Strength enhancement and failure mechanisms of polypropylene Fibre-Reinforced cemented backfills for underground load-bearing and support

• A fractal theory framework quantified crack patterns and pore distributions in cemented backfills under multi-factor coupling. • Comparative analysis revealed distinct mechanical behaviors of cemented paste and fibre-reinforced backfills, with optimal mix ratios determined for different curing ages.a • Fibre addition effectively redistributed stresses and transformed brittle tensile cracks into tensile–shear mixed fractures. • Quantitative characterization of hydration products clarified the cooperative mechanism between fibres and hydration gels. • The integrated micro–macro investigation deepens failure mechanism insights and provides guidance for safer, more reliable mine backfill design. Cemented paste backfill (CPB) made from waste mine materials is essential for goaf management and underground support but often faces instability due to mining activities. To enhance CPB stability, an orthogonal experifigment was conducted to analyze the mechanical properties of CPB and fibre-reinforced backfill (FRB) under various conditions. Uniaxial compression tests and digital image correlation (DIC) tests were performed on optimally proportioned samples at different curing times, and the damage mechanisms were further revealed through microscopic experiments. The findings indicate that fibres, as reinforcement materials, create a mesh structure that reduces slurry fluidity while enhancing the uniaxial compressive strength (UCS) of CPB. The improvement in UCS aligns with cement hydration patterns, and the fractal dimension of FRB shows a consistent relationship with UCS. Fibres inhibit microcrack formation during curing, optimize pore size distribution, and enhance peak strength. They also alleviate stress concentration in CPB, preventing crack propagation and premature failure. Additionally, the content of hydration products significantly influences CPB's mechanical properties over time. Fibres bond with hydration products, absorb fracture energy through friction, and are pulled from the matrix during loading. This study provides valuable insights for improving CPB strength and ensuring safer, more reliable load-bearing and support performance in underground mining operations.