Sulphate effect on the early age strength and self-desiccation of cemented paste backfill

• Sulphate can significantly decrease the strength gain rate of tailings backfill at early ages.
• Sulphate reduces the self-desiccation of Portland cement – cemented backfill.
• Sulphate content of tailings backfill is important for the barricades stability.
• Sulphate content of tailings backfill is important for mining cycle.

This paper presents the results of an experimental study on the sulphate effect on the early age (1, 3, 7 and 28 days) strength and self-desiccation of cemented paste backfill (CPB). The CPB specimens have an initial sulphate concentration of 0, 5000, 15,000 and 25,000 ppm and are cured at room temperature (20 °C). Tests on the mechanical properties were conducted on the CPB, while microstructural analyses were performed on the CPB and cement paste samples. Suction and electrical conductivity monitoring were also performed on the CPB specimens with different sulphate contents. The results show that sulphate has a significant effect on the early age strength and self-desiccation of CPB. In the early ages, sulphate can have negative effects, i.e., leads to a decrease in the CPB strength and reduction in the intensity and rate of self-desiccation within the CPB. The magnitude of these effects depends on the initial sulphate concentration. The inhibition of cement hydration by sulphate ions is a key reason for the observed decrease in CPB strength and self-desiccation intensity or rate. Ettringite formation, changes in the pore structure and sulphate absorption by C-S-H are found as additional negative factors that affect CPB strength. This study has demonstrated that the effect of sulphate on the early strength and self-desiccation of CPB is an important factor for consideration in the designing of cost-effective, safe and durable CPB structures as well as for reducing the mining cycle time; in other words, increasing mining productivity.