Regulation mechanism of Ca–Mg–Na on crystallization process of calcium sulphate hemihydrate whiskers and preparation verification based on steamed ammonia-calcium liquor

This study focuses on calcium resources from steamed ammonia-calcium liquor, using a pure CaCl2 system to simulate the waste liquid. Experiments and molecular dynamics simulations investigated the regulatory mechanisms of Ca–Mg–Na on calcium sulphate hemihydrate whiskers. Under optimized conditions (n(CaCl2):n(Na2SO4) = 1:1, reaction time of 3 h, ageing time of 5 h, Na+ concentration of 0.5 mol/L, and Mg2+ concentration of 0.002 mol/L), rod-like mixtures with an average aspect ratio of 20.26 ± 7.50 (comprising calcium sulphate hemihydrate and omongwaite (Na2Ca5(SO4)6·3H2O)) were obtained. Mechanistic analysis revealed that Na+ preferentially adsorbs onto (400) crystal plane, inhibiting a-axis growth, while Mg2+ adsorbs on (400), (200) and (020) crystal planes, suppressing multidirectional growth. Validation using a 2-fold concentrated solution yielded rod-like crystals with an average aspect ratio of 14.76 ± 5.87, confirming the experimental reliability. The mixture demonstrates superior thermal stability compared to pure calcium sulphate hemihydrate. This study achieves both waste valorization and high-value materials preparation.