Research progress of phosphogypsum-based backfill technology

China, a major producer of phosphogypsum, encounters significant environmental challenges owing to its accumulation. Employing phosphogypsum-based backfill is an effective solution for the safe and efficient mining of bulk solid waste from phosphate ores. It aligns with national ecological goals and promotes sustainable green mining practices. This method holds great potential for widespread application in China, where rapid advancements and significant scientific and technological achievements have been made. This paper systematically explores phosphogypsum-related progress, examining material characteristics, filling preparation processes, filling performance, filling pollution risks, and control strategies. (1) A quantitative analysis of relevant research results and policy trends has revealed a growing interest in phosphogypsum-based backfill, with an increasing number of technical papers and patents published annually. The rapid development of phosphogypsum-based backfill is closely linked to policy guidance. Since 2017, policies have explicitly encouraged the application of phosphogypsum in filling underground gob spaces. Phosphogypsum waste as a filling material for underground goaf is a general trend. (2) The study collated the chemical composition and physical properties of phosphogypsum from different regions, both domestically and internationally. It analyzed the types of cementitious materials, the theoretical optimal stoichiometric composition range, and summarized the factors and evolutionary mechanisms influencing the rheological and mechanical properties of phosphogypsum-based backfill. Typically, phosphogypsum appears gray-black or gray-white, with a pH value ranging from 1.2 to 6. Its main chemical components are CaO, SO₃, and SiO₂, and domestic phosphogypsum generally contains lower fluorine levels, ranging between 0.5% and 1.5%, compared to foreign samples. In addition to traditional factors such as slurry concentration, lime–sand ratio, and admixture, the composition and particle size of phosphogypsum, pretreatment methods, ambient temperature, and stirring time significantly affect its rheological properties. The development of hydration products plays a decisive role in the strength of phosphogypsum-based backfill. Common hydration products include calcium silicate hydrate (C–S–H) gel, calcium aluminate hydrate (C–A–H) gel, AFt, calcite, and calcium hydroxide (Ca(OH)₂). (3) The potential environmental pollution risk of phosphogypsum-based backfill was evaluated, and the existing phosphogypsum pretreatment technology and toxic element fixation mechanism were summarized. In the backfill system, hydration products interact with harmful impurities, thereby stabilizing and fixing toxic substances. This stabilization process occurs through mechanisms such as adsorption, physical encapsulation, chemical precipitation, and ion replacement. Pretreatment technologies for phosphogypsum are classified into physical, chemical, and thermal methods. (4) Future efforts in phosphogypsum-based backfill should prioritize long-term safety and environmental sustainability. This involves focusing on pretreatment, middle, and back-end treatments to overcome theoretical and technical challenges associated with eco-friendly applications. Research should be directed toward three key areas: advancing pretreatment technologies, optimizing target materials, and enhancing in situ monitoring techniques. The aim is to showcase the development achievements of phosphogypsum-based backfill and provide valuable references and insights for its large-scale application and future research efforts.