactioncontinuedformorethan40hat25

C,butitshortenedforabout20hat45

C.Thus,thesecondexothermicpeakwithahighbutnarrowshapewasformed.AsshowninFig.7(b),theearly-stagecumulativehydrationheatofblendedcementhadaconsiderableincreasecomparedtosampleshydratingat25

C.Itwasfoundthatthe12haccumulativehydrationheatincreasedby118.14%forPortlandcementwithincreasingtemperaturefrom25

Cto45

C,while124.01%, 156.12%,155.92%and119.60%forsamplesSS10,SS20,SS35andSS50,respectively.Itiselucidatedthatelevatedtemperaturepromotedtheearly-agehydrationofblendedcementmoreobviouslythanthatofPortlandcementandthepromotingeffectbecamegreatwithanincreaseinsteelslagcontentinblendedcement.Comparedtohydratingat25

C,after168hofhydration,thecumulativehydrationheatincreasedby2.22%,0.68%,18.14%,35.36%and27.99%forsamplesCemSS10,SS20,SS35andSS50,respectively.Itisindicatedthatthehydrationofblendedcementcontaininglargeamountofsteelslagdevelopedsignicantlyatlaterstageofhydration.ItalsocanbeseenfromFig.7(b)thattheFig.4.Hydrationheatevolutionrateofblendedcementcontainingsteelslagat25

C(a)therstpeakand(b)hydrationwithin168h.Fig.5.Cumulativehydrationheatofblendedcementcontainingsteelslagat25

C(a)theinitialhydrationtimeand(b)hydrationwithin168h.46F.Hanetal./ThermochimicaActa605(2015)4351cumulativehydrationheatofPortlandcementandblendedcementcontainingnomorethan35%steelslaghadlittledifferenceinthelaterperiod.ButtherewasstillcertaingapbetweenPortlandcementandblendedcementcontaining50%steelslagduetosmallmassfractionofcementandlowactivityofsteelslag.Figs.8and9showthehydrationheatevolutionrateandcumulativehydrationheatofblendedcementat60

C,respective-ly.ItcanbeseenfromFig.8(a)thatthevalueoftherstexothermicpeakwaslowerat60

Ccomparedtohydratingat25

C.Thiswasaconsequenceoftheacceleratedformationofprimaryettringiteonthesurfaceofcementgrainsat60

Casitwasexplainedbefore.Itshouldbenotedthatincreasingtemperatureto60

CpromotedobviouslytheinitialhydrationofsamplesSS20andSS35(Fig.8(a)andFig.9(a)).AsshowninFig.8(b),asexpected,anincreaseintemperaturefrom45

Cto60

Creducedtheendingtimeofdormantperiodandincreasedthevalueofthesecondexothermicpeakfurther,meanwhile,itshiftedthepeaktoearliertime.Afterabout10hofhydration,thehydrationofblendedcementcontainingsteelslagenteredadiffusioncontrollingprocess.Thecumulativehydrationheatalsoincreasedsignicantlyasincreasingtemperature(Fig.9(b)).Whentemperatureraisedfrom25

Cto60

C,the12haccumulativehydrationheatincreasedby179.10%,194.98%,251.32%,268.77%and233.43%forsamplesCemSS10,SS20,SS35andSS50,respectively.Theactivitiesofcementandsteelslagweregreatlystimulatedatthetemperatureof60

C.ElevatedtemperaturecouldobviouslypromotetheearlyhydrationofactivecomponentsinsteelslagsuchasC2SandC3S.Moreover,fastreactionofcementinashorttimeresultedinahydrationenvironmentwithhighalkalinity,whichwasbenecialtothehydrationofsteelslag(Fig.3).Thus,forblendedcementcontainingsteelslag,muchmorehydrationheatgeneratedattheearlystageofhydrationat60

C.Comparedtohydratingat25

C,thecumulativehydrationheatwithin168hincreasedby15.49%,7.47%,26.47%,22.97%and32.26%forsamplesCemSS10,SS20,SS35andSS50,respectively.ItcanbefoundthattheamplitudeofincreaseofhydrationheatofblendedcementinthelaterperiodwasrelativelysmallcomparedtoPortlandcementat60

