Solidification and removal of impurities from phosphogypsum for road applications: a review

Abstract

Phosphogypsum, a major by-product of phosphoric acid production, can be recycled. Nonetheless, phosphogypsum contains impurities such as heavy metals, fluoride, and phosphate, which can decrease the performance and contaminate the environment, calling for pretreatment or solidification methods. Here, we review phosphogypsum with emphasis on impurities and their hazards, solidification methods, and treatment methods for use in road construction. Solidification involves blending phosphogypsum with inorganic cementitious materials, adding additives, geopolymers, biological treatments, and biochar adsorption. Phosphogypsum can be blended with electrolytic manganese residues, granulated blast furnace slag, and inorganic cementitious materials. Additives comprise polymers, surface modifiers, and curing agents. We observe that solidification methods display more advantages than pretreatment methods. The combination of phosphogypsum with inorganic cementitious materials, polymer surface modifiers, curing agents, geopolymer materials, and biomass materials can effectively solidify various impurities, though the effectiveness varies across different solidification methods. There are four solidification mechanisms: physical encapsulation, chemical precipitation, ion exchange, and adsorption. When solidified in road engineering applications, phosphogypsum show reduced leaching levels of arsenic, lead, while maintaining a good road performance.