Phase relations in the system Ca₂SiO₄-CaO-CaSO₄-CaF₂ relevant to cement clinkering

Static and dynamic techniques, including mercury quenching and differential thermal analysis, have been used to determine phase relations in the CaSO₄-Ca₂SiO₄-CaF₂ system without excessive (⪖ 1 %) volatilization occurring up to ∼ 1250°C. Melting commences at a ternary eutectic between Ca₂SO₄, CaF₂ and 3CaSO₄·3Ca₂SiO₄·CaF₂ at 960 ± 5°C. Solid phases coexisting with melt at invariant points include α and α′ C₂S, CaF₂, CaSO₄, and an apatite-structured phase 3CaSO₄·3Ca₂SiO₄·CaF₂ (fluorellestadite). Phase relations are also presented for three pseudobinary systems: fluorellestadite-CaSO₄, fluorellestadite-CaF₂ and fluorellestadite-Ca₂SiO₄. Reconnaissances of the CaO-Ca₂SiO₄CaSO₄CaF₂ system were made at 1000 and 1100°C. C₃S and its fluorine-stabilized derivatives were not encountered at this temperature. However, extensive substitution of F⁻ and SO²⁻₄ in the dicalcium silicate phase results in a lowering of its α-α′ transformation temperature from 1447°C to as low as 1370°C.