Stability of silico-ferrite of calcium and aluminum (SFCA) in air-solid solution limits between 1240 °C and 1390 °C and phase relationships within the Fe2O3-CaO-Al2O3-SiO2 (FCAS) system

Quenching experiments were used to investigate the solid solution range, thermal stability, and selected phase relationships of silico-ferrite of calcium and aluminum (SFCA) within the Fe2O3-CaO-Al2O3-SiO2 (FCAS) system. SFCA was found to be stable within a plane that connects the end members CF3 (CaO·3Fe2O3), CA3 (CaO·3Al2O3), and C4S3 (4CaO·3SiO2). Chemical substitution in the four component system follows the coupled substitution mechanism 2(Fe3+, Al3+)↔(Ca2+, Fe2+)+Si4+ with the greatest range in chemical substitution occurring in the direction of the Al3+↔Fe3+ exchange (ranging from 0 wt pct Al2O3 to ∼31.5 wt pct Al2O3). The extent of Al3+↔Fe3+ substitution decreases with increasing temperature, and it was estimated that SFCA completely decomposes by ∼1480 °C. Coupled substitution involving Ca2+ and Si4+ for 2M3+ is not as extensive as the Al3+↔Fe3+ exchange, having a maximum range between 3 and 11 wt pct C4S3 component. Additional phases encountered in the experimental program included hematite; magnetite; quench liquid; dicalcium silicate; Fe-bearing gehlenite; calcium alumino-ferrite solid solutions, C(A, F)6 and C(A, F)2, plus an unidentified phase, possibly representing a solid substitution between SFCA-I and C(A, F)3. Schematic phase diagrams have been constructed to show the relationship of SFCA with these surrounding phases.