<p>Metals extraction and processing generate a substantial amount of industrial waste, posing significant environmental hazards. To reduce the accumulation of such waste, this paper incorporates steel slag (SS) into a lime (CH)–sodium sulfate (SN) composite-activated cementitious system. The mechanical properties, hydration products, and microstructure of the cementitious system with varying SS contents were characterized. The results indicate that the addition of SS not only enhances the mechanical properties of the cementitious system but also significantly reduces its cost and energy consumption. When the content of SS is lower than 30%, it does not reduce the generation of C-(A)-S-H and ettringite in the cementitious system, and can be used as a micro-filling to make the matrix dense, thus improving the mechanical properties of the cementitious system. However, as the SS content continues to increase, the formation of C-(A)-S-H and ettringite in the cementitious system decreases, leading to an increase in the number of harmful pores in the matrix and a subsequent reduction in the mechanical properties of the system. This study provides important insights into the sustainable development of metal extraction and processing.</p>