Experimental study on evolutive fracture behavior and properties of sulfate-rich fiber-reinforced cemented paste backfill under pure mode-I, mode-II, and mode-III loadings

Tensile and shear fracture behaviors of fiber-reinforced cemented paste backfill (FRCPB) play critical roles in the safe and cost-effective engineering design of underground mine backfill structures. Sulfate pore solution is a key factor affecting the time-dependent evolution of fracture behavior and properties of FRCPB. However, no studies have been carried out to systematically investigate its effects on the fracture behavior and properties of mine backfill used in underground mines. The objective of this study is to discover the effect of sulfate concentration (0, 5000, 15,000, and 25,000 ppm) on FRCPB's fracture behavior and properties at different curing times (3, 7, 28, and 90 days) and under pure mode-I, mode-II, and mode-III loadings. The results show that, except for the 3-day FRCPB, the increase in sulfate concentration can continuously improve pre-peak stiffness and peak fracture resistance force and significantly enhance the post-peak residual resistance, especially under mode-III loading. Moreover, fracture energy shows a higher sensitivity to the sulfate concentration with respect to fracture toughness and stiffness. Furthermore, sulfate concentration has a very limited effect on tensile crack trajectory under mode I loading, while a transition of shear cracks from a curved crack pattern to a sharp and straight one is detected under mode II and III loadings. In addition, the fracture toughness possesses a stronger correlation with material stiffness and thus is recommended for the design of FRCPB to offer a more effective immediate support to surrounding rock. The obtained results provide an in-depth insight into the sulfate-induced evolution of fracture behavior and properties of FRCPB under various loading conditions and thereby promote the safe implementation of mine backfill technology.