The approach to equilibrium of mixtures of CaO, A1₂O₃, and SiO₂ heated to various temperatures

Abstract

Equilibrium conditions in the CaO-Al₂O₃-SiO₂ ternary system as shown by the equilibrium diagram were established by allowing the solid phases to crystallize out of a liquid phase at specific temperatures. The purpose of this investigation was to determine whether equilibrium was attained or was closely approached when certain compositions of the anorthite-mullite-cristobalite and the anorthite-gehlenite-corundum compatibility triangles were fired to 1400° C. The effect on equilibrium of different raw materials used to form the compositions was also studied.

Based on the results of the present investigation the following conclusions seem justified:

(1) Equilibrium is attained by compositions in the anorthite-gehlenite-corundum compatibility triangle when held for twelve hours at 1400°C.

(2) Equilibrium is closely approached but is not attained by compositions in the anorthite-mullite-cristobalite compatibility triangle when held for twelve hours at 1400°C.

(3) The reactions in the anorthite-gehlenite-corundum sub-system take place essentially in the solid state.

(4) The transitory compounds that are formed at temperatures below 1400°C progress from the binary compounds of dicalcium silicate and calcium aluminate to binary compounds of higher silica and alumina contents, respectively, until gehlenite and anorthite are formed.

(5) The introduction of incompatible phases such as wollastonite and mullite does not deter the attainment of equilibrium in the anorthite-gehlenite-corundum triangle.

(6) Absorption and bulk density are not good indicators of the attainment of equilibrium in the compositions studied, but serve to show the marked differences in physical properties of samples having the same ultimate composition but made from different raw materials.