AbstractCopper slag (CS) is a by-product generated during the process of copper metal smelting. The influence of CS on the mechanical performance, hydrate assemblage, and mechanism of the cementitious system was investigated by multiple methodologies to accelerate the sustainable development of the copper industries. The results revealed that adding 10% by weight of CS was beneficial to developing compressive strength. However, the strength decreased significantly if too much CS was incorporated. This adverse effect can be mitigated by prolonging the curing age. Besides, the evolution of mechanical performance was explained by multiple microscopic studies. More ettringite (AFt) and monosulfate (AFm) were formed in the paste containing 10% by weight of CS at 28 days, which filled the space and improved the strength. For the paste containing 30% by weight of CS, the backscattered electron images suggested that more than 60% of incorporated CS participated in the hydration at 200 days and developed an additional C─ (A)─ S─ H with a higher atomic ratio of Fe/Si and Al/Si, resulting in a dramatic improvement in compressive strength. Finally, the hydration mechanism of CS in the cementitious system was established based on the dissolution test combined with the theory of cement hydration.