Salt additives for enhanced carbon sequestration and mechanical properties in fly ash-based filling slurry: a mechanistic study

Underground backfill and storage of CO 2 provides multiple benefits, including carbon emission reduction and improved geological stability of mined-out areas and overlying strata. It enables long-term CO 2 sequestration in the form of mineral carbonates, while optimizing the mechanical and seepage properties of the backfill system, thus demonstrating significant engineering value and climatic benefits. This study investigates the enhancement mechanisms of carbon sequestration efficiency and mechanical properties in fly ash–cement-based slurry. It systematically incorporates three salt additives (Na 2 CO 3 , CH 3 COONH 4 , NH 4 Cl) at varying dosages, employing a multi-scale experimental approach. Thermogravimetric analysis (TGA) assessed carbon sequestration efficiency. Uniaxial compressive strength (UCS) testing and CT scanning quantitatively characterized mechanical performance and pore evolution. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were employed to analyze the micro-phase interaction mechanism of additives within the hardened backfill. Results indicate that the optimized mix proportion (63% matrix mass fraction, fly ash-to-cement ratio of 9:1, and 7% Na 2 CO 3 ) achieved a CO 2 sequestration capacity of 86.69 kg·m −3 , accompanied by substantial generation of high-crystallinity calcite. The 28-day UCS increased to 5.39 MPa, representing a 50.14% increase compared to the blank control group. Concurrently, total porosity increased from 5.37 to 7.96%, with interconnected pores comprising 99.5% of the total, and the average pore diameter decreased from 22.06 to 20.53 μm. These findings verify the mechanism by which Na 2 CO 3 enhances both the carbon sequestration efficiency and mechanical properties of fly ash backfill slurry. The optimized system features a denser microstructure and superior permeability, effectively improving the stability and bearing capacity of the backfill. This lays a theoretical foundation for the safe mining of coal resources and the enhancement of geological engineering stability.