1、Effectofbiogenicsulfuricacidonbehaviorofalkali-activatedcememtconcreteAbstract：Alkali-activatedcement(AAC)hasshownveryhighresistancetoacidandsalt.Moreover,itisinexpensivethanordinaryPortlandcement(OPC).AACisexpectedtohaveagreatpotentialformarineapplications.Tosimulatemarineenvironmentsinthiswork,AAC
2、sampleswereimmersedinbiogenicsulfuricacidinsteadofchemicalsulfuricacid.Threedifferentconcentrationlevelswereused,i.e.pH=l.l,1.6and2.0respectively.Changesinappearance,massloss,compressivestrength,porefluidpHvalue,andthereleasequantityofCa2+weremeasure.ESEM-EDS,XRD,andIRwereusedtocharacterizemicrostru
3、cturalandmorphologicalchanges,elementprofile,andcorrosionproductswithinthetop5mmofspecimensection.Theresultsshowthattherewasnocorrosiondamagewhentheacidconcentration(namelypHvalue)waseither1.6or2.0.WhenpHwasreducedto1.1,thechemicalbondsonsurface,suchasCa-O,Si-O,andAl-Ostartedtobreakoff.SO42combinedw
4、ithCa2+toformgypsum,leadingtoalooselayeronthesurface.Fortheplace5mmawayfromthesurface,asthepHvalueishigherthan1.1,therewasonlyaslightcorrosion.Theresearchfindingsareusefultoimprovethedurabilityofreinforcedalkali-activatedcementconcreteinmarineapplications.Keywords:Alkali-activatedcementconcrete,Biog
5、enicsulfuricacid,Sulfuricacidconcentrations,Durability1.IntroductionBiogenicsulfuricacidhasadetrimentaleffectonconcrete1.Suchcorrosiondamageoftenoccurinsewagetreatmentfacilities,marineconstruction,oilwells,mining,andanyareaenrichedwithmicroorganisms2.Thecorrosionduetomicroorganismsaccountedfor10%20%
6、ofthedamageofGermanbuildingmaterialsand10.9%ofconcretesewagepipeinAmerica3.$11billionwasspenttorepairthesewagedamageintheUS3.Itwasreportedthat20%ofthedamageofconcretestructureswascausedbymicrobialcorrosioninmarineapplications4.Microbialcorrosionhasbroughthugeeconomiclossestomodernsocieties.Theacidre
7、sistanceofordinaryPortlandcement(OPC)ispoorerthanthatofalkali-activatedcement(AAC)6.Inaddition,theimpermeabilityofAACisbetterthanthatofOPC,thepriceofAACischeaperthanOPC5-9.Therefore,AACisexpectedtohaveagreatpotentialinmarineapplications.Paker10foundthatamongallkindsofmicroorganisms,sulfate-reducingb
8、acteria(SRB)andsulphur-oxidisingbacteria(SOB)havecausedthemostseriouscorrosiononconcrete.SulfateandorganicsulfurareturnedintoH2SbySRBinanaerobicenvironments,andthenH-2SisoxidizedintobiogenicsulfuricacidbySOBinaerobicenvironments.ThepHofbiogenicsulfuricacidcanbelowerthan1.0.Themainhydrationproductsof
9、OPCisC-S-H,Ca(OH)2andsoon,corrodedbyH2SO4,C-S-HandCa(OH)2areresolved,SiO2gelandgypsumisproduced,formingasoftenedlayerll.Monteny12foundthatthesoftenedlayerontheOPCconcretesurfaceactsasabarriertothetransportofchemicalacid,however,asforthebiogenicsulfuricacid,thesoftenedlayerbuildsacomfortableenvironme
10、ntforbacteriatogrowandtoproducemorebiogenicsulfuricacid,whichacceleratesthecorrosionrates.Alkali-activatedslagcementconcretewasimmersedinaH2SO4solution(pH=2)for6months,therewaslittlechangeofcompressivestrength13.However,thecompressivestrengthdeclinedrapidlywhenthesamplewasimmersedina5%H2SO4solutionf