C.Itisduetothefactthatmostactivephasesofsteelslaghadhydratedintheearlyperiodandtheinertcomponents(ROphase,Fe2O3)cannotreactevenintheconditionofstrongalkalinityandelevatedtemperature[21].Thehydrationofsteelslagwasdifferentfromthatofpozzolanicmineraladmixture,suchasgranulatedblastfurnaceslagoryash,whichwillreactwithCa(OH)2 producedbycementatthelaterstageofhydration,moreover,elevatedtemperatureprovidedenergytoactivatealkali-hydroxideattackontheslagory ashparticles[28]. Forblendedcementcontainingsmallamountofgranulatedblastfurnaceslag,thecumulativehydrationheatwasmorethanPortlandcementat60

Cduetothereleasedlatentheatofcrystallizationofamorphousphaseinslag[29].However,steelslagdidnotreactwiththehydrationproductsofcement.Steelslagandcementaffectedeachothers hydrationbychangingthehydrationenvironment[10].Thus,forsteelslaginblendedcement,increasingtemperaturejustpromotedtheFig.6.Hydrationheatevolutionrateofblendedcementcontainingsteelslagat45

C(a)therstpeakand(b)hydrationwithin168h.Fig.7.Cumulativehydrationheatofblendedcementcontainingsteelslagat45

C(a)theinitialhydrationtimeand(b)hydrationwithin168h.F. Hanetal./ThermochimicaActa605(2015)435147hydrationofactivecomponentsandmadethesepartsofheatemitaheadoftime.Therefore,forblendedcementcontaining10%steelslag,thecumulativehydrationheatwasclosetoPortlandcementat25

Cand45

Cinthelaterperiod(Fig.5(b)andFig.7(b)),andithadacertaingapwithPortlandcementat60

C(Fig.9(b).Meanwhile,thecumulativehydrationheatofblendedcementcontainingsteelslagcannotexceedthatofPortlandcementwithin168hatelevatedtemperatures(Fig.7(b)andFig.9(b)).3.3.HydrationkineticsofblendedcementcontainingsteelslagSeveralstudieshavedescribedthehydrationkineticsofPortlandcementandmanykineticsmodelshavebeenbuilttomimicthehydrationprocessofPortlandcement[30]. Inordertoinvestigatethehydrationkineticsofblendedcementcontainingsteelslag,ahydrationkineticsequationwasusedinthispaper.Thisequation,thegeneralformofwhichispresentedinEq.(1),hasalsobeenusedtostudythehydrationreactionofPortlandcementandblendedcementcontaininggranulatedblastfurnaceslag[31],thealkali-activatedreactionofslag[17,32]andthepozzolanicreactionofmetakaolin[33]orricehuskash[34].11a 1=3hinKt(1)wherea isthedegreeofhydration;Kistherateconstant;tisthehydrationtime;nisaconstantrelatedwiththehydrationmechanism.Thevalueofnissmallerthan1indicatesthatnucleationkineticscontrolsthereactionrate,whileitiscloserto1 indicatesthattheprocessisgovernedbyphaseboundarykinetics,andthevalueisequalorlargerthan2indicatesthatthekineticscorrespondstoadiffusionprocess.Thedegreeofreaction,a,whichcanberepresentedastheratioofthecumulativehydrationheatattime,t, totheultimatetotalhydrationheat,Qmax,anditisshowninEq.(2).a Qt Qmax(2)AformulaofhydrationkineticsproposedbyKnudson(Eq.(3))[19]isusedtodeterminetheultimatetotalhydrationheat,Qmax.1Qt 1Qmaxt50Qmaxtt0(3)wheret50isthehydrationreactiontimewhenthecumulativehydrationheatis50%ofthetotalhydrationheat(i.e.,half-lifeperiod);(tt0)isthehydrationtimestartingfromtheaccelerationperiod.Thecurvesofln[1-(1-a)1/3]vs. ln(tt0)relationshipforblendedcementcontainingsteelslagareshowninFig.10.ItcanbeseenfromFig.10thatEq.(1)hasbeensimulatedwellduringtheaccelerationperiod(Fig.10lineAB)anddecayperiod(Fig.10 lineCD),butEq.(1)wasnotapplicablefordecelerationperiodduetothefactthatthecorrespondinglineofdecelerationperiodwasacurve(Fig.10lineBC).Fordecelerationperiod,Equation[1-(1-a)1/3]=Kln(tt0)hasbeenttedontothe[1-(1-a)1/3]vs. ln(tt0)Fig.9.Cumulativehydrationheatofblendedcementcontainingsteelslagat60

