Effects of CO2 Concentration During Curing on Carbon Sequestration and Strength of Coal-Based Solid Waste Backfill

Coal development produces large amounts of coal-based solid wastes (coal gangue, slag, fly ash) and substantial CO2 emissions, posing severe ecological burdens on mining areas. This study integrated coal-based solid-waste cemented backfilling with CO2 mineral sequestration to co-dispose of coal-based solid wastes and CO2, experimentally investigating the CO2 sequestration performance and uniaxial compressive strength (UCS) of coal-based solid-waste cemented backfill (CSCB) under mineralization curing with different CO2 concentrations. TG-DTG, SEM-EDS, and XRD were adopted to investigate the CO2 mineral sequestration process and the strength reduction of CSCB. The TG-DTG-based calculated CO2 uptake of CSCB was positively correlated with the CO2 concentration, rising by 86.9% from 2.52% to 4.71% as the concentration increased from 1.5% to 10%. In contrast, CSCB UCS decreased drastically by 88.1% from 6.05 MPa to 0.72 MPa when concentration increased from 0 to 10%. The combined microcharacterization results suggest that CO2 reacted with alkaline substances and hydration products (CH, C-(A)-S-H gel, AFt) in CSCB to form carbonates. The UCS reduction is interpreted as arising mainly from a weakened alkaline environment (according to results and speculation), degradation of the C-(A)-S-H gel, and the AFt–CO2 substitution reaction. Within the investigated conditions (a single mix proportion and a 14-day curing age), this study provides a reference for coal-based solid-waste backfilling coupled with CO2 mineral sequestration; extrapolation to other mix proportions, longer curing ages, or field-scale applications requires further verification.