Potentialuseofphosphogypsuminconcrete

CementandConcreteResearch29(1999)1419–14250008-8846/99/$–seefrontmatter©1999ElsevierScienceLtd.Allrightsreserved.PII:S0008-8846(99)00107-6PotentialuseofphosphogypsuminconcreteMohammadM.Smadi*,RamiH.Haddad,AhmadM.AkourJordanUniversityofScience&Technology,CivilEngineeringDepartment,P.O.Box3030,Irbid,JordanReceived28May1998;accepted11May1999AbstractAnexperimentalstudywasconductedtoinvestigatethepotentialuseofphosphogypsum(PG)inconcrete.Thiswasaccomplishedbypreparingmortarmixesatawater/cementratioof0.6usingtwotypesofcement,ordinaryPortlandcement(OPC)andpozzolanicPort-landcement(PPC),andtwotypesoffineaggregate,naturalriversandandlimestone.Thesemixeswerepreparedatdifferentreplace-ments(byweight)ofPGandpurifiedPGrangingfrom10–100%.ThepurifiedPGwasobtainedbycalciningPG(washedinwaterandnotwashed)attemperaturesof170,600,750,850,and9508C.Thecompressive,tensile,andflexuralstrengthsofdifferenthardenedmor-tarswereobtainedfollowing3,7,and28daysofcuringinairandinamoistroom.Inaddition,thesettingtimeandthesoundnessofce-mentpastes,preparedusingOPCandPPCatawater/cementratioof0.6andatdifferentreplacementsofPG,wereevaluated.Atapar-ticularPGpercentageofreplacement,theresultsindicatedanincreasingtrendinstrengthgainovercuringtime.Thestrengthsofmortars,curedforaspecifictime,showedadecreaseinthestrengthwithfurtherreplacementwithPG.Thepurificationprocess,byheat-ingPGupto9008C,hasresultedinimprovingthestrengthsofthemortarmixes.ThehighestpercentageincreaseinstrengthwasfoundtobeforPGcalcinedattemperaturesof850and9008C.TheincorporationofPGinthecementpastehasdramaticallyincreaseditsinitialandfinalsettingtimesandhasincreasedthepercentageelongationofpasteprismsusedtoevaluatethesoundness.Theimprovementinstrengthuponcalcinationisprimarilyduetotheformationofanhydrateathighertemperatures,whichsubsequentlyhydratestogypsum;thelatterhassomecementingvalue.©1999ElsevierScienceLtd.Allrightsreserved.Keywords:Gypsum;Sulfate;Expansion;Mechanicalproperties;X-raydiffraction1.IntroductionPhosphogypsum(PG)isaby-productresultingfromthephosphoricacidprocessformanufacturingfertilizers.ItconsistsmainlyofCaSO4•2H2Oandcontainssomeimpu-ritiessuchasP2O5,F2,andorganicsubstances.Thequan-tityofPGproducedisverylarge:foreach1tonofphos-phate(P2O5)produced,thereisacoproductionof5tonsofcalciumsulfate(PG).Theannualworldproductionofthismaterialisupto180milliontonsandisexpectedtoreachupto300milliontonsbytheyear2000[1].Only15%ofthePGisutilizedbythecementandgypsumindustriesasasettingmoderatorforcementandformakinggypsumplas-ter.TheimpuritiesinPGhavebeenfoundtodelaytheset-tingtimeandreducetherateofearlystrengthdevelopmentofcementtoagreaterextentthanpuregypsum,althoughstrengthsatlateragesarenotaffected.Fortunately,eitherchemicalprocessingorheattreatmentcanreducetheseim-purities.Yet,cautionshouldbeexercisedwhenusingblendsofPortlandcementwithgypsumduetothehighpos-sibilityofvolumeinstability(expansion)causedbythefor-mationofettringateoncetheconcreteormortarhashardened.Theremaining85%ofPGisnotused,causinganenvi-ronmentalproblemandcreatinganeedforlargeareasfordisposal.Therefore,attemptsweremadetousePGinappli-cationssuchasroadandrailworksfills,stabilizationofbasecourse,andbuildingconstructions.Inaddition,thepo-tentialforusingPGinmakingroller-compactedconcrete,supersulfatedcement,andhighaluminacement,aswellasinordinaryconcrete,hasbeeninvestigated[2–9].PGhasbeenusedinroller-compactedconcrete(RCC)toconstructparkingfacilities[2].TheresultsindicatedthatPG,withoutbeingdetrimentaltostrength,contributedtogoodmixcompactionandsurfacefinishasaresultofitsfinegrad-ing.UsingPGinRCChasproventobebeneficialdueto:(a)theimprovementinbondingatconstructionjointsasaresultofPGsetretardationaction,and(b)thereductionintheoc-currenceofcrackingwhensawcutjointsarenotprovided,duetodryingshrinkagecompensationbythePG.UsingPGinsupersulphatedcement(SSC)hasresultedinshorteningitssettingtimewithoutaffectingitsultimatestrength.Nevertheless,curingSSCinseawaterhasresultedinreducingitsultimatestrengthcomparedtothatofspeci-menscuredindrinkingwater[3,4].*Correspondingauthor.Tel.:[phone],ext.2145;fax:[phone].E-mailaddress:[e-mail](M.M.Smadi)1420M.M.Smadietal./CementandConcreteResearch29(1999)1419–1425TheeffectsofPGandpurifiedPG(washedwithmilkoflimeorwater)onthehydrationofPortlandcementandtrasscementwereinvestigatedbymeasuringthesettingtimeandthestrengthofpastes,aswellasperformingchemicalanaly-sisandscanningelectronmicroscopic(SEM)onthehydra-tionproducts[5].ItwasconcludedthatPGcanbeusedasaretarderdirectlyintrasscement,whereasitshouldbepuri-fiedbeforebeingusedinPortlandcement.Inanotherstudy,ÖlmezandYilmaz[6]washedPGinwaterormilkoflime,thenexposedittothermaltreatmentsat140–1508Cfor30minandat130–1508Cfor60mintoimproveitschemicalandphysicalproperties.Theresultsshowedthatallofthewater-solubleimpuritiescanbere-movedbythesimplewashingtreatmentwithwaterormilkoflime,buttheP2O5,substitutedinthecrystallatticeofPG,andotherimpuritiescannotberemovedbythewashingpro-cessandneededathermaltreatment.Theamountofimpuri-tiesinPGw