Growth of large-sized α-HH via Mg2+-involved atmospheric salt bath-stirring strategy: crystallization mechanism and enhanced mechanical properties

This study proposes a method involving stirring after a constant-temperature water bath in a salt solution to enhance the crystal size, overall homogeneity, and hydration strength of α-hemihydrate gypsum (α-HH). Utilizing by-product gypsum from the chlor-alkali industry as raw material, the influence of Mg²⁺ ions on phase evolution and final morphology was systematically investigated. During the constant-temperature water bath process in salt solution, the neutral MgSO₄⁰ ion pairs formed by Mg²⁺ and SO₄^2- facilitated the dissolution of dihydrate gypsum (DH) while amplifying the solubility difference between α-HH and DH, thereby promoting preferential crystallization and precipitation of α-HH. Furthermore, Mg²⁺ inhibited crystal growth along the c-axis direction during α-HH development, effectively reducing the aspect ratio. The subsequent stirring process significantly increased secondary nucleation probability, enabling mutual adhesion among α-HH crystals of varying dimensions. This synergistic process ultimately yielded enlarged α-HH crystals with an average particle size of 143.21 μm and aspect ratio of 5.57. The flexural strength and absolute dry compressive strength of bulk samples prepared from these α-HH crystals are discussed in detail, demonstrating substantial improvements in mechanical properties compared to conventional preparation methods.