Abstract: To reduce the carbon footprint of the cement industry and recycle solid waste such as coal gangue and slag, this paper incorporates slag as a calcium source into alkali-activated coal gangue slurry to prepare alkali-activated coal gangue-slag (AACGS) cementitious materials. The effects of four factors—slag content, liquid-to-solid ratio, modulus of alkali activator, and alkali equivalent—on the setting time, fluidity, and compressive strength of AACGS specimens were studied to determine the optimal mix proportions. Additionally, the influence of slag content on the drying shrinkage and capillary water absorption properties of AACGS specimens was investigated. Furthermore, isothermal calorimetry (TAM Air), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) were employed to analyze the hydration heat, phase composition, pore structure characteristics, and micromorphology of AACGS specimens. The results indicate that AACGS gel materials exhibit optimal overall performance when the liquid-to-solid ratio is 0.32, the modulus of the alkali activator is 1.3, and the alkali equivalent is 14%. Increased slag content enhances the fluidity and compressive strength of AACGS slurry, improves capillary water absorption resistance, increases hydration heat, accelerates setting time, and refines the pore structure, but it also leads to greater drying shrinkage. When the slag content is 30%, the compressive strength of AACGS cementitious materials meets the P.O 42.5 cement standard, demonstrating significant potential as a replacement for Portland cement. This has important implications for the resource utilization of solid waste and the development of environmentally friendly building materials.