11、or1month,andtheconcretewasseriouslydamagedafter3months13.SunandWu14immersedalkali-activatedflyashcementpasteinvariousconcentrationsofH2SO4solutions,andfoundthattherewasslightdamageonthesurfacewhenimmersedin0.05%H2SO4for6months.Whenthesampleswereimmersedina3%H2SO4solution,thesurfacewasdamagedjustafte
12、r1week.However,thereisnostudyontheeffectofbiogenicsulfuricacidofdifferentconcentrationsonthebehaviorofAAC.Thispaperstudysthebehaviorofalkali-activatedcementconcreteunderbiogenicsulfuricacidwiththreedifferentconcentrationlevels.BetterunderstandingoftheacidcorrosionresistanceofAACisprerequisitetoawide
13、spreaduseinmarineapplications.!.Experimental2.1MaterialsThefollowingmaterialswereusedinthisstudy.Slag(Tables1&2)wasproducedbyFuzhouTaiyuconcretefactory.Flyash(Tables3&4)wasproducedbyFuzhouShuangtengbuildingmaterialslimitedcompany.Naturalcoarseaggregates(Tables5&6)andriversand(Table7)camefromMinjiang
14、river.ChemicalgradeNaOH(96%)wasproducedbyTianjinZhiyuanchemicalreagentcompany.2.2Specimenspreparation2.2.1 MixproportionTheC60concretewasdesignedaccordingtoSpecificationformixproportiondesignofordinaryconcrete15.ThemixcompositionsofalkaliactivatedconcreteisprovidedinTable8.2.2.2 Thepreparationproces
15、sThefollowingprocedureisemployed.1 .Drymixcoarseaggregates,sand,slag,andflyashforoneminute.2 .PremixNaOHwithwater,inablenduntilalltheNaOHisdissolved.3 .AddtheNaOHsolutionintothedrymixandmixforoneminute.4 .Castfreshconcreteintomoldsundervibration5 .Removesamplesfromthemoldsafter24hours,transferintost
16、eampressureequipment,andthenexperiencesanautoclavedcuring(hasavacuumpumpingfor30minutesfirst,andthenincreasedtemperatureto195andpressureto1.2MPafor1hour,thetemperatureandpressureshouldbemaintainfor6hours,andthenreducethepressurein2hours).2.3Testprocedures2.3.1 Simulationdeviceforbiogenicsulfuricacid
17、corrosionThedesignofsimulationdeviceforbiogenicsulfuricacidcorrosiontakesMontery,sdevice16asareferenceandweimprovesit,makingitavailableforthesimulationofbiogenicsulfuricacidcorrosioninmarineenvironment.ThedeviceisprovidedinFig1.InFigl,Part1isusedtoproduceH2S,andpart2isusedtosimulatethecorrosion.InPa
18、rt1,Na2ScontainerandHClcontainerisplacedonhigherplacethanH2Scontainer,Na2SsolutionandHClsolutionflowintoH2ScontainerandH2Sgasisgenerated.AirpumptransportsO2intoH2Scontainer,pumpingO2andH2Sgasintopart2.TheflowrateofNa2SsolutionandHClsolutioniscontrolledbyflowspeedcontroller,andtheoxygencontentiscontr
19、olledbytheairpump.Inpart2,bacterialiquidwasplacedinbacterialculturepool(ThiobacillusferrooxidansprovidedbyxiamenThirdInstituteofOceanographywasused)17.H2SgasandO2comefromPart1shouldbedispersedinthebacterialiquid.Concretespecimenswereimmersedinthebacterialiquid.Boththebacteriaintheliquidandonthesurfa
20、ceofconcretespecimenscanproducebiogenicacidandcorrodetheconcrete.H2StailgasshouldbedischargedintoZn(CH3COO)2solutionandwasabsorbedbyZn(CH3COO)2solution,ensuringthesafetyoftheexperiment.Tostudytheinfluenceofdifferentbiogenicsulfuricacidconcentrationsonconcrete,onlytheconcentrationofH2Swasdifferentand
