Enhanced immobilization of fluoride in phosphogypsum-based cement-free paste backfill modified by polyaluminum chloride and its mechanism

Phosphogypsum (PG)-based cemented paste backfill (PCPB) is a promising avenue for the large-scale disposal of PG. However, the toxicity and low strength of PCPB results in high costs and elevated fluoride pollution, limiting its potential applications. Steel slag and granulated blast furnace slag have been proven as cementitious materials for the preparation of PCPB to improve strength and decrease costs, but the problem of potential fluorine pollution remains unresolved. This study expands on previous work that investigated the feasibility of incorporating polyaluminium chloride (PAC) into PG-based cement-free paste backfill (PCFPB) to enhance fluoride immobilization and stabilization (S/S). The research assesses the strength, mineralogical composition, and microstructural properties and fluoride S/S capacities of PCFPB specimens utilizing a comprehensive analysis, including X-ray diffraction, scanning electron microscopy, and leaching tests. Results demonstrate that the incorporation of PAC significantly enhances the early strength development of PCFPB, and facilitates the generation of hydration products (ettringite and C-(A)-S-H gels). Meanwhile, the addition of PAC accelerated hydration rates and increased the cumulative heat release of PCFPB specimens. Moreover, the study reveals that PAC plays a crucial role in fluoride S/S through both physical encapsulation and chemical stabilization. These findings provide valuable insights into mitigating PG-related environmental risks, offering a theoretical basis for further advancing the sustainable management of PG.