Comprehensive Evaluation of Coal-Fired Bottom Slag as a Precursor for Alkali-Activated Materials

Bottom slag (BS), a coal combustion by-product, is an underutilized aluminosilicate resource with potential for alkali activation. In this study, the physicochemical characteristics of BS, including its chemical composition, mineralogy, morphology, and activity index, were systematically evaluated, and ground BS was blended with ground granulated blast-furnace slag (GGBFS) to prepare alkali-activated binders. The effects of BS content on mechanical properties, reaction products, and microstructural evolution were investigated using compressive strength tests and multi-scale characterization techniques. The results showed that BS contains abundant amorphous aluminosilicate phases with low residual carbon and exhibits considerable latent cementitious activity despite its relatively low intrinsic reactivity. The mechanical performance of the binders was strongly governed by precursor composition. Incorporating 20–40 wt.% BS achieved the optimum balance between precursor reactivity and strength development, whereas higher BS contents significantly inhibited alkali activation reaction because of the dilution of reactive glassy phases. Strength development was closely associated with the formation of C-(A)-S-H and N-A-S-H gels and the development of a dense microstructure. These results demonstrate that the synergistic activation of BS and GGBFS provides an effective route for producing low-carbon alkali-activated binders while enabling the high-value utilization of coal-fired BS.