Research on the process and mechanism of preparing high-strength α-hemihydrate gypsum from phosphogypsum with mellitic acid

Phosphogypsum, a voluminous by-product of wet-process phosphoric acid production, presents substantial environmental burdens due to its extensive accumulation. Transforming this waste material into high-value α-hemihydrate gypsum (α-HH) offers a promising avenue for resource recovery and environmental sustainability. This study delves into the innovative process and underlying mechanism for synthesizing high-strength α-HH from phosphogypsum utilizing mellitic acid as a crystal modifier. By employing single-factor and orthogonal experiments within a pressurized hydrothermal method, an optimal procedure was identified. The resultant product demonstrated impressive mechanical properties, achieving 2 h flexural and dry compressive strength of 9.84 MPa and 67.65 MPa, respectively, thereby exceeding the α60 level specified in JC/T 2038–2010. X-ray photoelectron spectroscopy (XPS) analysis confirmed the adsorption of mellitic acid onto the α-HH surface. Density functional theory (DFT) calculations further elucidated that the adsorption energies of mellitic acid on the (001) and (100) crystal planes were −10.45 eV and −1.80 eV, respectively, indicating stronger interaction with the (001) plane. This strong adsorption plays a crucial role in regulating the crystal morphology towards more desirable short columnar structures, thus enhancing the mechanical strength of the produced α-HH. These findings offer valuable theoretical and practical insights for advancing gypsum-based material engineering and expanding the high-value utilization of industrial by-products.