Carbon-negative backfill design: dual functionality of aluminum nanoparticles in strength enhancement and CO₂ fixation

The sustainable management of coal-based solid waste and effective CO₂ sequestration remain critical challenges in green mining practices. This study presents a novel aluminum nanoparticle-modified CO₂-carbonated backfill (ANCB) material that integrates mechanical enhancement with carbon capture functionality. Aluminum nanoparticles (Al-NPs) were incorporated at varying concentrations to investigate their effects on the unconfined compressive strength (UCS), CO₂ adsorption capacity, hydration and carbonation reactions, and microstructural evolution of the ANCB system. Results demonstrate that low-dose Al-NPs (0.02-0.06 wt%) significantly improve UCS by up to 73.8%, with early-age strength development accelerated due to enhanced nucleation and hydration kinetics. CO₂ adsorption was found to correlate strongly with strength gain, with the optimal Al-NPs concentration (0.06 wt%) achieving a balanced enhancement of both properties. Microstructural and spectroscopic analysis (SEM-EDS, XRD, FTIR) confirmed that Al-NPs promote denser C-A-S-H gel formation, facilitate CaCO₃ precipitation, and contribute reactive Al(OH)₄^- ions that drive pozzolanic activity. Excessive Al-NPs, however, led to particle agglomeration and microstructural heterogeneity, reducing material performance. This work establishes the dual role of Al-NPs in advancing the mechanical and environmental performance of coal-based backfill materials. The ANCB system offers a promising, scalable approach for carbon-negative backfilling in underground mining, bridging waste valorization with in-situ CO₂ sequestration.