DOI10.1617/s11527-013-0128-z
ORIGINALARTICLE
Cementpastecontentandwaterabsorptionofrecycledconcretecoarseaggregates
P.Belin•G.Habert•M.Thiery•N.Roussel
Received:8February2013/Accepted:14June2013ÓRILEM2013
AbstractInthispaper,wefirstshowthattheamountofresidualcementpasteinrecycledconcreteaggregates(RCA)dependsontheinitialpastecontent,onthemechanicalpropertiesoftheinitialcementpasteandonthemechanicalqualityofthetransitionzoneattheinterfacebetweentheinitialnaturalaggregatesandcementpaste.OurresultssuggestthatwaterabsorptionofRCAat24hcanbeseenasthesimplesumofthecapillaryabsorptionofbothresidualcementpasteandinitialnaturalaggregates.Theabsorptionkineticsissimilartothecapillaryabsorptionoftraditionaluncru-shedconcretesbut,ontheindustrialtimescaleofinterest,isstronglyaffectedbythesizeandconfigurationoftheresidualcementpastepatchesatthesurfaceoftheinitialnaturalaggregates.WefinallyproposeatentativeframeforaclassificationofRCAbasedonwaterabsorptionrateandwaterabsorptioncapacityat24h.KeywordsWastemanagementÁAggregateÁAbsorptionÁRecycledconcreteÁWorkability1Introduction
Concreteisthemostusedbuildingmaterialintheconstructionindustry.Itsworldproductionisesti-matedtobeabout6billionstons/year[1].
P.BelinÁG.HabertÁM.ThieryÁN.Roussel(&)
Universite
´ParisEst,IFSTTAR,Paris,Francee-mail:nicolas.roussel@ifsttar.frOnonehand,thispredominantmarketpositioninducesalargeconsumptionofnaturalresources.Forexample,threebillionstonsofaggregatesarepro-ducedeachyearinthecountriesoftheEuropeanUnion[2].Withinthisframe,theavailabilityofnaturalaggregatesourcesbecomesthereforeacrucialques-tion[3].
Ontheotherhand,theenvironmentalconcernrelatingtowastefromtheconstructionanddemolitionsector(C&Dwaste)isgrowing.Forexample,about450millionstonsofC&DwastesaregeneratedintheEuropeanUnioncountriesperyear.Theamountofinertmaterialssuchasbrickandconcreteisestimatedtobe180milliontons,whichrepresent480kg/capita[4].Mostofthesewastesaredirectlyburiedinground.Inthefollowing,wefocusonconcretewastesobtainedbyconcretecrushingafterdemolition.Wecalltheminthefollowingrecycledconcreteaggregates(RCA).Theyaremadeofcoarseparticlescontainingbothresidualcementpasteandnaturalaggregates.
IntegrationofRCAinconcretemixdesignhasthepotentialtoreducesimultaneouslytheamountofC&Dwasteburiedingroundandtopreservenaturalmineralaggregateresources.However,thispotentialcanonlybeturnedintoamarketrealityiftheaggregatesobtainedbyconcreterecyclingareusednotonlyin‘‘niche’’lowqualityproductapplicationsbutalsoindominantmarketapplicationssuchasstructuralconcrete.
ResearchonRCAstartedalmostsimultaneouslywithconcreteinvention.SaintLeger[5],forinstance,
initsconcretepatent,highlightedthefactthattheconcretefromtheSaint-MartinchannelinParis(France)hadbeenreused,afterhammercrushing,fortheproductionofnewconcretes.However,throughconcreteindustrialhistory,theuseofRCAinstruc-turalconcreteapplicationshasoftenbeenrestrictedonlytoveryspecificcases[6–11]andRCA,inpractice,aremostlyusedingranularbaseorsub-baseapplicationsforroadsandembankmentsconstruction.ThemainreasonforthatopenlooprecyclingliesinthefactthatRCAcontainsanon-neglectableamountofcementpastefromtheoriginalconcrete.Thisresidualamountoftheinitialpastedependsonthecharacteristicsoftheinitialconcrete,onthecrushingprocessandontheconsideredparticlesizeintheRCA[12–15].ThispasteisattachedtothenaturaloriginalaggregateandhasahigherporosityandlowermechanicalpropertiesthanNAandreducesthereforethequalityofRCAcomparedtoNA[12,16,17].Additionally,somechemicalcompatibilityissuesbetweentheresidualandnewpastecanberaised(relatedforexampletothepresenceofsulphatescomingfromgypsumnaturallypresentincement).Consequently,resultsfrom[11,18,19]show,forinstance,thatdensityofRCAisdecreasedby6–10%.Moreover,waterabsorptionofRCA,whenmeasuredinliterature,rangesfrom3to12%[20–24].ThesevaluesaresignificantlyhigherthanthoseofNA,whichusuallyrangefrom0.5to2.0%.AbrasionresistanceofRCAmeasuredbytheLosAngelestestcanbedecreasedbyupto70%comparedtoNA[19,24,25].TheelasticmodulusofconcretespreparedfromRCAisreportedtobe30–50%lowerthantheoneofnormalconcrete[26,27].
