Crystallization agents modifying conversion of phosphogypsum to α-hemihydrate gypsum: Effects of the charge and molecular polarity of carbon-based oxygen atoms

Abstract

To promote the resource utilization of phosphogypsum (PG) and reduce environmental pollution, this study utilizes the autoclave method with various organic acids and their sodium salts as crystallization agents to convert PG into α-hemihydrate gypsum (α-HH). The study investigates the effects and mechanisms of these crystallization agents on the crystal morphology and strength of α-HH using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), zeta potential measurements, and molecular dynamics simulations. The results indicate that adding various organic acids and their sodium salts as crystallization agents can effectively regulate the strength and crystal morphology of the α-HH obtained from PG to reduce the aspect ratio of the α-HH crystals. Sodium citrate demonstrated the optimal efficacy as a crystallization agent in controlling the crystal morphology of α-HH. Reducing the aspect ratio to 1.25:1. The absolute dry compressive strength increased to 40.3 MPa when sodium citrate was used, meeting the requirements for high-strength gypsum, specifically α40. Sodium salts of various organic acids demonstrate greater ability than their corresponding organic acids in regulating the crystal morphology of α-HH. This is mainly attributed to the fact that compared to organic acids, the carbon-based oxygen atoms in sodium salts have stronger electron-donating capacity and higher molecular polarity. These characteristics facilitate the formation of cyclic chelates with Ca²⁺ ions within α-HH crystals, thereby enhancing crystal morphology control and increasing α-HH strength.