21、otherparametersremainunchanged.Aftermanyattempts,theparameterswasdetermined:thevolumeofbacterialiquidis80L;theflowspeedofO2is2Lmin;thesituationsofdifferentbiogenicsulfuricacidconcentrations,pH=2.0,1.6and1.1(correspondingtothesampleSA2.0,SA1.6,SA1.1),werecorrespondingtothethreesituations:0MNa2Sand0MH
22、Cl;0.428MNa2Sand0.211MHCl;0.856MNa2Sand0.422MHCLTheflowspeedofbothHClsolutionandNa2Ssolutionis1.4mlmin.Bacterialiquidshouldbereplacedevery14dayssoastomaintaintheactivityofbacteria.2.3.2 AppearanceandmassUseEOS6Ddigitalcameratotaketheappearancepictureofconcrete.Afteracertainperiodofimmersion,thesofte
23、nlayeronthesurfaceofspecimenswerebrushedandremoved,thentheconcretespecimenswereplacedintheoventodryfor24hours.Finally,themassofconcretewasobtainedandtheweightlossrateWwascalculatedbyEq.(l).w=WtW100%(1)WhereW=weightlossrate;w0=initialweightofconcrete(g);wt=weightofconcreteattdaysofimmersion(g).2.3.3
24、CompressivestrengthThetestmethodofcompressivestrengthisconductedaccordingtoStandardfortestmethodOfmechanicalpropertiesofordinaryconcrete(GBT50081-2002)18,thesizeofconcreteis100mm100mm100mm.2.3.4 PorefluidpHvaluecurvePorefluidpHvaluecurveismeasuredaccordingtoASTMC311ofSolidSolutionExtractionMethod19.
25、Splitconcretespecimensatpredeterminedageintotwohalves,andthepowderofconcreteatthesplitsectionisobtainedusingadrillingmachine.Thedrillingpositionshouldbe0lmm,12mm,23mm,34mm,45mm,56mm,67mm,78mm,89mmand910mmawayfromthesurfaceofconcrete.Igpowdershouldbeobtainedineveryposition.MixIgpowderandIOgdistilledw
26、aterinatesttube,andarubberisusedtopreventcarbonization.Shakethetesttubeevery5minutesfor1hour,andthenfilterthesolutiontoobtainfiltrate.UsepHmeterproducedbyHangzhouAolilongCompanytomeasurethepHvalueoffiltrate,thepHvalueisthePorefluidpHvalue.2.3.5 ReleasequantityofCa2+Usemediumspeedfilterpapertofilterb
27、acterialiquid,whichhasimmersedconcretespecimensfordifferentage.Put50mlfiltratein250volumetricflask,andadd3dropsof1+1HClsolution(amixtureof1volumeHCland1volumedistilledwater),andheatandboilitfor30second.Coolingto50degreesbelowandadding5mlKOH(massfractionis20%),andaddabout80mgindicator(amixtureof0.2gc
28、alcein,0.07phenolphthaleinand20gKCl).Shakeandadd2mltriethanolaminetoshieldFeion.UseEDTAstandardsolutiontotitration,andfinallytheYellowgreenturnsintoredandnocolorreversionphenomenonappears.TakedowntheusageofEDTAsolutionV,thereleasequantityofCa2+canbecalculatedbasedonEq.(2):VVM100.81000A=(2)VWWhereX=R
29、eleasequantityofCa2+(mg/L);V=UsageofEDTAsolution(ml);M=ConcentrationofEDTAstandardsolution(molL);Vw=Vblumeoftestsolution(ml);100.8=MolarmassofCaCO3(gmol).Everyspecimensolutionshouldbetestfor2times,andthedeviationshouldbelessthan2mg/L.TheaveragevalueistakenasthefinalreleasequantityofCa2+.2.3.6 ESEM-E
30、DSUseS-3400NenvironmentalscanningelectronmicroscopeproducedbyJapanHitachicompanytoobservespecimen.Removeconcreteatpredeterminedagefromthebacterialiquid,anddryitinashadeandsplitintotwohalves.Thespecimencontainsplitsectionistakentoobservethecorrosiondegreein010mmawayfromthesurfacebyESEM,anduseX-raylin
