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1、UnitOneCementIntroductionCement,anymaterialthathardensandbecomesstronglyadhesiveafterapplicationinplasticform.Thetermcementisoftenusedinterchangeablywithglueandadhesive;inengineeringandbuildingconstructionthetermusuallyreferstoafinelypowdered,manufacturedsubstan
2、ceconsistingofgypsumplasterorportlandcementthathardensandadheresafterbeingmixedwithwater.Cementsareusedforvariouspurposes,suchasbindingsandandgraveltogetherwithPortlandcementtoformconcrete,forunitingthesurfacesofvariousmaterials,orforcoatingsurfacestoprotectthem
3、fromchemicalattack.Cementsaremadeinawidevarietyofcompositionsforawidevarietyofuses.Theymaybenamedfortheprincipalconstituents,suchascalcareouscement,whichcontainssilica,andepoxycement,whichcontainsepoxyresins;forthematerialstheyjoin,suchasglassorvinylcement;forth
4、eobjecttowhichtheyareapplied,suchasboilercement,orfortheircharacteristicproperty,suchashydrauliccement,whichhardensunderwater,oracid-resistingcement,orquick-settingcement.Cementsusedinconstructionaresometimesnamedfortheircommonlyreportedplaceoforigin,likeRomance
5、ment,orfortheirresemblancetoothermaterials,suchasPortlandcement,whichproducesaconcreteresemblingthePortlandstoneusedforbuildinginEngland.Cementsthatresisthightemperaturesarecalledrefractorycements.Cementsset,orharden,bytheevaporationoftheplasticizingliquidsuchas
6、water,alcohol,oroil,byinternalchemicalchange,byhydration,orbythegrowthofinterlacingsetsofcrystals.Othercementshardenastheyreactwiththeoxygenorcarbondioxideintheatmosphere.PortlandCementTypicalPortlandcementsaremixturesoftricalciumsilicate,tricalciumaluminate,and
7、dicalciumsilicate,invaryingproportions,togetherwithsmallamountsofmagnesiumandironcompounds.Gypsumisoftenaddedtoslowthehardeningprocess.Portlandcementismanufacturedfromlime-bearing'materials,usuallylimestone,togetherwithclays,shales,orblast-furnaceslagcontaininga
8、luminaandsilica,intheapproximateproportionsof60percentlime,19percentsilica,and8percentalumina,5percentiron,5percentmagnesia,and3percentsulfurtrioxide.Somerocks,called
