Alkali-Activated CO₂-Capturing Backfill Material for Coal Mining Incorporating Municipal Solid Waste Incinerated Bottom Ash

The ability of backfill materials to capture CO₂ in coal goaf would reduce atmospheric CO₂ concentrations and replace mining cement-based backfill materials to offset the considerable global CO₂ emissions of ordinary Portland cement production. This study investigated the feasibility of integrating CO₂ capture and backfill in coal goaf with municipal solid waste incinerated bottom ash (MSWI-BA) and granulated blast furnace slag (GBFS). The CO₂ capture ability and its effect on the mechanical strength and microstructure of alkali-activated materials incorporating MSWI-BA and GBFS were studied. Carbonation depth increased with increasing MSWI-BA incorporation. After 6 h of carbonation at 65 ± 2% relative humidity and 30% CO₂, alkali-activated material incorporating 40% MSWI-BA exhibited a CO₂ uptake of 4.46 wt %. However, compressive strength decreased following carbonation, yet the alkali-activated material sample with the highest CO₂ uptake showed 7-day, 14-day, and 28-day compressive strengths of 19.6, 20.3, and 19.7 MPa, respectively. While the surface areas and total pore volumes of carbonated samples decreased due to the formation of carbonates filling the pores, the porosity of carbonated samples incorporated with 10, 30, and 50% MSWI-BA decreased by 6.5, 14.2, and 22.2%, respectively, indicating that the carbonated MSWI-BA-incorporated sample still maintained the structural stability. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results revealed that increasing the MSWI-BA incorporation proportion resulted in more carbonates being generated. The CO₂ emission index was at least 51.9% lower than ordinary Portland cement (OPC), indicating that alkali-activated materials incorporating MSWI-BA can serve as a promising green, low-carbon emission material for backfilling coal goaf to mitigate rock burst disasters and capturing CO₂.