Coupled Thermochemical Effects on the Strength Development of Slag-Paste Backfill Materials

In this paper, the coupled effects of sulfate (chemical factor) and curing temperature (thermal factor) on the strength development of cemented paste backfill (slag-CPB) that contains ground granulated blast furnace slag (slag) as a mineral admixture is studied. Almost 200 slag-CPB samples, cured at temperatures of 2, 20, 35, and 50°C at 28, 90, and 150 days with sulfate concentrations of zero ppm, 5,000 ppm, 15,000 ppm, and 25,000 ppm are tested for uniaxial compressive strength (UCS). The results show that the effect of sulfate on the strength of slag-CPB is significantly dependent on the curing temperature, initial sulfate content, and curing time. The coupled effects of sulfate and temperature can lead to an increase or decrease of the slag-CPB strength and significantly influence the type and amount of minerals formed within the cemented matrix of slag-CPBs. The strength-increasing factors are (1) refinement of the pore structure of slag-CPBs due to the precipitation of ettringite within the empty pores of the cemented matrix; (2) faster cement hydration rate and pozzolanic reaction with higher curing temperatures; and (3) activation of the slag reaction by sulfate ions. The identified strength-decreasing factors include (1) sulfate absorption by calcium silicate hydrate (C-S-H) that could lead to the formation of weaker C-S-H gel; (2) increase of ettringite dissolution as the curing temperature increases that results in the coarsening of the pore structure; (3) inhibition of cement hydration by sulfate; and (4) formation of hydration rims around the slag grains at high temperatures. There is competition between the strength-decreasing and increasing factors. The dominant influencing factors depend on the curing temperature, curing time, and initial sulfate content. A comparison of the performance of slag-CPBs and portland cement CPBs shows that a partial replacement of ordinary portland cement with slag improves the resistance of the studied cemented paste backfill at advanced ages and curing temperatures of 2°C and 20°C, while negatively affecting resistance for curing temperatures that are ≥35°C. This study has demonstrated that the coupled effects of sulfate and temperature on the strength of cemented paste backfill are an important factor that should be considered in the designing of cost-effective, safe, and durable cemented paste backfill structures.