Decomposition and solid reactions of calcium sulfate doped with SiO2, Fe2O3 and Al2O3

The effects of SiO2, Fe2O3 and Al2O3 on the decomposition of calcium sulfate (CaSO4) were investigated by thermogravimetry, and the mineral phase composition of reaction products at high temperature was analyzed by X-ray powder diffraction when CaSO4was mixed with SiO2, Fe2O3, or Al2O3. Moreover, the solid-phase reactions between CaSO4 and the oxides as well as the decomposition kinetics of CaSO4 before and after mixing with Fe2O3 were well discussed. The experimental results show that the decomposition of CaSO4 is correlated with its crystal structure transformation, and the sensitive temperature zone for CaSO4 destabilization ranges from 1473 to 1523 K. All the additives including SiO2, Fe2O3 and Al2O3 can accelerate CaSO4 decomposition from the view of whole thermogravimetric analysis results. The initial decomposition temperature of CaSO4 can be reduced by the addition of SiO2 due to solid-phase reactions between SiO2 and CaO, and the formation of silicate mineral, while the formation of calcium sulphosilicate above 1473 K restrains CaSO4 decomposition to a certain extent. CaSO4 decomposition is promoted mainly by Fe ionic diffusion after adding Fe2O3. For the addition of Al2O3, the decomposition of CaSO4 is sped at the first stage by the formation of calcium aluminate, and then prevented after the formation of calcium sulphoaluminate. Thermal decomposition kinetic mechanism analysis indicates that the kinetic mechanism of pure CaSO4 decomposition is spherical symmetry phase boundary reaction, while the mechanism changes to random nucleation and subsequent growth after mixing with Fe2O3.