The integration of CO₂ sequestration with alkali-activated fly ash (FA) backfill slurry presents a promising strategy for simultaneous resource utilization of industrial waste and carbon capture. However, the efficiency of this process is often constrained by the inherently poor reactivity of FA and the rheology of FA-based slurry. This investigation applies acetic acid (AA) to modify FA to improve the CO₂ sequestration and rheological properties of alkali-activated FA-based backfill (AFB) slurry. Results reveal a non-monotonic relationship between AA dosage and the CO₂ sequestration capacity of AFB, characterized by an initial increase followed by a decline. At 2 wt% AA, the material achieves its optimal CO₂ uptake of 1.27% and minimum yield stress (9.28 Pa). This improvement originates from AA-driven surface modification, resulting in finer particles and higher reactivity compared to that of the control group. Furthermore, AA adsorbs onto the FA surface and functions as a lubricant and dispersant, thereby improving slurry rheology. Carbon life cycle assessment indicates that AFB employing 2 wt% AA-modified FA reduces total emissions by 65.76 kg t⁻¹ compared to that of the unmodified reference, underscoring the potential of this strategy for achieving negative carbon emissions in mining backfill applications.