Early-age strength evolution and brittle-to-ductile transition mechanism of basalt-fiber-reinforced cemented gangue backfill

High brittleness and rapid post-peak capacity loss are often observed in cemented gangue backfill (CGB) under uniaxial compression. The risk of sudden instability in deep mining panels is therefore increased. Chopped basalt fibers (0-0.60 wt%) were incorporated to improve early-age load capacity and ductility, and curing ages of 3–60 d were investigated. Uniaxial compression tests were performed. Crack evolution was interpreted mainly through Acoustic emission (AE) characteristics and surface failure observations, with digital image correlation DIC serving as an auxiliary qualitative tool. Scanning electron microscopy and discrete element simulations were used to clarify multiscale toughening mechanisms. An optimal dosage was identified at 0.3 wt%. At 28 d, uniaxial compressive strength (UCS) and peak strain were increased by 67.2% and 37.0%, reaching 5.71 MPa and 3.7%. A rapid strength-gain window occurred from 3 d to 7 d. For the 0.3 wt% mixture, UCS rose from 0.74 MPa to 3.23 MPa (436.49%), supporting mining initiation at approximately 7 d after backfilling. At 7 d, tensile cracks dominated the reference mixture (82.6%). After fiber addition, crack deflection and branching were promoted and shear sliding was activated. At 0.3 wt%, the shear-associated crack proportion increased markedly, while crack deflection and branching were promoted. Failure mode evolved from concentrated longitudinal splitting toward a more distributed oblique mixed-mode crack network, and post-peak softening was mitigated. A dense C-S-H-rich hydration layer was formed in situ on fiber surfaces, creating a fiber-hydration product-matrix interface. Strength and damage tolerance were jointly improved through bridging-based load transfer, stress redistribution, and crack-tip blunting. The results clarify the early-age strengthening and quasi-brittle toughening mechanism of basalt-fiber-reinforced cemented gangue backfill and provide a basis for mixture optimization and early-age support design.