Mechanical behavior and failure mechanisms of waste rock–phosphogypsum cemented backfill

This study investigates the mechanical behavior and failure mechanisms of cemented backfill composed of waste rock and phosphogypsum. Slump tests were conducted on slurries containing fine-grained waste rock (Type A aggregate, 0–3 mm), coarse-grained waste rock (Type B aggregate, 3–5 mm), and varying proportions of phosphogypsum to determine the optimal mix concentration. Uniaxial compression tests, scanning electron microscopy (SEM), and numerical simulations were then employed to examine stress–strain responses, acoustic emission characteristics, energy evolution, and fracture development. The results demonstrate that phosphogypsum addition effectively improves slurry workability. At a mass concentration of 78%, all slurry mixtures exhibited slump values within the favorable pipeline transport range of 27.5–28.5 cm. For the same phosphogypsum content, Type B aggregate specimens exhibited peak strengths 144%–183% higher and elastic moduli 125%–245% higher than those of Type A specimens. As phosphogypsum content increased from 30% to 50%, the peak strength and elastic modulus of both aggregate types decreased by 74%–76% and 65%–76%, respectively. Input energy density and elastic energy density increased with larger aggregate particle size but decreased with higher phosphogypsum content, whereas dissipated energy density showed the opposite trend. Type A specimens predominantly failed in tension, whereas Type B specimens transitioned from tensile to mixed tensile–shear failure as phosphogypsum content decreased. For both aggregate types, the ratio of tensile to shear cracks decreased with declining phosphogypsum content. These findings provide new insights into optimizing waste rock–phosphogypsum backfill design within the tested ranges.