Revealing maleic acid role in the preparation of α-hemihydrate gypsum from titanium gypsum through experiments and DFT calculations.

Titanium gypsum (TG) is rarely used to produce α-hemihydrate gypsum (α-HH) because of its poor crystallinity and high impurity and moisture contents. Here, a method is proposed to prepare α - HH by adjusting the reaction temperature, CaCl solution concentration and maleic acid dosage based on acid leaching and heat-treated TG as raw material. The effect of maleic acid and Fe ions on the preparation of α-HH were systematically analyzed using density functional theory (DFT) and typical materials characterization methods, X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). Under the optimal conditions (CaCl concentration of 23 % and reaction temperature of 95 °C), the maleic acid is chemically adsorbed on the crystal surfaces of α-HH, the strongest adsorption is in the (111) surface. Increasing the maleic acid concentration from 0 to 0.15 % decreased the aspect ratio of the α-HH crystals from 8.26 to 0.96, respectively, where the optimal dosage was 0.1 %. The theoretical results proved that the substitution energy of Fe was greater than that of Ca, and Fe ions can spontaneously enter the α-HH lattice to replace Ca ions. Furthermore, the adsorption energy of maleic acid on the (111) surface increased after the substitution of Fe to generate a synergistic effect that hinders α-HH growth along the c-axis, resulting in the preferred morphology. The results of this study provide a new method for using waste TG to produce a high-value-added product.