Mechanical properties of phosphogypsum-recycled soil under varying dosages and curing times

Soil shear failure is a primary cause of slope instability. With insufficient shear resistance, the detached soil not only undermines slope integrity but also creates safety risks for roadway infrastructure. Phosphogypsum, a solid waste generated from the production of wet-process phosphoric acid, poses environmental risks if landfilled. However, its cementing properties could be employed to enhance the shear strength of the slope soils, enabling sustainable reuse of this by-product. In this study, the effects of phosphogypsum dosage and curing time on the mechanical properties of phosphogypsum-recycled soil (PRS) were investigated by direct shear tests. Results indicate that phosphogypsum induces a transition in the stress–strain response from strain softening to strain hardening. Shear strength, cohesion, and the internal friction angle increased with curing time. Using two gradient-boosted decision tree models (GBDT-C and GBDT-IFA) for variable importance analysis, phosphogypsum dosage was identified as the dominant factor, accounting for 81.9% and 57.0% of the feature importance for cohesion and friction angle, respectively. This is consistent with the results of the two-way ANOVA. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) revealed that the PRS strength was increased by rod-shaped CaSO 4 ·2H 2 O crystals filling the voids and forming a network between the soil particles. The pore size distribution shifted from a bimodal pattern dominated by macropores to a unimodal pattern dominated by capillary pores. The median pore size and most probable pore size gradually decreased before the phosphogypsum dosage reached 25%. This indicates that phosphogypsum addition optimized the pore structure, facilitating more uniform stress distribution within the soil. This research demonstrates that adding phosphogypsum and extending the curing time effectively enhances the shear properties of slope protection soils, providing a viable solution for high-value applications of phosphogypsum in slope protection.