Portland Cement
Portland CementPORTLAND 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 as a specific stoichiometry, with the formula C3S2H3, it does not at all form an ordered structure of uniform composition. C-S-H is actually an amorphous gel with a highly variable stoichiometry. The ratio of C to S, for example, can range from 1:1 to 2:1, depending on mix design and curing conditions.
Structural properties.
The strength developed by portland cement depends on its composition and the fineness to which it is ground. The C3S is mainly responsible for the strength developed in the first week of hardening and the C2S for the subsequent increase in strength. The alumina and iron compounds that are present only in lesser amounts make little direct contribution to strength. Set cement and concrete can suffer deterioration from attack by some natural or artificial chemical agents. The alumina compound is the most vulnerable to chemical attack in soils containing sulfate salts or in seawater,...
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