5.1PortlandCementandItsMajorConstituentPhases學號：姓名：5.1.1PortlandCementGeneral5.1.2TypesofPortlandcement5.1.3CementChemicalNomenclatureandOtherAbbreviations5.1.1PortlandCementGeneralPreparationofPortlandCement:Portlandcementismadebyheatingamixtureoflimestoneandclay,ortheothermaterialsofsimilarbulkcompositionandsufficientreactivity,ultimatelytoatemperatureofabout1450℃.Partialfusionoccurs,andnodulesofclinkerareproduced.Theclinkerismixedwithafewpercentofcalciumsulfateandfinelyground,tomakethecement.Alite,Belite,AluminateandFerrite:Alite

(Ca3SiO5):AliteisthemostimportantconstituentofallnormalPortlandcementclinkers,ofwhichitconstitutes50%～70%.Itistricalciumsilicate(Ca3SiO5)modifiedincompositionandcrystalstructurebyionicsubstitutions.Belite

(Ca2SiO4):Beliteconstitutes15%～30%ofnormalPortlandcementclinkers.Itisdicalciumsilicate(Ca2SiO4)modifiedbyionicsubstitutionsandnormallypresentwhollyorlargelyastheβpolymorph.Aluminate

(Ca3Al2O6):Aluminateconstitutes5%10%ofmostnormalPortlandcementclinkers.Itistricalciumaluminate(Ca3Al2O6),substantiallymodifiedincompositionandsometimesalsoinstructurebyionicsubstitutions.Ferrite

(Ca2AlFeO5):Ferritemakesup5%15%ofnormalPortlandcementclinkers.Itistetracalciumaluminoferrite(Ca2AlFeO5),substantiallymodifiedincompositionbyvariationinAl/Feratioandionicsubstitutions.5.1.2TypesofPortlandcement

Rapid-hardeningPortlandcements

Rapid-hardeningPortlandcementshavebeenproducedinvariousways,suchasvaryingthecompositiontoincreasethealitecontent,finergrindingoftheclinker,andimprovementsinthemanufacturingprocess,e.g.finergrindingorbettermixingoftherawmaterials.ThealitecontentsofPortlandcementshaveincreasedsteadilyovertheoneandahalfcenturiesduringwhichthelatterhavebeenproduced,andmanycementsthatwouldbeconsideredordinarytodaywouldhavebeendescribedasrapidhardeningonlyafewdecadesago.InASTMspecifications,rapid-hardeningcementsarecalledhighearlystrengthorTypeⅢcements.Forbothordinaryandrapid-hardeningcements,bothlowerandupperlimitsmaybeimposedonstrengthat28days,upperlimitsbeingasafeguardagainstpoordurabilityresultingfromtheuseofinadequatecementcontentsinconcrete.

WhitePortlandcementsWhitePortlandcementsaremadebyincreasingtheratioofAl2O3toFe2O3,andthusrepresenttheoppositeextremeincompositiontosulfate-resistingPortlandcements.Thenormal,darkcolourofPortlandcementisduetotheferrite,formationofwhichinawhitecementmustthusbeavoided.ItisimpracticabletoemployrawmaterialsthatarecompletelyfreefromFe2O3andothercomponents,suchasMn2O3,thatcontributetothecolour.Theeffectsofthesecomponentsandbyrapidquenching.Inadditiontoalite,beliteandaluminate,someglassmaybeformed.5.1.3CementChemicalNomenclatureandOtherAbbreviationsChemicalformulaeincementchemistryareoftenexpressedassumsofoxides;thustricalciumsilicate,Ca3Sio5,canbewrittenas3CaO·SiO2.Thisdoesnotimplythattheconstituentoxideshaveanyseparateexistencewithinthestructure.Itis