Understanding the mechanism on Na ⁺ ‐mediated crystallization of α‐hemihydrate gypsum in Na ₂ SO ₄ –water solutions

Abstract Gypsum crystals are influenced by environmental factors, leading to significant variations in strength among different gypsum product forms. The α‐hemihydrate gypsum, characterized by its well‐defined shape and high length‐to‐width ratio, has attracted attention for its compact structure and excellent mechanical properties. This study employs density functional theory to explore the crystallization mechanism of sodium ions from inorganic salts on α‐hemihydrate gypsum (α‐HH). The findings indicate that the adsorption capacity of Na + ions on the α‐HH (002) crystal plane surpasses that on other crystal planes. This difference is due to the bonding and charge transfer occurring between Na + ions and sulfate ions, primarily resulting from the interactions between the Na 2 s and Na 2 p orbitals with O 2 p orbitals. Moreover, the presence of water molecules further enhances the adsorption of Na + ions, particularly on the (002) crystal plane. When Na + ions substitute for Ca 2+ ions in the crystal structure, they are more likely to replace Ca atoms on the (002) crystal plane, which weakens the adsorption capacity of Ca 2+ ions. The mechanisms of selective adsorption and doping of Na + ions on the α‐HH (002) crystal plane provide valuable theoretical insights for the preparation of α‐hemihydrate gypsum crystals with optimal morphology.