C(a)theinitialhydrationtimeand(b)hydrationwithin168h.Fig.8.Hydrationheatevolutionrateofblendedcementcontainingsteelslagat60

C(a)therstpeakand(b)hydrationwithin168h.48F.Hanetal./ThermochimicaActa605(2015)4351relationshipforblendedcementcontainingsteelslag.Fig.11presentsanexampleofSS20toelucidatethisrelationship.ThekineticsparametersofblendedcementcontainingsteelslagatdifferenttemperaturesareshowninTable3.AsshowninFig.10,apparently,forallsamplesstudiedinthispaper,thereactionfastenteredtheaccelerationperiod,andaftertheintensereaction,thetimeofenteringthedecayperiodwasalsoshortenedwithincreasingtemperatures.Theseresultswereagreementwiththehydrationheatevolutionrateandcumulativehydrationheat(Figs.49).ItcanbeseenfromTable3thatthevaluesofnforblendedcementscontainingsteelslagweresmallerthan1inaccelerationperiod,whichisindicatedthatthehydrationreactionratewascontrolledbynucleationkinetics.Whilethevaluesofnbecamelargerthan2indecayperiod,elucidatingthatthehydrationprocessbecameadiffusioncontrollingprocessatlaterstageofhydration.Duetothedifferentcontrollingmechanism,thereactionrateinaccelerationperiodwasabout1020timesofthatindecayperiod.Thevalueofnwasalsorelatedtothereactionresistanceofblendedcement.AsshowninTable3,thevalueofnincreasedwithanincreaseinreplacementratioofsteelslagatthreeexaminedtemperatures.Itisindicatedthatincreasingsteelslagcontentincreasedthereactionresistanceofblendedcement.TheactivitiesofactivephasesinsteelslagsuchasC2SandC3Swerelowduetotheslowlycoolingresultinginaperfectcrystallization.C3Sisthemaincementitiousphaseofcement,butcluster-gatheredC2Swithlargesizeandngerstructureisthemaincementitiousphaseofsteelslag[6].Thus,thereactionresistanceofsteelslagFig.10.Curvesofln[1-(1-a)1/3]vs.ln(tt0)andsegmentallysimulatelinesfor:(a)Cem,(b)SS10,(c)SS20,(d)SS35and(e)SS50.Symbolsrepresenttheexperimentaldataandtheredsolidlinesshowthestimulatedlines.(Forinterpretationofthereferencestocolorinthisgurelegend,thereaderisreferredtothewebversionofthisarticle.)F. Hanetal./ThermochimicaActa605(2015)435149washigherthanthatofPortlandcement.Asexpected,thereactionrate,K, decreasedwithincreasingsteelslagcontentinthreehydrationperiods(Table3). Itwasfoundthatincreasingtemperaturereducedthevaluesofninaccelerationperiod.Thiswasrelatedtotheoverallaccelerationofhydrationkineticswithincreasingtemperature.ElevatedtemperaturepromotedthehydrationofPortlandcementandactivecomponentsofsteelslag(Figs.69) andincreasedthereactionrate,K, ofblendedcementaswell(Table3).However,increasingtemperatureincreasedthevaluesofninthedecayperiodbecausethatdiffusionofreactantsthroughthereactionproductlayertooccurreactionsonthesurfacesofunhydratedparticlewasthemaincontrollingfactor.Elevatedtemperatureleadedtoformadensemicrostructurewithplentyofhydrationproducts,soitresultedinahigherreactionresistancecomparedtohydratingat25

