Durability Evolution of Coal Gangue Sand Cemented Paste Backfill under Combined Freeze-Thaw Cycles and Sulfate Attack

To address the durability degradation of cemented filling materials under the coupled effects of freeze-thaw cycles and sulfate attack in cold-arid mining areas, this study prepared coal gangue paste backfill specimens and conducted alternating experiments (50 freeze-thaw cycles and 50 days of sulfate erosion) on control (C0) and coupled (FS) groups. 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 revealed pore coarsening, with the proportion of harmful pores (> 1 µm) increasing from 5%-15–22%-26%, and the median pore diameter expanding 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 (3:1, FS-P2 group) effectively inhibited pore expansion, with harmful pores accounting for only 15% after 50 cycles, and strength retention rates improved by 15.4%-29.0% compared 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.