Calcium Orthosilicate, Ca₂SiO₄

Dicalcium Silicate, or Calcium Orthosilicate, 2CaO.SiO₂ or Ca₂SiO₄, does not occur in nature but is obtained by fusing together silica and calcium oxide in suitable proportions, or by dissolving silica, alone or with calcium oxide, in calcium chloride. It is frequently produced in slags on which it confers its own property of disintegrating on cooling. It is a polymorphic compound. α-Ortho-silicate melts at 2080° C., crystallises in the monoclinic system, has a density of 3.27, and hardness 5.6. By slow cooling it changes at 1410° C. to the orthorhombic β-compound, of density 3.28, and at 675° C. to the monoclinic γ-compound which has a decidedly smaller density, namely, 2.97. The last change must therefore involve a considerable increase in volume, and it is this increase which causes the pulverisation on slow cooling noticed by Le Chatelier. Rapid cooling fixes α-orthosilicate at the ordinary temperature. There is also a fourth unstable form, β'-orthosilicate, obtained by cooling hydrated orthosilicate from 1425° C. α-Orthosilicate has the power of setting with water, but this property is not possessed by the γ-variety. The β-compound cannot be kept at ordinary temperatures.

The tendency to form the γ-orthosilicate, and therefore to pulverise, can be reduced by the presence of certain foreign substances, for example, magnesium, aluminium, and iron oxides. It has been suggested that the difference between hydraulic and non-hydraulic dicalcium silicate is to be accounted for by a difference in the structure of the molecule, the latter being a true orthosilicate, , and the former a basic metasilicate, , which is readily hydrolysed, yielding lime and normal metasilicate.

The heat of formation from the metasilicate is 8.7 Cal., or from silica and lime 24.8 Cal. The orthosilicate is more readily attacked by acid than the metasilicate.

With manganese orthosilicate in varying proportions a continuous series of solid solutions is formed. With calcium chloride a compound, 2CaO.SiO₂.CaCl₂, is obtained. The freezing-point curve of binary mixtures of lithium and calcium orthosilicates indicates a compound Li₄SiO₄.Ca₂SiO₄, stable below 932° C. and of density 2.847.