To address the environmental pollution caused by industrial solid waste storage and the urgent need for low-carbon transformation in the concrete industry, a synergistic activation via active complementation and graded complementation technical strategy was proposed. This approach overcame the limitations associated with single solid waste admixtures, such as early-age strength reduction and deterioration of the interfacial transition zone. A composite mineral admixture was developed by incorporating coal gasification slag (inert skeletal component), fly ash (source of reactive Al₂O₃), and ground granulated blast-furnace slag (material with latent hydraulic properties) at a mass ratio of 2∶3∶5. Through mechano-chemical synergistic activation, a high-reactivity ultrafine mineral admixture was obtained. C50 concrete mixtures were prepared using cement replacement levels of 0, 10%, 20%, 30%, and 40%. Systematic macroscopic performance evaluations and microscopic mechanism analyses were conducted. Results indicate that the early-age (7 d and 28 d) compressive and flexural strengths decrease with increasing replacement levels. However, at later ages (56 d and 90 d), strength development exhibits an initial increase followed by a decline, with optimal mechanical performance achieved at a 10% replacement level. Durability indicators, including resistance to carbonation and chloride ion penetration, initially improve with higher replacement levels before declining, reaching peak performance at a 30% replacement rate. At the micro level, the incorporation of the three-component solid waste system does not generate new hydration products. Instead, it promotes secondary hydration reactions through the continuous consumption of Ca(OH)₂ to form additional C-S-H gel. This process refines the pore structure and improves the morphology of the interfacial transition zone. The inverse contradiction between replacement level and mechanical performance commonly observed in solid waste-based admixtures is resolved, an optimal replacement range of 10%~30% is identified, and the synergistic strengthening mechanism among multiple solid wastes is revealed. These findings provide a theoretical foundation for the large-scale utilization of industrial solid waste in high-strength concrete applications and support the construction industry’s pursuit of carbon neutrality.