Severaltechnicalsolutionscanbefoundinlitera-tureallowingforanimprovementofthequalityofbothRCAandconcretespreparedfromRCA.Thefirstoneconsistsintreatingtheaggregateswithaballmillinordertoremovetheresidualcementpaste[28].ItwasshownthattheabrasionprocessismoreefficientifRCAisheatedpreviouslytothecrushing[18].ThisheatingandcrushingtechniqueisnowusedinJapantoproducehighqualityrecycledaggregatesthathavethesamepropertiesasNA[29].Notethat,inthistechnique,thefineparticlesresultingfromtheabra-sionprocesscanberecycledasrawlow-CO2materialincementplants[30].Mechanicalabrasioncanalsobereplacedeitherbychemicalabrasionwithacidsolu-tion[31]orbyultrasonicwaves[32].IthashoweverMaterialsandStructures
tobekeptinmindthattheabovetechniquesarenotenvironmentallyneutral.Forinstance,theamountofCO2generatedbytheheating/crushingprocessisthreetimeshigherthantheoneneededtoproducecrushednaturalaggregates[18].Itissuggestedin[18]thatthistechniquecouldpresentsomeenvironmentalbenefitsonlyifthefinestparticlesgeneratedbytheprocessarerecycledaslow-CO2materialsincementplants.
WithoutreducingtheamountofpasteinRCA,mechanicalpropertiesofconcretespreparedfromRCAcanalsobeimprovedbydoingasilicafumeimpregnationoftheparticles.ThistechniqueimprovestheinterfacequalitybetweenthenewcementpasteandtheRCA[32].ItisalsopossibletotreattheRCAsurfacewithasilane-typeagent[33].Finally,animprovedmixdesignprotocolofconcretespreparedfromRCAcanallowforanincreaseinmechanicalproperties[34].
AlltheabovetechniquescanimprovemechanicalpropertiesofbothRCAandconcretespreparedfromRCAbuttheydonotdealspecificallywiththedetrimentalinfluenceofthehighwaterabsorptionofRCAontherheologicalpropertiesofconcretespreparedfromRCAinthefreshstateandespeciallyonworkabilityorslumplosswithtime[26,35].
TheinfluenceofwaterabsorptiononslumplossofconcretepreparedwithRCAwasstudiedin[25]bymixingconcreteswithnaturalaggregatesandRCAatvariousmoisturestates.WithsaturatedRCA,theslumplossesoftheconcretecontainingRCAandtheonecontainingnaturalaggregatesweresimilar.With100%dryRCA,aslumplossof10cmforaninitialslumpof14cmwasmeasuredover1haftertheendofmixing.TheseresultsshowthatwaterabsorptioninRCAcontrarilytonaturalroundedaggregatesisnotinstantaneous.Itthereforeinducesworkabilityorslumplossthatcouldbenonacceptableatthebuildingsiteifthewaterabsorptionkineticsaftertheendofmixingisnotcompatiblewiththebuildingprocess.SaturationofRCAcouldappearasapotentialtechnicalsolutiontothisissue.ItshouldhoweverbekeptinmindthatsaturatingRCAwithoutanyexcessofwaterisaverycomplextaskwithintheframeofindustrialproductionofconcrete.Asallotherwatercompensationtechniquesaredifficulttoimplementintheconcreteproductionplant,absorptionkineticsofRCAseemslikeacriticalfeatureofthesematerials.Despitethis,onlyfewmeasurementsofabsorptionkineticscanbefoundinliterature.NealenandSchenk
MaterialsandStructures
[35]measuredadifferenceof20%betweentheabsorptionofRCAafter30minandafter24h.Tametal.[31]developedamethodallowingforthemeasurementofabsorptionkinetics.ThismethodconsistsinintroducingthetestedRCAinapicnometerandadjustingthewaterlevelatregulartimeintervals.Themassofwaterneededtoadjustthelevelisweightedandcorrespondtothewaterabsorption.Recently,acontinuousmeasurementofwaterabsorp-tionwasproposedin[36]basedonhydrostaticweighing.
Inthispaper,wefirstmeasuretheamountofresidualcementpastesinRCAresultingfromthecrushingofvariousconcretes.Wethenproposeanexperimentderivedfromtheexperimentalsetupin[36]allowingforthecontinuousmeasurementoftheabsorptionkineticsofRCA.Usingthistest,wemeasurethiskineticsforvariousRCA.WeshowthattwoRCAwithsimilar24habsorptionvaluescanhaveverydifferentkineticsandhavethereforeverydiffer-entconsequencesontheslumplossofconcretespreparedwiththesedryRCA.Inordertocapturethisdifference,wesuggestthatitisneededtointroduceanabsorptionkineticscharacteristictime.WemoreoverproposeasimpleandconceptualclassificationofRCA
basedonboth24habsorptionvalueandabsorptionkineticscharacteristictime.Wefinallyconcludeonthetypeofconcretethatseemstocomplywiththeaboverequirementsanditsavailabilityinexistingconcretestructures.
