Thermodynamics of the cao-si02 system

In order to maintain consistency, analytical expressions for the free energy of mixing of phases should reproduce not only the phase diagrams but also the experimentally determined activities. Information on the partial molar free energies and the phase boundaries, in turn, can be used to estimate the free energy of formation of compounds.

An examination of thermochemical data in the CaO-SiO₂ system showed that ΔGδf values for -Ca₂Si0₄, which are stable at temperatures above 1710°K, are limited a maximum of 1800°K. The free energy of formation in a temperature range from about 1700 to 2400°K was estimated from the phase boundary and the activity of silica to be as follows:

2Ca0(s) + Si0₂(cristo.) = Ca2Si0₄() ΔG°f = −86303.50 − 34.338 Tjoules

An analytical expression for the free energy of mixing of the liquid phase was obtained for the entire composition range in the CaO-Si0₂ system. Confidence in the estimated G‡f for -Ca₂Si0₄ was demonstrated by good agreement of the calculated phase diagram and the experimentally determined activity of silica.