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AbriefhistoryofconcreteandtheroleofSCMsinreducingenvironmentalimpact
Concreteisavitalhumaninventionthathascometodefinemanycivilisations.Fromancienttimesitwasutilisedtobuildstructuresofmassandpermanence.Structuressuchastheaquaducts,sewersandbridgesconstructedbytheRomansareatestamenttotheversatilityandlongevityofconcrete.Aftermanyhundredsofyearsinservice,theycontinuetofunctionandprovideconfidencetofuturebuildersthatconcreteisapracticalanddependablematerialforconstruction.
Ourmodernstructuresarestrongerandmoredurableduetocontinuedinnovationinthedesignandmanufactureofconcrete.Asforthefuture,scientistsarealreadyworkingonaformofconcretethatwillbeusedtoestablishacivilisationonanearbyplanetsuchasMars.Thefirstcolonistwillbringmostofwhattheyneedwiththemincludingtemporarystructures,butapermanentcolonywillrequirepermanentstructures.Thebasisoftheirresearchistomanufacturetheconcretefromingredientsfoundinabundanceontheredplanetitself.
So,whatisconcrete?
Unlikesteel,aluminiumorplastic,theword“concrete”doesnotrefertoasinglematerial.Itisacompositemadeupofmanymaterials.Insimpletermsitismadebycombiningaggregate(mostcommonly,stoneandsand)withabinder(mostcommonlycementandwater).Theaggregateisnormallyinertandactsasapassivespacer,relyingonthebindertolockitintoasolidmatrix.Thebinderistheactiveingredientthatimpartsthelargestinfluenceoverthe
propertiesofthehardenedmaterial.Forthisreason,thebinderreceivesagreatdealmoreattentionandbecomesthefocusofwhatistofollow.
So,whatiscement?
Moderncement(mostcommonlyPortlandCement)isbasedonJosephAspdin’spatentof1824,ondevelopmentofamaterialthat(oncehardened)resembledthenaturallyoccurringstonequarriedontheIsleofPortland,offtheBritishCoast.
Manyimprovementhaveoccurredsince1824,withchangesinPortlandcement’schemistryandproductionthatledtobetterconsistency,qualityandeconomiesofscale–particularly,aftertheintroductionofthecontinuousrotarykilninthelate1850’s.
Todaycementisdefinedasa“hydraulic”material(asitreactswithwater).Itismadebycombiningpredominantlylimestone(ancientorganicsedimentaryrockthatformsfromtheaccumulationofshell,coral,algae,andfaecaldebris)withclayandotherearthbearingingredientsintoakilnoperatedattemperaturesabove1,300degreesCelsius.Atsuchhightemperatures,theingredients“sinter”(compactandformasolidmasswithoutmelting).Thissolidmass,intheformofglassynodules,iscalled“clinker”.Aftercooling,theclinkerisfinelygroundandmixedwithothermineralstoproducecementpowder.
Initspowderform,cementishighlyreactiveandmustbecontainedtopreventexposuretowater.Evenvapourintheairiscapableofstartingthechemicalreactionsthatwill(givenenoughtime)returnittoarock-likehardenedstate.
Portlandcementhasevolvedintonumeroustypes.ThemostcommonlyusedinconcretemanufactureinAustraliaare:GeneralPurposeGrey(GP)andOffWhite(OW).Othermorespecialisedcementsarealsoused,suchas:HighEarlyStrength,LowHeat,ShrinkageLimited,SulfateResistant,HighAlumina,OilWellandMasonry.
So,whatisSCM?
Bythe1960’s,Australianconstructionprojectswerecommonlysupplementingcementuseinconcretewithothermaterialsfoundtoexhibitcementitiousproperties-suchasflyash,slagandsilicafume.Thesematerials,knowncollectivelyasSecondary(orSupplementary)CementitiousMaterials(SCMs)wererecognisedforimprovingthedurabilityofconcretebeyondthatmadewithPortlandCementalone.
InthebeginningSCMswereoftenfarcheaperthancementandavailableinabundantquantitiesbecausetheywerewasteproductsofotherindustrieskeentorelievethemselvesofthelogisticandfinancialburdenofdealingwiththeirstorageanddisposal.
AlsoconcretemanufacturingcompanieswerekeentotakeadvantageofingredientsthatcouldpotentiallyreplaceexpensivePortlandcement.FurthermoreitwasclearthatSCMswereessentialforproducingveryhighstrengthconcretes(80MPaandabove).Buttherewasaproblemforeverydaylowgradeconcretesusedincommonapplicationssuchasslabs,suspendedfloorsandpaving.
SCMsaresecondarycementitiousbynature.PortlandCementsareprimarycementitious.Insimpleterms,thismeansSCMsdonotcontributetothesettimepropertiesofconcrete.Thoughthelongtermpropertiesoftheconcretearesuperior,theshorttermissuesfortheconcreteplaceraredelaysinsettingandslowergrowthofstrength.Thesefactorsincreaseprojectconstructiontimeandcosts.AcommensurateapproachtotheuseofSCMswasadoptedbytheconcretemanufacturingindustrytooptimisethebenefitswhilstmanagingthelimitations.WhatfollowsisabriefexplanationofeachofthethreemostcommonlyusedSCMsinAustralia.