2Materialsandprotocols2.1Materials
2.1.1Componentsandmixdesign
InordertoproducethetestedRCA,sixdifferentconcreteswerederivedfromthecombinationoftwocrushednaturalcoarseaggregatesofsize4/10and10/20,threeroundednaturalcoarseaggregatesofsize4/10,5/12and10/20andtworoundednaturalsandsofsize0/4and0/5(cf.Table1).ACEMItypecementwasusedand,inthecaseofthehighmechanicalstrengthmaterials,acommercialpoly-carboxylatesuperplasticizerwasaddedtothemixture.Themixdesignswerechosentoreproducesometypicalconcretemixdesignsrangingfromlowstrengthhousingconcrete(C25R)tohighstrengthmodern
Table1Mixdesign,mechanicalstrengthandcrushingprocessesforthetestedRCA
C25R
Crushedcoarseaggregate10/20(kg/m3)
Crushedcoarseaggregate4/10(kg/m3)
Roundedcoarseaggregate10/20(kg/m3)
Roundedcoarseaggregate4/10(kg/m3)
Roundedcoarseaggregate5/12(kg/m3)
Roundedsand0/5(kg/m3)Roundedsand0/4(kg/m3)Cement(kg/m)Nonabsorbedwater(kg/m3)W/Cratio(-)
Super-plasticizer(kg/m)Compressivestrengthet28days(MPa)Crushingprocess
33C30C––650461––7403001910.64–32
Semi-industrial
C35R––834200––7923101580.511.2443.6Industrial
C35C636245––––7633881960.480.4846.36Industrial
C50R––509–4284064004101930.47–54.8Laboratory
C65R––734179––7295331790.3413.3167.45Industrial
C65C564217––––6765942020.3414.8569.7Industrial
––619–3884534462301820.79–24Laboratory
MaterialsandStructures
concretes(C65RandC65C).InTable1,25,30,35,50and65correspondtothetargetedmechanicalstrengthofthetestedconcreteswhereastheletterRorCcorrespondstothenatureofthenaturalaggregatesusedtopreparetheconcretes,RforRounded,CforCrushed.
2.1.2Crushingprocess
Threedifferenttypesofjawcrusherswereusedtoobtain,aftersieving,RCAfromeachconcretefromTable1.Inthisstudy,itisimportanttokeepinmindthatonlythe10/20granularclassesarestudied.OurRCAisthereforeacoarseaggregate.Oneofthecrusherwasasmalllaboratorycrusher(FritschÒ1100W).Thesecondcrusherwasaconstructionsiteportablecrusher(calledheresemi-industrial)whereasthelargestcrusherwasaroadportableplant(calledhereindustrial)(cf.Fig.1).Onlyonecrushingcyclewascarriedoutforeachconcrete[16].Aftercrushing,allRCAsampleswereprotectedfromcarbonationbycuringthemunderwaterandstoringtheminsealedtanksalongwithsodalime.Allsampleswerestoredfor6monthsbeforetesting.
2.2Measurementsofcementpastecontent
inRCAthroughchemicalanalysis
ThepurposeofthisarticleistostudytheabsorptionkineticsofRCA.SincethispropertyismainlygovernedbytheporosityofthecementpastephaseattachedtoRCA,itisthereforeofprimeimportancetobeabletocharacterizethecementpastecontentofRCA.Itcanbenotedthat,inliterature,thequantifi-cationoftheresidualmortarcontentismoreoftenmeasuredthantheresidualcementpastecontent[12,13,37].
WhereasthedeterminationoftheresidualmortarcontentinRCAisusuallydeterminedbysuccessive
Fig.1Industrialroadportablecrusher.(Left)duringcrushingprocess(right)crushingjaws
thermaltreatments(cyclesoffreezingandheating)[12,37],chemicalanalysisisgenerallycarriedoutinordertodeterminethecementpastecontent.
First,thedeterminationofthecementpastecontentintheRCArequirestheassessmentofthecementcontentintheRCA.Forthatpurpose,twohundredsgramsofdryRCAweregroundedinaringrollmillfrom10/20mmdownto315lm.Onegramoftheresultingpowderwasdilutedinanitricacidsolution(1/50).Theobtainedsolutionwasthenfilteredandanalysed.Thesolublecationicoxides(SiO2,Al2O3,TiO2,Fe2O3,CaO,MgOandMnO)wereproportionedbyatomicemissionspectrometry.Thefollowingmaintracersofcement,SiO2,Fe2O3andAl2O3,aregenerallyused[38].IthastobenotedthatCaOisnotappropriateinthecaseoflimestoneaggregatesbecauseacidalsoattackstheaggregates.SiO2isarelevanttracerofcementsinceamorphousSiO2fromaggregatesisnotdissolvedin1/50dilutednitricacidsolution.NotethatAl2O3andFe2O3canalsobeusedbutleadstohighlyvariablemeasuredcementcontentsduetothelowamountofthisoxideincementwithrespecttoSiO2.MoreoverSiO2hastobeavoidedasatracerifthecementitiousmatrixiscarbonatedsinceinthatcaseSiO2fromcarbonatedC–S–H(silicagel)cannotbeextractedbya1/50nitricacidsolution[39–41].InthisstudyallsampleswereprotectedfromcarbonationandSiO2wasthereforeusedtodeterminethecementcontentMcoftheRCAfromthemassofSiO2pergramofRCAaccordingtoMc¼MSiO2=%SiO2,whereMSiO2istheSiO2contentintheRCAdeterminedbyatomicemissionspectrometryand%SiO2isthemassfractionofSiO2containedinthecement.
