Effects of curing time, cement content, and saturation state on mode-I fracture toughness of cemented paste backfill

Due to the wide existence of tensile stresses in cemented paste backfill (CPB) mode-I fracture toughness (KIC) plays a crucial role in the mechanical stability of CPB under complex field loading conditions. However, the studies on KIC of CPB materials and associated influential factors are very limited. Hence, the main objective of this study is to experimentally investigate the effects of curing time (3, 7, 28, and 90 days), cement content (2%, 4.5%, and 7%) and saturation state (unsaturated and saturated states) on the evolution of KIC through single-edge-notch bending tests on CPB specimens. The obtained results show that KIC displays a strongly time-dependent and nonlinear evolution with curing time, especially during the early age. Moreover, the development of KIC is sensitive to changes in cement content, which can be attributed to the precipitation of cement hydration products as well as the self-densification process in CPB. In addition, through the vacuum-based rapid re-saturation approach proposed in this study, it has been concluded that the re-saturated CPB has a lower KIC compared with the CPB without resaturation treatment and thus confirm the dependence of KIC on the saturation state.