Grouting materials are essential for trenchless road repair. However, conventional cement-based grouting materials suffer from considerable shrinkage and low early-age strength. To address these challenges, this study utilizes industrial solid wastes (phosphogypsum and slag) for the synergistic synthesis of a phosphogypsum–slag-based geopolymer (PBG). Using PBG as a binder and fine sand as an aggregate, a sustainable grouting material was developed. The influence of binder-to-sand and water-to-solid ratios on PBG workability was systematically evaluated, identifying the optimal water-to-solid ratio. Based on this, the effects of the binder-to-sand ratio on mechanical strength at various curing ages, durability, and leaching of toxic substances were analyzed. The mechanism of strength development mechanism and immobilization behavior of toxic substances were revealed through SEM. The results indicate that the material exhibits excellent performance when the water-to-solid ratio is 0.28 and the binder-to-sand ratio ranges from 0.70 to 0.75. The material exhibits fluidity of 160–240 mm, initial setting time > 30 min, and final setting time < 400 min, a bleeding rate < 0.4%, and 28-day compressive strength ≥ 9.0 MPa. Both the impermeability and freeze–thaw resistance of PBG grouting material improve with a higher binder-to-sand ratio. Toxic substance leaching complies with Class III groundwater quality standards. Carbon footprint analysis indicates that the material significantly reduces carbon emissions.

The large-scale and high-value utilization of industrial solid waste has become a key research area in sustainable building materials. However, ensuring effective backfilling quality in narrow or irregular spaces remains challenging in civil engineering. Developing flowable solidification materials from industrial solid waste not only resolves issues inherent in traditional backfilling techniques but also enhances efficient resource utilization. In this study, phosphogypsum was used to prepare geopolymers, which served as binders replacing cement in producing phosphogypsum-based fluidized solidified soil (PFSS). The workability, mechanical strength, and toxic substance leaching of PFSS were evaluated. Moreover, the underlying mechanisms of strength formation and toxic substance immobilization were investigated. The optimal PFSS composition was determined to have a water-to-solid ratio of 0.48–0.50 and a geopolymer content of 12–18% (by mass). Under these conditions, the material exhibited fluidity ranging from 160 to 220 mm, a 28-day compressive strength of 0.86 MPa, a California Bearing Ratio (CBR) of 8%, and a resilient modulus of 40 MPa. These parameters satisfy the performance standards required for backfilling in high-grade highways. The leaching concentrations of heavy metals (As, Pb, and Cr) complied with China’s Class III groundwater quality standards. Microstructural analyses indicated the occurrence of hydration, pozzolanic reactions, geopolymerization, and carbonation. Microstructural analyses indicated the formation of an interlocking three-dimensional network, composed of C-S-H, C-A-S-H gels, and ettringite (AFt), which contributes significantly to the strength development and immobilization of heavy metals. These products collectively formed an interlocking three-dimensional network structure, significantly contributing to PFSS strength development. Heavy metals were effectively immobilized within the matrix due to the combined effects of physical adsorption and chemical bonding.