ThecementpastecontentintheRCAMPcanthenbecalculatedfromthecementcontentusingthefollowingequationMP¼Mcð1þ0:25aÞ
ð1Þ
MaterialsandStructures
whereaisthedegreeofhydrationofthecementintheresidualcementpaste.0.25standsfortheamountofwater,whichischemicallyboundtothecement.acorrespondstoalongtermdegreeofhydration(atleast28days)inthecaseofwatercuringconditions.acanbeinferredaccordingtothefollowingempiricalequation[42]:a¼1ÀeÀ3:3W
C:
ð2Þ
Inthepresentresearch,wechoosetoexpressourresultswitharelativecementpastecontentMP/MP0withrespecttothecementpastecontentoftheoriginalconcreteMP0,whichcaneasilybecomputedfromtheconcretemixdesign:M1þ0:25a
P0¼
1þWCþAð3Þ
CwhereAstandsfortheaggregatescontentoftheoriginalconcrete.Accordingly,MP/MP0canbewrittenas:
MPM¼MWA
P0
c1þCþC:ð4Þ
Itisinterestingtonotethatthehydrationdegreeof
thecementplaysnoroleinthepreviousequation.Asaconsequence,therelativecementpastecontentdoesnotdependontheaboveassumptionsonthedegreeofhydration.Inthefollowing,wewillalsoassesstheporosityofthecementpaste.Thisparameterisestimatedbyusingthesimplerelationsbelow[43].TheinitialporosityofthefreshcementpasteP0canbecomputedfromthewater-to-cementratio:
qW
À1
P0¼1À1þcCð5Þ
whereqcisthecementpowderspecificgravity.
TheporosityofthepastePforadegreeofhydrationaisthengivenby[43]
P¼P25aC
01À0:W:ð6Þ
2.3Absorptionkineticsmeasurement2.3.1Pre-treatmentoftheaggregates
Apre-treatmentwascarriedoutinordertodrytheaggregatestoareferencestatebeforesubmittingthem
tothewaterabsorptiontest.TheRCAwereheatedat45°Cundervacuum.Thisprocedurewassimilartothedryingmethodusedtoprepareconcretesamplesformercuryintrusionporosimetryin[44].Inthispaper,thispre-treatmentwascarriedoutatanabsolutepressureintherangeof30–50mbar,withsilicagelandsodalimetoavoidcarbonationofthecementitiousmatrix.Thepre-treatmentlasteduntilmassequilib-riumwasreached(i.e.thedailymasslosswaslowerthan0.05%).Beforetheabsorptiontest,thesampleswerecooled1dayatroomtemperatureandatmo-sphericpressureinadessicatorwithsilicagelandsodalime.Althoughthispre-treatmentwaspreferredtohighertemperatureovendrying(classically105°C)inordertoreducethesamplesalteration(dehydration,macro-cracking,etc.),itstillhastobenotedthatthispre-treatmentmayhaveconsequencesontherecycledaggregates(forexample,somemicro-crackswerereportedin[45]).
2.3.2Waterabsorptionmeasurement
Aglassjarwasfirstimmersedina22°Ctemperatureregulatedbathuntiltemperatureequilibriumwasreached(i.e.nosignificantmasschangeofthefullyimmersedemptyjarwasmeasured).Thetestedaggregatesurfacewascleanedfromfineparticleswithcompressedairandthesamplemassm0wasmeasured(Fig.2).
Thejar,partiallyfilledwithdistilledwater,waswithdrawnfromthetemperatureregulatedbathandtheaggregateswereintroducedintothejarattimet=0.Thesamplequantitywasequivalenttooneortwolayersofaggregates(around150ginthiswork)inordertoallowairbubblestoeasilycirculatethrough
Fig.2Waterabsorptionkineticsmeasurementsetup
MaterialsandStructures
thesample.Theaggregateswerethenagitatedtoremoveanytrappedairbubbles.Thejarwasthenhungtoabalanceconnectedtoadatarecordingsystemandimmersedinthetemperature-regulatedbath(Fig.1).TherecordingofthemassofthejarcontainingthesampleM(t)wasstartedapproximately70saftertheintroductionoftheaggregatesintowater.Asaconsequence,theinitialabsorptioncannotbemea-suredandcanonlybededucedfromtheevolutionbetween70sand24halongwithabsolutevalueofthewaterabsorptionat24h.However,beingabletomeasurewaterabsorptionafter70smeansthatwehaveaccesstowhatshallhappeninpracticemoreorlessaftertheendofthemixingprocess.Theevolutionofthemasswasthenrecordedfor24h.The24hmeasurementofthemassofthejarcontainingthesampleisnotedhereM24.
Attheendofthetest,theaggregateswereextractedfromwater,driedwithabsorbentclothsuntilallvisiblefilmsofwaterwereremovedbuttheaggregatestillhadadampappearance[46].Themassofthesamplem24after24himmersionwasthenmeasured.Itcanbenotedthatthemoisturestatecriterionusedhereisidenticaltotheoneusedforthesaturatedsurfacedriedmethodforwaterabsorptionmeasurement[47].Asaconsequence,itcanbeestimatedthattherelativemeasurementerrorduetotheoperatorevaluationofthemoisturestateshouldbesimilarandaround5%[47].
Fig.3ResidualcementpastevolumefractionintheRCAasafunctionoftheW/Cratioforthevariousconcretesandjawcrushersstudiedinthispaper
Assumingthatthereisnovolumechangeoftheaggregates,themassofabsorbedwaterDmw(t)wasthencalculatedfrom:
DmwðtÞ¼m24Àm0ÀðM24ÀMðtÞÞ:
ð7Þ
AndthewaterabsorptionofthetestedRCAwasdefinedas:AdsðtÞ¼
DmwðtÞ
:m0
ð8Þ
Thistestallowsforthecontinuousmeasurementofwaterabsorptionofcoarseaggregates(diameterlargerthan8mm)from70saftertheintroductionoftheaggregatesinwaterupto24h.Itcanbenotedthat,whenusedinthecaseofsmalleraggregates(i.e.lowerthan8mm),airbubblestrappinginthesamplepreventedfromacontinuousmeasurementandfre-quentjolting,abletoperturbthecontinuousmassmeasurement,wasnecessary.