31、eanalysis(EDS)isusedtoobtaintheelementsdistributioninareaof05mm.2.3.7 XRDX/PertProMPDXraypowderdiffractionproducedbyHollandPhilipsCompanyisused.Crushtheconcreteatpredeterminedage,andtaketheconcretefromplacetobetested.Removetheaggregateandgrindthecementpasteinagatemortar.Filterwith0.075mmsieveandceme
32、ntpowderisgo注en.Drythepowderin60for3hoursandthepowderavailablefortestisobtained.2.3.8 IR360intelligentInfraredspectrumanalyzerproducedbyUSANigaoliCompanyisused,andthepreparationofpowderspecimenissameasXRD.3.Results3.1 ChangeofappearanceInFig.2,surfaceofSA1.1wassoftenedin4weeks,andmanysmallholesappea
33、red.Partofthesoftenedsurfacewaspeeloffandcoarseaggregatecanbeseen,andthecolorofsurfaceturnsintopale.At12weeks,morecoarseaggregatewasexposed,andtheaggregatecame3mmoutfromthecorrosionsurface,andobviousgapappearedbetweenthecoarseaggregateandcementpaste.Removethesoftenedlayerandthehardnessofinternalmatr
34、ixwasnotdeclined.AsforSAI.6,moreholesappearedonthesurfacein20weeks,butthesurfacewasn,tsoftened,andthesurfacewascoveredwithalayerofyellowsubstance,whichshouldbetheyellowprecipitatecausedbythehydrolyzationofFeion17(Fe2+H2O=Fe(OH)2;Fe3+H2O=Fe(OH)3).AsforSA1.1,itssurfacedidn,tturnyellow,asthesurfacewass
35、oftenedandpeeledoff.Moreover,inanacidoflowerpH,itisharderforFeiontohydrolyze.AsforSA2.0,thecorrosiondegreewasweakerthanSAI.6.Therewaslittlechangeonthesurface,andmoreyellowprecipitatewasproduced.WhenthepHwashigher,thehydrolyzationofFeionwasmoreserious.3.2 ChangeofmassInFig.3,themasscurveofSAI.6andSA2
36、0approachstraightline,therewasonlyaslightlossofmass.At20weeks,thelossofmasswas4.29%and3.18%respectively.AsforSA1.1,therateofthelossofmassincreasedobviouslyafter2weeks.At20weeks,thelossofmasswas13.1%,whichwassignificantlyhigherthanSAI.6andSA2.0.3.3 ChangeofcompressivestrengthInFig.4,thecompressivest
37、rengthcurveofSAI.6andSA2.0approachstraightline.At20weeks,thelossofcompressivestrengthwas18.2%and11.3%respectively.AsforSA1.1,therateofthelossofmassincreasedobviouslyafter4weeks.Theshapeofcurvefrom4weeksto20weekswasstraightline.At20weeks,thelossofcompressivestrengthwas36%,whichwassignificantlyhighert
38、hanSAI.6andSA2.0.3.4 PorefluidpHvaluecurveAcidificationdepth:AcidificationdepthcomesfromporefluidpHvaluecurve,theinitialporefluidpHvalueofAACis11.18,theplacewhosepH11.18wasacidized,thedistancebetween0mmandtheplacepH11.18isacidificationdepth.InFig.5,theacidificationdepthat2weekissmall,andpHvalueincre
39、asesfast.Withtimewenton,theacidificationdepthwasgettinglarger,theincreasedspeedofpHbecameslower.At20week,asthesurfacewassoftenandpeeledoff,theformerpartwasahorizontalline,namelypHwassamewiththecorrosionacid.ThecurveofSAI.6andSA2.0weresimilarasthatofSA1.1,butasthesurfacewasnotsoftened,sotherewerenoho
