Failure behavior of rock and steel slag cemented paste backfill composite structures under uniaxial compression: Effects of interface angle and steel slag content

• Incorporating steel slag into cemented paste backfill improves plastic behavior and reduces brittleness, making the stress-strain response smoother as the slag content increases. • At an interface angle of 45°, the composite exhibits a bimodal stress-strain curve and failure shifts from Y-shaped fractures to interface and axial splitting. • Steel slag reduces hydration products (e.g., Ca(OH)2) and increases harmful pores, weakening bond strength; thus, the recommended slag content should not exceed 45%. • Increasing the interface angle lowers the strength of rock-backfill composites, and the critical interface slip angle first decreases then increases with the ES/ER ratio.