Revealing the Mechanisms of the Morphological Response of α-Calcium Sulfate Hemihydrate to Aluminum Hydroxide by Density Functional Theory Simulations and Experimental Methods

The morphology of α-calcium sulfate hemihydrate (α-HH) crystals is a critical determinant of their applicability. This study systematically explored the regulatory effect of aluminum hydroxide (Al(OH)₃) on the morphology of α-HH. Our findings indicated that Al(OH)₃ substantially suppressed the growth of α-HH crystals along the <i>c</i>-axis. As the concentration of Al(OH)₃ increased from 0% to 0.125%, the morphology of the α-HH crystals progressively transitioned from fibrous to columnar, platelike, and then granular forms. Density functional theory (DFT) calculations revealed that at lower concentrations of Al(OH)₃, Al(OH)₂⁺, and Al(OH)₄^- ions preferentially adsorb on the (002) end plane, leading to a reduction in the surface energy of this plane. This adsorption inhibits its growth and enhances the exposure of the (002) plane, ultimately resulting in a morphological transformation of the α-HH crystals from a fibrous to short columnar. At elevated concentrations of Al(OH)₃, the selective adsorption of Al(OH)₂⁺ and Al(OH)₄^-_, alongside the nonselective precipitation of excess Al(OH)₃ as colloidal forms on the surface of α-HH, facilitated the growth of specific crystals exhibiting platelike and granular morphologies. This work will provide a foundational reference for the preparation of α-HH with diverse morphologies derived from industrial gypsum.