Formation and transition of calcium aluminate and calcium silicate compounds from pre-synthesized mullite in low-calcium system by solid-state reaction

The formation and transition of calcium aluminate and calcium silicate compounds from pre-synthesized mullite in low-calcium system were systematically studied by solid-state reaction at 1350–1500 °C using XRF, XRD, FTIR, SEM and PSD methods. Ca₃Al₂O₆, Ca₁₂Al₁₄O₃₃, CaAl₂O₄, Ca₂SiO₄ and Ca₂Al₂SiO₇ can form via direct reaction of mullite with CaO at the beginning of the reactions, then Ca₁₂Al₁₄O₃₃ and Ca₃Al₂O₆ react with mullite to form CaAl₂O₄ and Ca₂SiO₄, and finally Ca₂Al₂SiO₇ reacts with CaO to generate Ca₂SiO₄ and calcium aluminate compounds as the sintering process proceeds. Elevating the sintering temperature is in favor of the formation of Ca₁₂Al₁₄O₃₃, Ca₃Al₂O₆ and Ca₂Al₂SiO₇ at the initial reaction stage, but Ca₂Al₂SiO₇ cannot totally transform to calcium silicate and calcium aluminate compounds in the low-calcium system, which deteriorates the pulverization performance of the final products. Increasing the calcium dosage accelerates the transformation of Ca₂Al₂SiO₇ to Ca₁₂Al₁₄O₃₃, CaAl₂O₄ and Ca₂SiO₄, which enhances the pulverization performance. If the CaO/Al₂O₃ molar ratio exceeds 1.4, Ca₃Al₂O₆ will generate by the reactions of pre-formed CaAl₂O₄ and Ca₁₂Al₁₄O₃₃ with excessive CaO.