CO2 nanobubble-enhanced cement–fly ash backfill: Optimizing aggregate gradation and microstructure

• Synergistic optimization of fractal aggregate grading (dimension 2.65) and CO2 nanobubble technology significantly increases CFB density and mechanical performance, with uniaxial compressive strength and elastic modulus improving by up to 13.46% and 27.47%, respectively. • CO2 nanobubble water enhances hydration and carbonization, promoting the formation of C–S–H and C–A–S–H gels and CaCO3, which strengthen the microstructure of cement–fly ash-based backfill. • The CO2 sequestration capacity of cement–fly ash-based materials treated with CO2 nanobubble water increases by 12.4 wt% to 99.8 wt% across different fractal dimensions, supporting low-carbon mine filling. • The research provides a scientific basis for designing high-performance, low-carbon backfill materials and advancing sustainable green mining practices.