3Experimentalresultsandanalysis3.1PastecontentintheRCA
ThemeasuredrawdataofpastecontentfromchemicalanalysisaregatheredinFig.3asafunctionoftheW/Cratio.Unsurprisingly,theresidualpastecontentintheRCAdependsonthepropertiesofthecement
MaterialsandStructures
paste.Themeasurementofmechanicalpropertiesofpurecementpastesbeingadelicateprocess,wedecideheretogodeeperintheunderstandingofthecrushingprocessbyevaluatingtheporosityofthepaste,whichcanbeassociatedtobothfracturestrengthandelasticmodulus.WethenplotinFig.4therelativeresidualcementpastecontent(i.e.theratiobetweentheresidualcementpastemassproportionandtheinitialcementpastemassproportionintheoriginalconcrete)asafunctionoftheporosityoftheinitialpaste.
WecanseeinFig.4thattherelativeresidualcementpastecontentdecreaseswiththeporosityoftheinitialpaste.Allresultsobtainedinthisstudyseemtogatheronamastercurve,theonlydeviationfromwhichappearsinthecaseoftheconcretespreparedwithnaturalcrushedaggregates,forwhichthecementcontentintheRCAisequaltothecementcontentintheoriginalconcrete.
Ourresultssuggestthereforethatthejawcrushertype(laboratory,semi-industrialorindustrial)doesnothaveanymajorinfluenceontherelativeresidualcementpastecontent.ItcanhoweverbekeptinmindthatonlyonecrushingcyclewasusedheretoproducetheRCA.Additionalcrushingcyclescoulddecreasetheresidualpastecontent[12].However,fromanindustrialpointofview,thiswouldgenerateadditionaleconomicandenvironmentalcosts[12].
Ourresultsmoreoversuggestthatthemainparam-etersdictatingtherelativeresidualpastecontentare
Fig.4RelativeresidualcementpastecontentintheRCAasafunctionof
porosityoftheinitialcementpasteforthevarious
concretesandjawcrushersstudiedinthispaper
themechanicalpropertiesoftheinitialcementpaste(i.e.itsporosity)andthenatureoftheaggregates(i.e.roundedorcrushed).Fromamoregeneralpointofview,itcanbeexpectedthatitisthemechanicalqualityoftheinterfacialtransitionzone(ITZ)betweentheinitialnaturalaggregatesandtheinitialcementpastethatshallmatter.Fromamechanicalpointofview,itwasshownthatsmoothnaturalroundedaggregatesdisplayweakerITZthannaturalcrushedaggregatesalthough,forthelatter,themechanicalpropertiesoftheinterfaceshowedstrongdependencyontheirroughness,theirproductionprocessandtheirchemicalcomposition[48].ThisisillustratedinFig.5,inwhichtheRCAfromC35R(highporositypasteandnaturalroundedaggregates)andC65C(lowporositypasteandnaturalcrushedaggregates)areshown.ItcanbeseenthatthecrushingprocessinducesafractureattheinterfacebetweentheaggregateandthepasteinthecaseofC35Rasthisinterfaceismechanicallyweakerthanthepaste.Itcanbenotedthatthesizeoftheresidualcementpastepatchesatthesurfaceofthegrainsissmallcomparedtothesizeoftheinitialnaturalaggregates.Mostcementandmortarparticlesresultingfromthecrushingaresmallerthan10mmandarethereforenotpresentintheRCAstudiedhere.Inindustrialpractice,crushingthistypeofconcreteshouldallowforapartialseparationoftheresidualcementphasefromtheinitialcoarsenaturalaggregatewithouttheneedforaspecifictreatment
MaterialsandStructures
suchastheonesdescribedintheintroductionsection.InthecaseofC65C(Fig.5right),thefractureoccursinthepasteitselfandaggregatesofcentimetresizecomposedonlyofamortarphasecanbefound.Inthiscase,itcanbenotedthatthevalueofthedensityoftheRCAisclosetothevalueofthedensityoftheinitialconcrete.Thecrushingprocessonlybreakstheconcreteinhomogeneousfragmentswithoutanyseparationofthecomponents.
Fromtheaboveresults,itseemsthereforepossibletoproduceRCAthatonlycontainsvolumefractionsintheorderofseveral%oftheinitialcementpaste(cf.Fig.3)aslongastheoriginalconcreteispreparedfromroundednaturalaggregates(i.e.weakITZ)andfromaninitialpastewithahighwatertocementratio(i.e.lowmechanicalproperties).3.2Waterabsorptionat24hours
WeplotinFig.6thewaterabsorptionratioasafunctionoftimeforthevariousRCAtestedherealongwiththeabsorptionofthenaturalroundedandcrushedaggregates,fromwhichtheRCAwereprepared.ItcanbeseeninFig.6that,formostmaterialsstudiedhere,aplateauisreachedat24horevenbeforeinthewaterabsorption.Itcanbeexpectedthat,whentheRCAreachsaturation,theporosityofboththeinitialnaturalaggregatesandtheresidualcementpasteisfilledwithwater.WeplotthereforeinFig.7the24hwaterabsorptionasafunctionoftheassessedporosityofthepastecontainedintheRCA(i.e.the
productoftheintrinsicporosityofthepastecomputedfromEq.(5)andtheresidualpastevolumefraction).OurresultssuggestthatwaterabsorptionofRCAat24hcanbeseenasthesimplesumofthecapillaryabsorptionofboththeresidualcementpasteandtheinitialnaturalaggregates.Thissumisstronglyaffectedbytheamountofresidualcementpaste,whichpondersthehighwaterabsorptionofthepaste.However,forhighRCAporosity,24habsorptionbecomeslowerthantheassessedporosityoftheRCAasshowninFig.8.Thiscouldbeexplainedbythefactthat24hisnotsufficienttoreachthefullsaturationoftheporosityoftheseRCAasmeasuredin[36].
