Coupled freeze-thaw and sulfate attack on coal gangue-based cemented backfill: pore evolution and damage modeling

To address the durability degradation of cemented backfill materials under coupled freeze-thaw and sulfate attack in cold-arid mining areas, this study prepared coal gangue paste backfill specimens. Alternating experiments (50 freeze-thaw cycles and 50 days of sulfate erosion) were conducted on control (C0) and coupled (FS) groups, and the mass loss rate, mechanical properties, dynamic elastic modulus, and pore structure evolution were systematically analyzed. The results indicated that: under coupled conditions, the mass loss rate increased exponentially with cycles, reaching 8.57-10.18% after 50 cycles; the compressive and flexural strength loss rates (73.2-98.6% and 70.8-94.7%, respectively) were significantly higher than those under single-factor conditions, exhibiting three-stage attenuation characteristics. Mercury intrusion porosimetry showed pore coarsening: the proportion of harmful pores (> 1 μm) increased from 5-15 to 22-26%, and the median pore diameter expanded from 0.18 μm to 0.53 μm. A 'pore-mechanics coupled damage model' (R² = 0.94) was established to quantify the synergistic effects of harmful pores and cycles. The high-cementitious ratio group (3:1, FS-P2) effectively inhibited pore expansion, with harmful pores at only 15% after 50 cycles and strength retention improved by 15.4-29.0% relative to other groups. This research elucidates the chain mechanism of freeze-thaw-sulfate coupled damage and establishes a pore-threshold-based design method for backfill durability, providing theoretical support for material optimization and engineering applications in cold-arid mining regions.