• Adding recycled steel fibers (RSF) to cemented-gangue–fly-ash backfill (CGFB) reduces uniaxial compressive strength by 3.86%–15.59% but significantly increases toughness by 69%–123%.
• Acoustic emission and energy evolution analyses revealed that RSF addition reduces post-peak stress energy release and requires higher external energy for failure, improving crack resistance.
• An optimal RSF content of 0.5% is recommended for engineering applications due to its excellent damage resistance and overall mechanical performance.
• SEM analysis showed that hydration products embed in RSF damaged areas, increasing interfacial friction and interlocking, which strengthens the bridging effect and enhances durability.