Thismeansthat,similarlyasintheprevioussection,itseemspossibletoproducecoarseRCAwithlow24hwaterabsorptionaslongastheoriginalconcreteispreparedfromroundednaturalaggregatesandfromaninitialpastewithahighwatertocementratio.Thesetwofeatureslimittheamountofresidualcementpasteandthereforethewaterabsorption.Theporosityoftheresidualpastewillbehighbutitwillbetemperedbythelowamountofresidualpasteduetotheweakmechan-icalinterfacebetweentheaggregateandthepasteandtotheseparatingeffectofthecrushingprocess.3.3Waterabsorptionkinetics
ItcanbenotedfromFig.6thatthetestedRCAsdisplayverydifferentabsorptionkineticsindependentoftheirwaterabsorptionat24h.Asaconsequence,theamountsofwaterabsorbedduringmixingor
Fig.5Fracturetypefor(left)C35R(highporositypasteandnaturalroundedaggregates)and(right)C65C(lowporositypasteandnaturalcrushedaggregates)
MaterialsandStructuresFig.6WaterabsorptionasafunctionoftimeforthevariousRCA8%C25R7%C30RWater absorption (%)6%5%4%3%2%1%C35RC35CC50RC65RC65CNatural crushed aggregatesNatural rounded aggregates0%110100100010000100000Time (s)Fig.7Waterabsorptionat24hasafunctionoftheassessedporosityoftheresidualpaste9%24 hours absorptionNatural rounded aggregates based RCANatural rounded aggregates8%7%Natural crushed aggregates based RCANatural crushed aggregates6%5%4%3%2%1%Total paste porosity (-)0%-2%-1%0%1%2%3%4%5%6%7%8%betweenmixingandcastingshallstronglydiffer.Itcanthereforebeexpectedthat,fromapracticalpointofview,workabilitylossduetowaterabsorptionwillalsobedifferentfromoneRCAtoanother.Weassumeherethatwaterabsorptionduringconcretemixingissimilartothewaterabsorptionofaggregatesimmersedindistilledwater.Wethereforeneglectanypotentialinfluenceofthewaterbindingcapacityofthecementgrains.ThisbindingcapacityfindsitsorigininthevanderWaalsinteractionsbetweencementgrains.Theenergylevelperunitvolumeoftheseinteractions(inJ/m3orinPa)isdimensionallycorrelatedtotheyieldstressofthepaste(i.e.thecriticalstressabovewhichthematerialflows,which
MaterialsandStructures
Fig.8Waterabsorptionat24hasafunctionoftheassessedporosityoftheRCA.Thedottedlineisthey=xplot10%9%8%7%6%5%4%3%2%1%24 hours absorptionTotal RCA porosity (-)0%0%1%2%3%4%5%6%7%8%9%10%11%correspondstotheenergyperunitvolumeneededtocanceltheseinteractions).Theyieldstressoftheconstitutivecementpastevariesbetweenfluidcon-cretessuchasself-compactingconcretesandfirmconcretes.However,itispossibletoconsiderthatitsorderofmagnitudeisbetween10and100Pa.Ontheotherhandthemagnitudeofthecapillarypressureattheoriginofwaterabsorptionisintheorderofc/rwherecisthesurfacetensionofwaterandristhecharacteristicsizeoftheRCAporosity.Ifweassumethatthischaracteristicsizeisintheorderofonemicrometre,thecapillarypressurecompetingwiththebindingcapacityofthecementgrainsshallbeintheorderof105Pa.ItisfarhigherthanthevanderWaalsenergylevelperunitvolumeandwaterabsorptionshallnotthereforebesloweddownbythebindingcapacityofthecementgrains.Itcanhoweverbekeptinmindthat,locally,waterabsorptioncouldincreasethesolidconcentrationatthesurfaceofthegrainsandthereforethelocalvanderWaalsbindingenergy.ThisincreaseinconcentrationwouldmoreoverpromotethejammingoffineparticlesintheRCAporositydecreasingthereforewaterabsorption.Wehoweverassumeinthefollowingthatthemeasuredwater
absorptionisrepresentativeofthewaterabsorptionoftheRCAwhentheyareimmersedinacementpaste.Ifweconsiderthat,throughtheabsorptionprocess,waterinvadestheporosityofanhomogeneousmate-rialofporosityp(i.e.thepatchesofcementpaste),themassofwaterabsorbedatatimetM(t)shouldbeproportionalto[49]:
pffi3
MðtÞ%p2fðs;rÞtð9Þwheref(s,r)isafunctionofthetortuosityandoftheporesizedistribution.Asaconsequence,acharacter-isticabsorptiontimeshouldbeproportionaltoTabs%
1
pÀ3:fðs;rÞ
ð10Þ
Weneglectherethewaterabsorptionoftheinitialnaturalaggregates.WemoreoverdefinearbitrarilyacharacteristicabsorptiontimeTabsasthetimeneededtoreach80%ofthefinalabsorptionat24h.WeplotinFig.9thischaracteristicabsorptiontimeasafunctionoftheintrinsicporosity(asopposedtothetotalporosityoftheresidualpaste)computedfromEq.(5)alongwithafittedfunctionofp-3.Thegood
MaterialsandStructures
agreementobtainedsuggeststhattheabovesimpledimensionalapproachisabletocapturethemeasuredwaterabsorptionkineticsand,therefore,thatwaterabsorptionkineticsofRCAisdictatedbythesamephysicalparameters(i.e.thecharacteristicsofthecapillaryporosity)asinotherporousbuildingmate-rials(cementpaste,mortar,bricks,etc.)[50].
