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PortlandCementPortlandcementismadebyheatingamixtureoflimestoneandclay,ortheothermaterialsofsimilarbulkcompositionandsufficientreactivity,ultimatelytoatemperatureofabout1450℃.Partialfusionoccurs,andnodulesofclinkerareproduced.Theclinkerismixedwithafewpercentofcalciumsulfateandfinelyground,tomakethecement.Thecalciumsulfatecontrolstherateofsetandinfluencestheratestrengthofdevelopmentis.Itcommonlydescriedgypsumas,butthismaybepartlyorwhollyreplacedbyotherformsofcalciumsulfate.Somespecificationsallowtheadditionmaterialsofotheratthegrindingstage.Theclinkertypicallyhasacompositionintheregionof67%CaO,22%SiO,5%AlO,3%FeO22323and3%othercomponents,andnormallycontainsfourmajorphases,calledalite,belite,aluminateferriteand.Severalotherphases,suchasalkalisulfatesandcalciumoxidearenormallypresentinminoramounts.Hardeningresultsfromreactionsbetweenthemajorphasesandwater.AliteisthemostimportantconstituentofallnormalPortlandcementclinkers,ofwhichitconstitutes50%～70%.Itistricalciumsilicate(CaSiO)35modifiedincompositionandcrystalstructurebyionicsubstitutions.Itreactsrelativelyquicklywithwaterin,andnormalPortlandcementisthemostimportantoftheconstituentphasesforstrengthdevelopment;atupagesto28daysis,itbyfarthemostimportant.Beliteconstitutes15%～30%ofnormalPortlandcementclinkers.Itisdicalciumsilicate(CaSiO)modifiedbyionicsubstitutionsandnormallypresent24whollyorlargelyastheβpolymorph.Itreactsslowlywithwater,thuscontributinglittletothestrengthduringthefirst28days.Butsubstantiallytothefurtherincreaseinstrengththatoccursatlaterages.Byoneyear,thestrengthsobtainablefrompurealiteandbeliteareaboutthesameundercomparableconditions.Aluminateconstitutes5%10%ofmostnormalPortlandcementclinkers.Itistricalciumaluminate(CaAlO),substantiallymodifiedincompositionand326sometimesalsoinstructurebyionicsubstitutionsreacts.Itrapidlywithwater,andcauseundesirablyrapidsettingunlessasetcontrollingagent,usuallygypsum,isused.Ferritemakesup5%15%ofnormalPortlandcementclinkersis.Ittetra
