Investigation into the individual effects of temperature and sulphate ions on the fracture toughness of waste-based cemented paste backfill containing superplasticizer

Waste-based cemented paste backfill (WCPB) sustains long-term high dynamic loads in goafs, making modes I and II fracture toughness ( K IC and K IIC ) essential indicators for stability assessment. To optimize the transportation performance, superplasticizers are added to enhance slurry fluidity in engineering practice. Concurrently, complex underground environmental factors (variable temperatures and sulphate contents in mine water) significantly affect the fracture toughness and its evolution. Therefore, to comprehensively evaluate the long-term performance of superplasticizer-containing WCPB in the field, this study investigates the evolution of fracture toughness under individual influences of variable temperatures and sulphate concentrations, respectively. Key findings include: (i) except for advanced-age (28 days) specimens cured at 35 °C, fracture toughness generally increases with curing time; (ii) superplasticizers negatively impact the fracture toughness of early-age (≤14 days) samples, but optimal dosages (0.05%, 0.125%) enhance the fracture toughness of advanced-age samples; (iii) higher temperatures improve early-age fracture toughness, yet specimens cured at 35 °C exhibit reduced fracture toughness after 28 days; (iv) sulphate ions inhibit the cement hydration, thereby reducing the K IC and K IIC of early-age samples. The generated ettringite refines particle gaps, enabling the fracture toughness of 2,500 ppm specimens to exceed that of sulphate-free specimens. These findings provide a reference for WCPB mix design in sulfate-bearing mine water environments.