Effects of potassium aluminium sulfate additive and calcination temperature on crystallinity, morphology and properties of calcined gypsum

Calcined gypsum (CG) produced by calcining desulfurised gypsum can be widely used in gypsum-based building materials, which has great economic and environmental benefits. In this work, CG was obtained by calcining desulfurised gypsum with the addition of potassium aluminium sulfate dodecahydrate (KAl(SO4)2.12H2O henceforth called PASD). The effects of calcination temperature and PASD content on the properties of the CG were systematically investigated and the morphological control mechanism of PASD on the CG was revealed. The properties of the CG were the optimum when desulfurised gypsum with 0.3% PASD was calcined at 170°C for 2 h. The initial and final setting times were 11.3 min and 15.2 min, respectively, and the flexural and compressive strengths at 2 h were 3.46 MPa and 9.38 MPa, respectively. The addition of PASD can promote the transformation of dihydrate gypsum into well-crystallised hemihydrate gypsum and regulate the morphology of hydration products. The addition of PASD not only caused the dihydrate gypsum to dehydrate and crack by layer, but also made the diameter of hydration products more uniform, thus improving the strength of the CG. High-quality CG will improve the performance of downstream building products, thereby broadening the utilisation of desulfurised gypsum.