Abstract: Ordinary Portland cement is the most widely used cementitious material in tailings cementitious backfill, but its production causes severe environmental pollution. The alkali-activated slag-steel slag cementitious system exhibits excellent mechanical properties and workability, making it a potential substitute for ordinary Portland cement. To clarify the macro- and micro-characteristics of the slag-steel slag cementitious system under the action of a composite activator, lime-sodium carbonate was used as a compound activator to activate the slag-steel slag cementitious system, producing a low-carbon and environmentally friendly alkali-activated slag-steel slag cementitious material. The effects of the composite activator dosage and steel slag content on the setting time and compressive strength of the cementitious system were investigated. The hydration characteristics and microstructure of the slag-steel slag cementitious system under the lime-sodium carbonate composite activator were characterized through XRD, FTIR, SEM, and other testing methods.The results show that the setting time of the lime-sodium carbonate composite-activated slag-steel slag cementitious system was within 255 minutes and was not significantly affected by the steel slag content. The 3-day and 28-day compressive strengths of the cementitious system could reach 23.0 MPa and 30.4 MPa, respectively. The steel slag content did not alter the types of hydration products, which mainly consisted of C−(A)−S−H gel, hydrotalcite, and calcite. Increasing the steel slag content slowed down the hydration heat release rate, reduced the cumulative hydration heat, and promoted later-stage hydration, improving the proportion of harmless pores in the matrix. At a 20% steel slag content, the formation of C−(A)−S−H gel was the highest. However, when the steel slag content exceeded 40%, the amount of C−(A)−S−H gel continued to decrease with increasing steel slag content, microcracks appeared in the matrix, and the compressive strength declined. This study provides a reference for the preparation of novel low-carbon cementitious materials and the resource utilization of steel slag.