Resistance Mechanism of Hybrid Fiber-Reinforced Tailing Mortar under Freeze–Thaw and Sulfate Attack

The continuous increase in the emissions of iron ore tailings (IOTs) has caused serious pollution to the environment and raised costly IOT management costs. For protecting the environment and decreasing the economic cost, the fiber-reinforced tailing mortar was prepared with 50% IOTs, polyacrylonitrile fiber (PANF), and basalt fiber (BF). The mass loss, porosity, ultrasonic velocity, mechanical strength, and pore structure of tailing mortar after different treatments, including 100 freeze–thaw (FT) cycles and 90 sulfate attack (SA) cycles were analyzed to investigate the effect of PANF and BF on the durability. The test results indicated that BF and PANF have great bridging effects and cracking resistance to improve the durability properties. The B0.25P0.05 (BF: 0.25% and PANF: 0.05%) specimens possess the minimum mechanical strength loss, particularly the uniaxial compressive strength (UCS) only decreased by 5.58% after 100 FT cycles and 2.73% after 90 SA cycles. The B0.25P0.05 specimens also have the smallest dynamic elastic modulus loss, and the reduction was only 7.24% after 100 FT cycles and 0.76% after 90 SA cycles. For 120-day long-term tests, the B0.25P0.05 specimens have the maximum increment in the UCS (22.04%) and dynamic elastic modulus (5.60%). Besides, the mechanical strength of tailing mortar undergoing FT and SA cycles can be predicted accurately by dynamic elastic modulus (prediction error ≤5%). From a microscopic view, the nuclear magnetic resonance (NMR) test demonstrated that fibers significantly decrease the large pores and improve the pore microstructure, and thus raise the mechanical properties. Finally, the ranking of durability is B0.25P0.05 > B1.00P0 > B0P0.08 > B0P0. The hybrid fibers of rigid BF (6 mm) and flexible PANF (12 mm) in tailing mortar cross-climb to generate a denser network structure. Besides, BF and PANF in the mortars can effectively fill the pores and bear expansion stress caused by FT and SA cycles. Therefore, these effects remarkably limit the deformation with different sizes.