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PORTLAND CEMENT Chemical composition Portland Cement is made up of four main compounds tricalcium silicate 3CaOSiO2 dicalcium silicate 2CaOSiO2 tricalcium aluminate 3CaOAl2O3 and a tetra-calcium aluminoferrite 4CaO Al2O3Fe2O3 In an abbreviated notation differing from the normal atomic symbols these compounds are designated as C3S C2S C3A and C4AF where C stands for calcium oxide lime S for silica A for alumina and F for iron oxide Small amounts of uncombined lime and magnesia also are present along with alkalies and minor amounts of other elements Hydration The most important hydraulic constituents are the calcium silicates C2S and C3S Upon mixing with water the calcium silicates react with water molecules to form calcium silicate hydrate 3CaO 2SiO23H2O and calcium hydroxide Ca OH 2 These compounds are given the shorthand notations C-S-H represented by the average formula C3S2H3 and CH and the Hydration reaction can be crudely represented by the following reactions 2C3S 6H C3S2H3 3CH 2C2S 4H C3S2H3 CH During the initial stage of hydration the parent compounds dissolve and the dissolution of their chemical bonds generates a significant amount of heat Then for reasons that are not fully understood hydration comes to a stop This quiescent or dormant period is extremely important in the placement of concrete Without a dormant period there would be no cement trucks pouring would have to be done immediately upon mixing Following the dormant period which can last several hours the cement begins to harden as CH and C-S-H are produced This is the cementitious material that binds cement and concrete together As hydration proceeds water and cement are continuously consumed Fortunately the C-S-H and CH products occupy almost the same volume as the original cement and water volume is approximately conserved and shrinkage is manageable Although the formulas above treat C-S-H
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