The crystal transformation characteristics from dihydrate gypsum to α-hemihydrate gypsum in Mg(NO3)2 solution

The preparation of high-strength α -hemihydrate gypsum ( α -HH) from dihydrate gypsum (DH) is a potential way to improve the utilization rate of industrial gypsum. α -HH with a low aspect ratio was prepared by atmospheric salt solution method, using Mg(NO 3 ) 2 solution as the salt medium. The effects of reaction temperature, reaction time, Mg(NO 3 ) 2 concentration, pH value, and the solid-to-liquid ratio on the purity, yield and aspect ratio of the product were investigated systematically. Under the optimal reaction conditions of 95 °C, 4 h, 40% (mass) Mg(NO 3 ) 2 , pH 5, and a solid–liquid ratio of 1:5, the yield and purity of the product could reach 89.67% and 99.85%, respectively. Additionally, the average aspect ratio of this product was 2.02, and the compressive strength reached 58.2 MPa. The regulation mechanism was studied by calculating the adsorption energies of Mg(NO 3 ) 2 on different crystal planes of α -HH, which indicated that Mg(NO 3 ) 2 exhibited the strongest adsorption on the (1 1 1) plane, and this preferential adsorption retarded the axial growth of α -HH, resulting in a reduced aspect ratio of the crystals. These findings suggest that the Mg(NO 3 ) 2 solution is an effective approach for preparing high-strength α -HH with controlled morphology.