Upcycling sulphidic copper tailings into alkali-activated slag materials: effect of the sulfur content.

Sulfidic copper tailings (SCTs) with excessive sulfur content are prone to oxidation, leading to the generation of sulfates and causing compatibility issues with cement. To address this problem, this paper proposes upcycling SCTs into alkali-activated slag (AAS) materials to fully utilize the produced sulfates for slag activation. The influence of the sulfur content of the SCTs compound (quartz, SCTs, and fine pyrite) on the properties of AAS was investigated from various aspects including setting time, compressive strength, hydration products, microstructure, and pore structure. The experimental results showed that adding SCTs compound enabled the generation of S-rich expansive products, such as ettringite, sodium sulfate, and gypsum. Moreover, nano-sized spherical particles were formed and well-distributed in pores or micro-cracks in the microstructure of AAS mortars. Consequently, AAS mortars with SCTs compound developed higher compressive strength at all ages than the blank ones, with an increase of 40.2-144.8% at 3 days, 29.4-115.7% at 7 days, and 29.3-136.3% at 28 days. Furthermore, AAS mortars with SCTs compounds enjoyed significant economic and environmental benefits, as demonstrated by cost-benefit and eco-efficiency analyses. The optimal sulfur content of the SCTs compound was found to be 15%.