Belite

Belite is an industrial mineral important in Portland cement manufacture. Its main constituent is dicalcium silicate, Ca₂SiO₄, sometimes formulated as 2 CaO · SiO₂ (C₂S in cement chemist notation).

The name was given by Törneborn in 1897 to a crystal identified in microscopic investigation of Portland cement. Belite is a name in common use in the cement industry, but is not a recognised mineral name. It occurs naturally as the mineral larnite, the name being derived from Larne, Northern Ireland, the closest town to Scawt Hill where it was discovered.

The belite found in Portland cement differs in composition from pure dicalcium silicate. It is a solid solution and contains minor amounts of other oxides besides CaO and SiO₂. A typical composition:

Based on this, the formula can be expressed as Ca_1.94Mg_0.02Na_0.01K_0.03Fe_0.02Al_0.07Si_0.90P_0.01O_3.93. 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.

Dicalcium silicate is stable, and is readily prepared from reactive CaO and SiO₂ at 300 °C. The low temperature form is γ-belite, or lime olivine. This form does not hydrate, and is avoided in cement manufacture.

As the temperature rises, it passes through several polymorphic states:

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