Abstract: Sludge particles are fine and have a high water content. The strength of solidified sludge is predominantly provided by cementation strength, with insufficient frictional resistance. Phosphogypsum-based aggregates were synthesized using phosphogypsum, slag, and quicklime as raw materials. These aggregates were combined with cement for sludge solidification, and the physical and mechanical properties of the solidified sludge were assessed. Results show that the phosphogypsum-based aggregates have a water absorption rate of 13.4%, which locally separates free water in the sludge, enhancing cement solidification. After compaction, the leaching concentrations of soluble phosphorus (P) and fluorine (F) in the phosphogypsum are significantly reduced compared to the raw phosphogypsum. Both concentrations are below the discharge standards for class III surface water (the leaching concentration of P is ≤ 0.2 mg/L, the leaching concentration of F is ≤ 1.0 mg/L), meeting environmental protection requirements. When the phosphogypsum aggregates are ≤5 mm, their uniform distribution enhances the skeletal effect, significantly increasing the strength of solidified sludge by 22.3%, exceeding 1 MPa. This occurs because reactions among phosphogypsum, slag powder, and lime generate calcium silicate hydrate (C-S-H) gel and acicular ettringite. These products intertwine to form a spatial network structure, filling the pores inside the phosphogypsum-based aggregates and the transition zone. This immobilizes soluble phosphorus and fluorine and strengthens the bonding capacity of the aggregate-sludge interface transition zone. The research conclusions provide a reference for the comprehensive utilization of phosphogypsum and offer a novel approach to sludge solidification.

Key words: solidified sludge, phosphogypsum-based aggregate, skeleton effect, interfacial cementation