<p>As the second most important solid waste produced by coal-fired power plants, the improper management of coal-fired slag has the potential to result in environmental pollution. It is therefore imperative that high-value utilization pathways for coal-fired slag should be developed. In this study, modified magnesium slag (MMS), produced by a magnesium smelter, was selected as the alkali activator. The activated silica-aluminum solid wastes, namely coal-fired slag (CFS) and mineral powder (MP), were employed as pozzolanic materials in the preparation of alkali-activated cementitious materials. The alkali-activated cementitious materials prepared with 50 wt% MMS, 40 wt% CFS and 10 wt% MP exhibited favorable mechanical properties, with a compressive strength of 32.804 MPa in the paste sample cured for 28 d. Then, the activated silica-aluminum solid waste consisting of CFS-MP generated a significant amount of C-S(A)-H gels, AFt, and other products, which were observed to occupy the pore structure of the specimen. In addition, the secondary hydration reaction of CFS-MP occurs in high alkalinity environments, resulting in the formation of a mutually stimulated and promoted reaction system between CFS-MP and MMS, this will subsequently accelerate the hydrolysis reaction of MMS. It is important to emphasize that the amount of MMS in alkali-activated cementitious materials must be strictly regulated to avert the potential issue of incomplete depolymerization-repolymerization of active silica-aluminum solid waste containing CFS-MP. This in turn could have a deleterious impact on the late strength of the cementitious materials. The aim of this work is to improve the joint disposal of MMS, CFS and MP and thereby provide a scientific basis for the development of environmentally friendly and low-carbon modified magnesium slag alkali-activated coal-fired slag based cementitious materials for mine backfilling.</p>