Effect of CO2 concentration on carbon sequestration and mechanical strength of coal-based solid waste cemented backfill

Coal production generates large quantities of coal-based solid wastes (coal gangue, blast furnace slag, and fly ash) and significant CO 2 emissions, creating severe ecological challenges in mining regions. This study integrates coal-based solid waste cemented backfilling with CO 2 mineral sequestration to manage both solid wastes and CO 2 . The CO 2 uptake and uniaxial compressive strength (UCS) of coal-based solid waste cemented backfill (CSCB) were investigated after 14 days of mineralization curing at CO 2 concentrations of 0–10 vol%. Thermogravimetric analysis (TG), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffraction (XRD) were used to examine the sequestration mechanism and the associated strength reduction. CO 2 uptake increased with CO 2 concentration: raising the concentration from 1.5 to 10 vol% increased the uptake from 2.42 to 4.05 wt% (a 67.4% increase), based on a single TG measurement per group. In contrast, UCS decreased from 6.05 MPa (0 vol%) to 0.72 MPa (10 vol%), an 88.1% reduction. Microstructural and mineralogical analyses indicate that CO 2 reacts with alkaline phases and hydration products (CH, MH, C-S(A)-H gel, and AFt) to form calcite, gypsum, and possible Mg-bearing carbonate (XRD peaks tentatively assigned to dolomite). The strength loss is attributed to weakening of the alkaline environment, decalcification of C-S(A)-H gel, and carbonation of AFt to gypsum. A concentration of 1.5 vol% provided a reasonable balance, with 2.42 wt% uptake and a 13.2% strength loss. These results show a clear trade-off between carbon uptake and mechanical performance in CSCB.