Impact damage and energy-absorption mechanisms of basalt–steel hybrid fibre-reinforced low-carbon shotcrete for underground roadways

Underground roadway linings require sprayed materials that combine reduced embodied carbon, reliable delivery, early load resistance, and stable energy dissipation under rockburst-type impact. This study examined eight shotcrete mixtures: an ordinary Portland cement control, a fibre-free low-carbon matrix containing 30% ground granulated blast-furnace slag and 15% fly ash, two basalt fibre mixtures, two steel-fibre mixtures, and two basalt–steel hybrids. Static, dynamic, imaging, acoustic, mineralogical, and micromechanical measurements from three independent batches were integrated through mixed-effects models. The hybrid containing 0.10 vol.% basalt fibre (BF) and 1.00 vol.% steel fibre (SF) reached a 28-day compressive strength of 63.1 MPa, a flexural strength of 9.90 MPa, a fracture energy of 523 N/m, and a specific absorbed energy of 3.05 MJ/m³ at 80 s⁻¹. Its cumulative drop-weight absorbed energy to through-cracking was 2.31 kJ, compared with 0.56 kJ for the fibre-free low-carbon mixture. Micro-computed tomography showed that hybridisation reduced crack volume from 4.20% to 1.35% and connectivity from 0.773 to 0.356. The mixture retained a 29.4% material-stage carbon reduction relative to ordinary shotcrete. The multitechnique observations were consistent with, but did not directly prove, a proposed sequence in which BFs restrain distributed microcracks and hooked SFs sustain widening macrocracks. LC denotes lowcarbon; LCH1 and LCH2 denote lowcarbon hybrid mixtures 1 and 2, respectively. LC-H1 is recommended where delivery tolerance governs, whereas LC-H2 is preferred for severe impact demand under the tested robotic spraying conditions, subject to full-scale field validation.