C.Itisnotedthatincreasingtemperatureincreasedthereactionrateinthedecayperiodalthoughhighertemperatureleadedtohigherreactionresistance.Itmightbebecausethatelevatedtemperatureprovidedsufcientenergytoovercomethereactionresistance.Equation[1-(1-a)1/3]=Kln(tt0)wasthederivationofempiricalformuladx/dt =K/tanditwasapplicableforthedualcontrollingreactionofchemicalreactionanddiffusion[31]. ItcanbeseenfromFig.10thatblendedcementcontainingnomorethan35%steelslagexperiencedtheprocessofdualcontrollingreaction.Butforblendedcementcontaining50%steelslag,thevalueofnwascloseto1inaccelerationperiodanditbecamelargerthan2indecayperiod.Moreover,asshowninFig.10(e),curvesBCwerealmoststraightlinesatthreeexaminedtemperatures.Itiselucidatedthattheearly-stagekineticsbehaviorofsampleSS50ismostlydominatedbyphaseboundaryreaction,afterwhichitdirectlyentereddiffusioncontrollingprocess.ForsampleSS50,thehydrationheatevolutionratewasrapidintheinitialhydrationtime(Figs.4,6,8(a)),thenlotsofhydratesdepositontheunhydratedparticles,thereactionsoccurredonthephaseboundarybetweencrystalandporesolution.Moreover,furtherimprovementofhydrationdegreewashinderedduetolowmassfractionofcementandlowactivityofsteelslag(Fig.3).Theapparentactivationenergy,Ea,ofblendedcementcontainingsteelslagwasdeterminedfromtheEq.(4):K01K02 t502t501expEaT1 T2RT1T2

(4)wheret501 andt502 arethehydrationtimewhenthecumulativeheatreleasedis50%ofthetotalhydrationheatatcuringtemperatureT1 andT2,respectively.Risgasconstant(8.314J/mol).Table4presentstheapparentactivationenergyoftheoverallreactionofblendedcementcontainingsteelslaginthetempera-turerangeof2560

C.Itisevidentthatanincreaseinreplacementratioofsteelslagincreasedtheapparentactivationenergy.Itwasrelatedtothelowactivityofsteelslagandlargeamountofinertphasesasitwasexplainedbefore.ItisnotedthatthedifferenceinapparentactivationenergybetweenPortlandcementandblendedcementcontaining10%steelslagwassmall.ItisindicatedthatsmallamountofsteelslagaffectedlittlethehydrationprocessorpropertiesofPortlandcement,whichwasconsistentwiththehydrationheatevolutionrateandcumulativehydrationheat(Figs.49) aswellaspreviousinvestigations[8,11].However,Wuetal.[35]determinedEa=49kJ/molforblendedcementcontaining50%granulatedblastfurnaceslag(GBFS),whichwashighercomparedtoEa=46.39kJ/molwithrespecttoblendedcementcontaining50%steelslag.Thehydrationpropertiesofsteelslagwerepoorwhenitwasmixedwithwater(Fig.3),butforGBFS,almostnoreactionsoccurredasitcontactedwithwater.TheglassphaseofGBFShadcertainactivityandthecrystallizationactivationenergyofvitreousbodywasabout421256kJ/mol[36], whichmeansthatmuchmoreenergyshouldbeprovidedforthehydrationofGBFS.However,elevatedtemperatureandalkalinityenvironmentcansignicantlyactivatethehydrationofGBFS,whichwillreactwithCa(OH)2 producedbycementhydrationinblendedcement[29]. ComparedtoGBFS,thehydrationmecha-nismofsteelslaginblendedcementwasdifferent,steelslagdidnotreactwithhydrationproductsofcement.IncorporationofsteelslaginPortlandcementcouldbeseenasincorporationofakindofmaterialwithlowhydrationactivity,inotherwords,blendedcementcontainingsteelslagcouldbeseenasalowqualityofcement.Therefore,theapparentactivationenergyofblendedcementcontainingsteelslagwaslowerthanthatofblendedcementcontainingGBFSatthesamelevelofreplacement.Fig.11.Curvesof[1-(1-a)1/3]vs.ln(tt0)andsimulatelinesindecelerationperiodforsampleSS20.Symbolsrepresenttheexperimentaldataandtheredsolidlinesshowthestimulatedlines.(Forinterpretationofthereferencestocolorinthisgurelegend,thereaderisreferredtothewebversionofthisarticle.)Table3Kineticsparametersofblendedcementcontainingsteelslagatdifferenttemperatures.SampleT(

