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