40、rizontallineintheformerpart.InFig.6,boththeacidificationdepthsofSAI.6andSA2.0weresimilarandmuchsmallerthanSA1.1.At20weeks,theacidificationdepthsofSAI.6andSA2.0was8mmand6mmrespectively.TheacidificationdepthofSA1.1wasmuchlarger,whichwas16mmat20week.3.5 ReleasequantityofCa2+InFig.7,theCa2+releasequanti
41、tyofSA1.1wasmuchlargerthanthatofSAI.6andSA2.0.During0-2week,theCa2+releasequantitywas501mgL,anditincreasedwiththetimewenton.TheCa2+releasequantityofSAI.6andSA2.0weresimilar,andduring0-2week,theCa2+releasequantitywas204mg/Land149mg/Lrespectively,anditincreasedinthefirstseveralweeksandthendecreasedwit
42、hthetimewenton.3.6 ESEM-EDS3.6.1 Appearanceof0-5mmcorrosionsectionInFig.8,SA1.1hasathickcorrosionlooselayer,andthecementinthenon-looselayerwasstillhard.ThecorrosionlooselayerofSAI.6andSA2.0was0.2mmand0.1mmrespectively,whichweremuchthinnerthanSA1.1andevencanbeignored.BoththenonlooselayerofSAI.6andSA2
43、0wascomplete.Aboveall,therewasnosignificantdamageinthenonlooselayer.3.6.2 EDSElementdistributioncurveof0-5mmcorrosionsection(1) Theeffectofbiogenicsulfuricacidconcentrationontheelementdistributionof0-5mmcorrosionsection.1.SInFig.10（八）,therewasmanySelementinthe0-2mmcorrosionlooselayer,andtherewaslit
44、tleSelementin2-5mmarea.AsforSAI.6andSA2.0,littleSelementwasfoundfromOmm.ItcanbespeculatedthatScamefromgypsumaftercorrosion,andcorrosionlooselayerhasmuchmoregypsum,andSA1.1hasathickerlooselayer.2.CaInFig.10(b),theCaelementin0-3.3mmareaofSA1.1decreasedobviously,andSAI.6andSA2.0hasasimilardistribution,
45、andtheCaelementinO-Immdecreasedobviously.ItshowsthatthebrokendepthofCa-O(comesfromC-S-H,gmeliniteetal.)ofSA1.1waslargerthanSAI.6andSA2.0.NoneoftheCaelement5mmawayfromthesurfaceofSA1.1,SAI.6andSA2.0wasdecreased,whichmeansthatCa-O5mmawayfromthesurfacewasnotbroken.3.NaInFig.10(c),theNaelementin0-5mmare
46、aofSA1.1increasesslightly,andtheTheNaelementofSAI.6in0-2.5mmwasataverylowlevelanditincreasesin2.5-5mmarea.TheNaelementofSA2.0in0-1.5mmwasataverylowlevelanditincreasesin1.5-5mmarea.ItcanbeseenthateventhesurfaceofSAI.6andSA2.0werenotsoftened,therewasdissolvingoutofNaelementandtheremoreNaelementwasdiss
47、olvedoutatalowerpH.4.SiInFig.10(d),theSielementinO-ImmareaofSA1.1wasataverylowlevel,asforSAI.6andSA2.0,therewasnosignsofdecreasefromthe0mm.ItshowsthattheSi-Ostructure(C-S-H,gmeliniteetal.)inthesurfaceofSA1.1wasbrokenandtheinternalstructurekeptcomplete,asforSAI.6andSA2.0,therewaslittledamageofSi-Ofro
48、mOmm.5.A1InFig.10(e),theAlin0-2mmareaofSA1.1wasataverylowlevel,andSA1.6,AlelementinO-Immwasataverylowlevel,andtheAlelementofSA2.0hasnodecreasedfromOmm.ItshowsthattheloweristhepHvalue,themoreseriousistheAldissolvedout.ThesurfaceofSA1.1hadamuchmoreseriouscorrosion,andSAI.6andSA2.0hadaslightcorrosion.(
49、2)ComparisonofCaandNa,AlandSiAsforSA1.1,CaandNaelementhasalowcontentinthesurfacebutthedissolvedoutdepthofNa(morethan5mm)waslargerthanCa(3.3mm),whichshowsthatNawaseasiertobedissolvedoutthanCa,andCaismorestablethanNa.AsforSiandAlelement,SihasalowcontentinO-ImmareaandAlhasalowcontentin0-2mm,sothedissolveddepthofAlislargerthanthatofSi,whichmeansthatAlO
