Effects of Functional Auxiliary Components on the Performance and CO2 Mineralization Response of CGS–GGBS-Based Backfill Binders

Coal-based solid-waste binders provide a promising route for integrating mine backfilling with CO2 mineralization, but auxiliary components can affect flowability, strength, and mineralization response differently. In this study, coal gasification slag (CGS) and ground granulated blast-furnace slag (GGBS) were used as the base binder, while soda residue, carbide slag, phosphogypsum, desulfurization gypsum, and red mud were introduced as functionally distinct auxiliary components. The binders were subjected to CO2 injection mixing, and their flowability, compressive strength, apparent CO2 uptake, reaction products, and pore-related characteristics were evaluated. The response to CO2 treatment depended strongly on the auxiliary–component combination. CO2 injection mixing reduced the early-age strength of most formulations. In contrast, the carbide slag–red mud formulation, CGS9, showed favorable compatibility between cementitious reactions and mineralization. Its 3 d and 7 d strengths increased by 36.0% and 14.4%, respectively, while its 28 d strength remained nearly unchanged. Its apparent CO2 uptake and carbonation degree reached 3.825% and 13.46%, respectively. XRD showed stronger calcite diffraction peaks after CO2 injection mixing, while FTIR showed enhanced carbonate absorption bands. SEM-EDS and LF-NMR indicated matrix densification and refinement of the water-filled pore environment. These findings show that functionally distinct auxiliary solid wastes can help coordinate cementitious reactions and CO2 mineralization, providing a feasible route for producing low-carbon mine backfill materials from coal-based solid wastes.