Sulfate-Dependent Shear Behavior of Cementing Fiber-Reinforced Tailings and Rock

A better understanding of the shear behavior of the interface between cemented paste backfill (CPB) and the surrounding rock is critical for constructing cost-effective, durable, and reliable CPB structures. In practice, CPBs suffer sulfate attack during their service life, and as a typical cementitious material, the CPB itself has disadvantages, such as high brittleness, easy cracking, and insufficient durability, which restrict the further popularization and application of CPB technology. Thus, in this study, direct shear tests, electrical conductivity (EC) and thermal gravity/differential thermal gravimetric (TG/DTG) analyses were conducted to research the effects of different amounts of monofilament polypropylene fibers (0%, 0.1%, 0.3%, and 0.5%; by mass of the sum of the dry tailings and cement) and initial sulfate concentrations (0 mg/L, 5000 mg/L and 25,000 mg/L) on the shear behavior of the fiber-reinforced CPBs and rock (FR-CPB/rock) interface, and the Mohr-Coulomb shear envelop was used to fit the shear strength of specimens with various periods (1 day, 3 days, 7 days, and 28 days) under various stresses (50 kPa, 100 kPa, 150 kPa). The experimental testing results indicated that the fibers generally enhance the performance of the shear behavior of the FR-CPB/rock interface and the optimal fiber content correlates to the initial sulfate concentration. For the same treatment time (7 days), a fiber content of 0.1% contributes to the best shear performance for the FR-CPB/rock interface with a sulfate concentration of 5000 mg/L. For the sulfate-free and 25,000 mg/L concentration specimens, 0.3% is the optimal fiber content. Furthermore, for the studied interface specimens, sulfate content can play a positive (the refinement of the pore structure) or negative (the sulfate retardation effect) role in the interface shear behavior between the FR-CPB and rock, depending on the treatment time, the initial sulfate concentration, and the fiber content. For the specimens treated for 7 days and 28 days, the specimens with initial sulfate concentrations of 5000 mg/L and 25,000 mg/L achieved the highest peak shear strengths, respectively. The outcomes of this paper present a substantial reference for the design and optimization of underground FR-CPB structures under sulfate attack.