Itishoweverworthnotingthat,inthecaseofthematerialsstudiedhere,theinfluenceoftheW/Cratioonporesizedistributionandtortuosityoftheresidualpasteseemstobeneglectable,asasimplep-3functionseemstobeabletofitourdata.Thisisanintriguingfeatureandcouldsuggestthatsomemicro-cracksduetothecrushingprocess,whichwoulddependonpasteporosityandlocaldefects,mayplayaroleonkinetics.Thisnetworkofmicro-crackswouldthenbypassthecementitiousmatrixanddisplaythesametortuosityandsizedistributionindependentoftheW/Cratio.Thisexplanationwouldhoweverbeincontradictiontothefactthatthetotalamountofabsorbedwaterisequaltothesumoftheassessedporosityofthepaste(withoutmicro-cracks)andporosityoftheinitialnaturalaggregates.
Fromapracticalpointofview,thismeansthatlowporosityconcretes(i.e.highmechanicalstrength)shoulddisplayslowwaterabsorption.Moreover,asshownabove,thehighamountofresidualcementpasteintheseconcretesshouldallowfortheabsorp-tionoflargeamountofwaterintheRCAaftermixingandbeforecastingthatcouldstronglyaffectwork-abilityloss.
Ithashowevertobenotedthattheaboveapproachreachesalimitatshorttimescales.WemeasuredthewaterabsorptionkineticsoftheconstitutivecementpasteofC25RandC50R(cf.Fig.10)alongwiththeconstitutivenaturalaggregates.Themeasurementsonthecementpasteswerecarriedoutonsieved10/20mmcrushedparticles.
FromthedatainFig.10andthemeasuredresidualamountofpaste,wecalculatethewaterabsorptionasafunctionoftimebyaddingtherespectivecontributionofthecomponents.TheresultsareplottedinFig.11.Althoughthekineticsonlargetimescalesiscorrectlypredicted,atshorttimescales,thereexistsastrongdiscrepancybetweenthecalculatedandmeasuredvalues.Wesuggestthatthisdiscrepancyisduetothecharacteristicsizeofthecementpasteelements.Inthetestedcrushedcementpaste,theaveragesizeoftheparticlesisbetween10and20mmwhereas,inthecrushedRCAfromC25RandC50R,thecementpatchesatthesurfaceofthegrainsarefarsmallerandthinner.Onthetimescaleofinterest[i.e.betweentheendofmixing(100s)andendofcasting(acouplehours)],thekineticsseemsthereforetobe
Fig.9Waterabsorptioncharacteristictimeasafunctionoftheintrinsicporosityoftheresidualcementpaste10000Characteristic time for 80% final absorption (s)1000100100.10.150.20.250.30.350.40.450.50.550.6Intrinsic paste porosity (-)MaterialsandStructures
Fig.10WaterabsorptionasafunctionoftimefortheconstitutivecementpastesandconstitutiveaggregatesofC25RandC50RWater absorption (%)100%10%constitutive paste of C25Rconstitutive paste of C50R1%constitutive natural rounded aggregates0%10100100010000100000Time (s)Fig.11WaterabsorptionasafunctionoftimeforC25RandC50R.Thecontinuouslinesarethepredictedvaluesfromthewaterabsorptionofeachcomponent5.0%4.5%4.0%3.5%Absorption (%)3.0%2.5%2.0%1.5%1.0%0.5%0.0%101001000100001000001000000C25RC50RTime (s)stronglyaffectedbythesizeandconfigurationoftheresidualcementpastepatchesatthesurfaceoftheinitialnaturalaggregates.Thisdistributionishoweverdifficulttopredictand,inordertopredictworkabil-ityloss,directmeasurementofwaterabsorptionkineticsseemsunavoidableforthequalificationofagivenRCA.
