The alite found in Portland cement differs in composition from simple tricalcium silicate. It is a solid solution and contains minor amounts of other oxides besides CaO and SiO2. A typical composition is:
Based on this, the formula can be expressed as Ca2.90Mg0.06Na0.01Fe0.03Al0.04Si0.95P0.01O5. In practice, the composition varies with the bulk composition of the clinker, subject to certain limits. Substitution of calcium ions or orthosilicate ions requires that electric charges be kept in balance. For instance, a limited number of orthosilicate (SiO44-) ions can be replaced with sulfate (SO42-) ions, provided that for each sulfate ion, two aluminate (AlO45-) ions are also substituted.
As the temperature changes, it passes through several polymorphic states:
The polymorphs differ structurally by minor deformations from the basic hexagonal structure.
2Ca3SiO5 + 6H2O → 3CaO.2SiO2.3H2O + 3Ca(OH)2
The hydrate is referred to as the "C-S-H" phase. It grows as a mass of interlocking needles that provide the strength of the hydrated cement system. High alite reactivity is desirable in Portland cement manufacture, and this is achieved by retaining, as far as possible, high temperature polymorphs, in crystals that are small, distorted and highly defective. Defects provide sites for initial water attack.
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