C)AccelerationperiodDecelerationperiodDecayperiodnKKnKCem250.490.027430.199962.700.00147SS100.530.022490.209042.440.00134SS200.500.023580.192022.450.00142SS350.580.017500.147032.350.00104SS500.910.006590.106712.340.00068Cem450.350.074700.262053.850.00277SS100.370.064800.235403.890.00188SS200.420.055650.201523.940.00131SS350.540.035110.135773.030.00081SS500.770.018460.139144.210.00074Cem600.290.112750.296042.890.00433SS100.340.100810.282624.300.00280SS200.440.074190.248805.110.00154SS350.480.066400.246641.380.00170SS500.700.031130.237751.290.00146Table4Apparentactivationenergyoftheoverallreactionofblendedcementcontainingsteelslag.SampleW/B Temperaturerange(

C)Ea(kJ/mol)Cem0.4256040.01SS10256041.43SS20256042.14SS35256043.58SS50256046.3950F.Hanetal./ThermochimicaActa605(2015)43514.Conclusion(1)ThehydrationprocessofsteelslagcanbedividedintovestagesanditissimilartothatofPortlandcement.Butcomparedtocementhydration,theendingtimeofdormantperiodandtheoccurringtimeofsecondexothermicweremuchlonger,andthevalueofsecondexothermicpeakwasmuchsmaller.TheactivityofsteelslagwasstilllowwhenpHvalueofsolutionwas13.0,butincreasingalkalinityofsolutiontopH13.6stimulatedsignicantlytheactivityofsteelslag.(2)Thehydrationheatevolutionrateandcumulativehydrationheatincreasedwithincreasingsteelcontentinblendedcementintheinitialhydrationtime.Atthetemperatureof45

Cor60

C,thepeakvalueoftherstheatevolutionwaslowerthanthatat25

CduetothefastreactionofC3Aatelevatedtemperatures.(3)Increasingsteelslagcontentinblendedcementincreasedtheoccurringtimeofsecondexothermicanddecreasedthevalueofsecondexothermicpeakaswellasreducedthecumulativehydrationheat.Anincreaseintemperaturefrom25

Cto45

Cor60

Cincreasedthevalueofsecondexothermicpeak,shiftsthepeaktoearliertimeandobviouslyincreasedthecumulativehydrationheatofblendedcement.Thepromotingeffectofelevatedtemperaturewasobviousforblendedcementcontaininghighcontentofsteelslag.(4)SmallamountofsteelslagaffectedlittlethehydrationprocessofPortlandcement,buttheretardingeffectofsteelslagonthehydrationofPortlandcementbecameobviouswhenthereplacementratioofsteelslagwashigh.(5)Increasingtemperaturecouldpromotethehydrationofactivecomponentsinsteelslagandmadethesepartsofheatemitaheadoftime.ThecumulativehydrationheatofblendedcementcontainingsteelslagcannotexceedthatofPortlandcementwithin168hatelevatedtemperatures.(6)Forblendedcementcontainingnomorethan35%steelslag,thehydrationreactionratewascontrolledbynucleationkineticsintheaccelerationperiodandthenbydiffusioninthedecayperiod,butinthedecelerationperiod,thehydrationexperienceddualcontrollingreactionofchemicalreactionanddiffusion.Theearly-stagekineticsbehaviorofblendedcementcontaining50%ofsteelslagwasmostlydominatedbyphaseboundaryreaction,afterwhichitdirectlyentereddiffusioncontrollingprocess.(7)Anincreaseinsteelslagcontentinblendedcementincreasedthevalueofnanddecreasedthereactionrate,K. 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