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Electrolytic manganese residue (EMR) is a solid waste generated during the production of electrolytic manganese metal through wet metallurgy, accumulating in large quantities and causing significant environment pollution. Due to its high sulfate content, EMR can be utilized to prepare supersulfate cement when combined with Ground Granulated Blast furnace Slag (GGBS). In this process, GGBS serves as the primary raw material, EMR acts as the sulfate activator, and CaO powder, along with trace amounts of cement, functions as the alkali activator. This results in the preparation of CaO-modified electrolytic manganese residue-based supersulfate cement (Abbreviated as “SSC”), facilitating the harmless and resourceful utilization of EMR. This study aims to determine the optimal dosage of CaO as the alkali activator for GGBS in SSC. A comprehensive analysis was conducted on four groups, including a control group. The mass ratio of EMR, GGBS, and cement in SSC was fixed as 35:60:5, and the optimum mixing ratio of lime powder as an external admixture was investigated through mechanical tests and microscopic experiments. The hydration products and mechanism of the cementitious materials were analyzed using X-ray diffraction (XRD), pH measurements, thermogravimetric and differential thermogravimetric analysis (TG-DTG), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). The results indicated that, under the combined influence of trace cement and raw lime powder, EMR effectively activated GGBS. The primary hydration products of the SSC are AFt and hydrated calcium silicate (C-S-H), which contributed to the mechanical strength of the SSC. At a hydration age of 3 days, the optimal CaO blending ratio was found to be 8% by mass of dried EMR. With this ratio, the compressive strength of SSC reached 18.2 MPa, the pore size of hardened slurry was refined, the structure became dense, and hydration products increased. It could be concluded that CaO enhances the early strength of SSC when used as an alkali activator. Full article