Development of Mode I and Mode II fracture toughness of cemented paste backfill: Experimental results of the effect of mix proportion, temperature and chemistry of the pore water

Abstract

Despite the comprehensive understanding of the development of the modes I & II fracture toughness of brittle materials, the evolution of the fracture toughness of cemented paste backfill (CPB) under different conditions remains unknown. Given the importance of modes I & II fracture toughness in analyzing the fracture initiation and propagation, many semi-circular CPB samples (with different water-cement (w/c = 6.5, 7.35, 8) ratios, cement contents (3%, 4.5%, 6%), curing temperatures (2 °C, 20 °C, 35 °C) and sulphate contents (0 ppm, 5000 ppm, 15000 ppm, 25000 ppm)) are prepared and subjected to semi-circular bending (SCB) test after being cured for 3 days, 7, 14, and 28 days. Irrespective of the mixing recipe or the curing conditions, all samples for mode I fracture toughness measurement generally experience a sudden failure, while the cracks in the samples for measuring the mode II fracture toughness propagate gradually. The obtained results show that a decrease of w/c ratio contributes to the modes I & II fracture toughness acquisition. Besides, owing to the acceleration effect of temperature on the cement hydration and the self-desiccation, the modes I & II fracture toughness of CPB increases with temperature as well. However, the effects of sulphate ions on the growth of the modes I & II fracture toughness of CPB depend on the initial sulphate content and the curing age. In other words, there is a competition effect between the factors contributing to the fracture toughness increase (refinement of the capillary structure by cement hydration products) and decrease (inhibition of the cement hydration by sulphate ions). The findings of this study provide critical information in the stability assessment of CPB structures, which is beneficial to the improvement of the mining safety.