3.4Aframeforapracticalwaterabsorption
classificationofRCAInpractice,itisnotacharacteristictimethatmattersbuttheamountofwaterthatisabsorbedaftertheendofthemixingprocessandbeforecasting.Thisamountofabsorbedwaterwillbeattheoriginofthe
MaterialsandStructuresFig.12ClassificationofRCAasafunctionoftheir24hwaterabsorptionandtheirbetweenmixingandcastingabsorptionrate
workabilityorslumploss.Weconsiderherethatatypicalmixingprocesslastsintheorderof100sandthatcastingtypicallyoccurs1haftermixing.Duringthisperiod,itcanbeestimatedthatanacceptablewatercontentvariationshouldstaylowerthantheuncer-taintyonthewaterdosageinconcretemixingplant(i.e.10–15L).Theamountofcoarseaggregatesinatypicalconcretebeingoftheorderof1,000kg/m3,thismeansthatthewaterabsorptionbetweentheendofmixingandcastingshouldnotbehigherthan1.5%.Itcanmoreoverbeexpectedthathigh24habsorptionvaluesarerepresentativeofahighamountofporouscementpaste.Thisshoulddecreasethemechanicalpropertiesoftheaggregatepreventingtheproductionofrecycledconcreteswithadequatehardenedmechanicalproperties.Thelimitvalueforthe24habsorptionisstilltobediscussed.However,alimitvaluearound6%canbeextracted
´nchezdeJuanfromthestateoftheartpaperofSa
´rrez[12].ThisvaluecanbecomparedtoandGutie
the10%limitvalueintheRILEMrecommenda-tions[51]andtothe7%limitvalueinJapaneseindustrialpractice[52].
WegathertheaboverequirementsinFig.12.TheRCAcomplyingwiththeserequirementsinourstudyareallbasedonroundednaturalaggregates(i.e.weakITZ).TheRCA,whichdisplaysthebestproperties,isC25R(i.e.aconcretetypicaloflowstrengthhousingconcretes).Itispossibletoextrap-olatefromFig.12thatthereshallexisttwotypesof
idealsourceofRCAconcerningwaterabsorption.Thefirstonewouldbe,asintheC25Rcase,aconcretepreparedwithhighwatertocementratioandnaturalroundedaggregates.CrushingshallinthiscaseleadstotheproductionofRCAcontainingaverylowamountofresidualcementpaste.Aswaterabsorptionseemstoresultfromtheaddedcontributionofthewaterabsorptionoftheresidualpasteandtheinitialnaturalaggregates,thewaterabsorptionandwaterabsorptionkineticsofsuchRCAshallbeveryclosetowaterabsorptionandwaterabsorptionkineticsoftheinitialnaturalaggregates.Thesecondidealsourcewouldbeaconcretewithalowwatertocementratio.CrushingshallleadinthiscasetotheproductionofRCAcontaininganamountofresidualpasteclosetheamountofcementpasteintheuncrushedconcretebutthispasteporosityshallbesufficientlylowtoleadtotolerablewaterabsorption.Moreover,atlowW/Cratio,itcanbeexpectedthattheveryfineporositymayleadtoabsorptionkineticsslowenoughtooccurmainlyaftersetting.
4Conclusion
Inthispaper,wefirstshowedthattheamountofresidualcementpasteinRCAmainlydependsonthepastecontentoftheinitialconcrete,onthemechanicalpropertiesoftheinitialcementpaste(i.e.itsporosity)
andonthemechanicalqualityoftheinterfacebetweentheinitialnaturalaggregatesandcementpaste.
WethenmeasuredthewaterabsorptionkineticsofRCAwithanewlydevelopedsimpletest.OurresultssuggestthatwaterabsorptionofRCAat24hcanbeseenasthesimplesumofthecapillaryabsorptionofbothresidualcementpasteandinitialnaturalaggregates.
WemoreovershowedthattwoRCAwithsimilar24habsorptionvaluescanhaveverydifferentabsorptionkineticsand,therefore,verydifferentconsequencesontheslumplossofconcretepreparedwiththesedryRCA.Thiskineticsseemstostronglydependontheporosityofthepastesimilarlytocapillaryabsorptionoftraditionaluncrushedcon-cretes.Onthetimescaleofinterest[i.e.betweentheendofmixing(100s)andendofcasting(acouplehours)],thekineticsisalsostronglyaffectedbythesizeandconfigurationoftheresidualcementpastepatchesatthesurfaceoftheinitialnaturalaggregates.WefinallyproposedatentativeclassificationofRCAbasedonwaterabsorptionrateandwaterabsorptionat24h.
Itcanbekeptinmindthat,ifalowandfastabsorptionistargeted,thebestrawmaterialstocrushareconcreteswithnaturalroundedaggregatesandhighW/Cratio.ThehighW/Cratioandtheweakpasteaggregateinterfaceshouldensurethattheresidualpastevolumeiskeptataminimumthroughthecrushingprocess.ThiswillnotonlyreducethefinalabsorptionoftheRCAbutalsoimproveitsmechanicalpropertiesallowingfortheproductionofstandardmechanicalstrengthsandelasticmodulusconcretes.Thehighwatertocementratio(i.e.highporosity)willalsoensurethatwaterabsorptionisfastandthatmostofitoccursthroughthemixingprocessoftherecycledconcrete.Workabilitylossandrheologyissuesshallthereforebereduced.
Itisfinallyinterestingtonotethattheserequire-mentscorrespondtothehousingconcretesthatweremassivelyusedinEuropeaftertheSecondWorldWar.Thishugepotentialdepositshallbeavailableinthenext20yearsasamineralresourcefornewconcreteproduction.
AcknowledgmentsTheauthorswouldliketothankT.SedranandF.deLarrardforprovidingsomeofthecrushedaggregates.SupportfromtheAgenceNationaledelaRecherche(ANR)isalsoacknowledged(GrantNo.ANR-09-JCJC-0074).
MaterialsandStructures
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