New insight into the salt role during preparing α-hemihydrate gypsum in atmospheric salt solution: Particle size control

The synergistic control of the crystal morphology and size of α-hemihydrate gypsum (α-HH) was achieved by adjusting the Na₂SO₄ concentration and maleic acid content. Maleic acid primarily regulated the α-HH morphology through surface adsorption. The concentration of Na₂SO₄ determined crystal size by tailoring the maximum relative supersaturation (S_max) during α-HH crystallization. Higher Na₂SO₄ concentrations led to higher S_max and smaller crystal sizes. The S_max of α-HH in a 9% Na₂SO₄ solution was 4.5 times that in a 6% solution, resulting in a significant decrease in crystal size from approximately 40–18 µm as the Na₂SO₄ concentration increased from 6% to 9%, while maintaining similar crystal morphology (average aspect ratio of about 1.0). This study filled the gap in research on particle size control during the preparation of α-HH using the salt solution method under atmospheric pressure, contributing to a more systematic and comprehensive understanding of this method.