Flyashisaby-productofcoalcombustioninpowerstations.Itimprovestheoverallperformanceandqualityofconcrete.Itaffectstheplasticpropertiesofconcretebyimprovingworkability,reducingwaterdemand,controllingbleeding,andloweringheatofhydration.Itincreasesstrengthdevelopmentatlaterstagesofthecure,reducescorrosionofreinforcingsteel,andgenerallyimprovesresistancetochemicalattackandmobilitythroughareductioninpermeability.Overthelast20years,currentpracticehasdevelopedtoastagewhereover90%ofconcreteplacedcontainssomeproportionofSCM,ofwhichflyashisthemostcommonlyused,formingthebenchmarkofNormalClassConcrete,whichinturnisdefinedunderAS1379asastandardtypeofconcretesuitableformostapplications.
Groundgranulatedblastfurnaceslag(GGBFS),ormoresimplyknownasslag,isproducedusingmoltenironslag(aby-productofironandsteelmaking)fromablastfurnace,andquenchingitinwatertoproduceagranularproductthatisthendriedandgroundintoafinepowder.Itistypicallyusedinconcreterequiringalowerheatofhydration,andwhereaggressivegroundwaterandadverseenvironmentalconditionsarepresent.Aswithflyashblends,earlyagestrengthdevelopmentisslowerthanwithstraightPortlandcementconcrete.Howeverstrengthdevelopmentfrom28daysonwardsisequivalentorbetter.Anotherbenefitisitscolour.Itisaverylight(almostwhite)powderthatiswellsuitedforspecialistdecorativeprojects,particularlywhencombinedwithwhiteoroff-whitePortlandcement.
Silicafume,alsoknownascondensedsilicafumeormicro-silica,isafinelydividedresidueresultingfromtheproductionofelementalsiliconorferro-siliconalloys,thatiscarriedfromthesteelblastfurnacebyexhaustgases.Silicafumeisoftenused,withorwithoutflyashorslag,tomakehighstrengthconcrete.ItwasthelastofthethreeSCMsdiscussedheretobedevelopedandsuccessfullyusedinconcrete.Itsmainstreamuseisasrecentasthe1980s.ItiscommonlythefavouredSCMforproducingmarinegradeconcrete,orforlow-reboundhighperformanceshotcrete.
OvertimethemorewidespreaduseanddependenceuponthesematerialshasdrivenupthepriceanderodedthesubstantialpricedifferencesthatonceexistedwithPortlandcement.
NowadaystheenvironmentalmovementandconcernovertheimpactofhumandevelopmentontheplanetcontinuestodrivetheuseofSCMsinconcrete.
In2012theAustralianGovernmentintroducedacarbonpricingscheme.Cementproducerswereidentifiedaslargescalepolluters,dueprimarilytotheoperationoftheirkilns.Itwasestimatedthatforeverytonneofcementproduced,onaverageasimilaramountofCO2isreleasedintotheatmosphere.
Thoughtheschemewasrepealedin2014,alastingconsequencewastopublicallydrawattentiontoindustrieswiththegreatestimpactonthereleaseofcarbonpollution.Inraisingtheawarenessofpollutiongeneratedbycementproduction,therehasbeengreateracceptanceanddemandfortheuseofSCMsinreducingthequantityofPortlandcementusedineverydayconcrete.
TheGreenStarSchemeisoneofmanyratingsystemsdesignedtoencourageandrecognisethereductioningreenhousegasemissions,resourceuseandwaste.Theschemeisbasedonawardingcreditpointsforcompliancewithspecificcriteria.
Thearmoftheschemethatdealsspecificallywithconcreteistitled‘Mat-4Concrete’.Itaddressesallconcreteusedinaprojectincludingstructuralandnon-structuralelements.UptotwocreditpointsareavailablewherethePortlandcementcontentinallconcreteusedontheprojecthasbeenreducedbyreplacingitwithsupplementarycementitiousmaterials:•OnepointisawardedwherethePortlandcementcontentisreducedby30%.•TwopointsareawardedwherethePortlandcementcontentisreducedby40%.
Morerecentdevelopmentsintheproductionofconcrete,withoutanyPortlandcementatall,suggestthattheGreenStarcriteriadonotgofarenough.Butavailabilityofthesenewgenerationproducts,termedGeopolymerconcrete,islimitedtospecificconcretemanufacturersproducingunderlicencetotheownersofthetechnology.ThetechnologyisbasedonthecommercialisationofproprietarychemicalsthatchemicallyactivateSCMsandenablethemtoperformmorelikeprimarycementitiousmaterialssuchasPortlandcement.
Populationgrowthperpetuallyincreasesdemandforconcreteandotherresources.ReducingoreliminatingPortlandcementwithSCMsmakesverygoodenvironmentalandeconomicsense.SCMsare,fortheforeseeablefuture,thewayofthefuture.
ByJoePietrosantoVicMixPtyLtd