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 are critical challenges for the mining industry. To address this, a novel aluminum nanoparticle-modified CO₂-carbonated backfill (ANCB) material iwas developed that synergistically enhances mechanical properties with carbon capture functionality. The effects of varying aluminum nanoparticles (Al-NPs) concentrations (0.02wt%–0.1wt%) were investigated on the unconfined compressive strength (UCS), CO₂ adsorption capacity, hydration and carbonation reactions, and microstructural evolution of the ANCB. Results demonstrate that low-dose Al-NPs (0.02wt%–0.06wt%) enhance UCS by up to 73.8%, with early-age strength development accelerated by promoting nucleation and hydration kinetics. Notably, CO₂ adsorption was strongly correlated with this strength gain, with an optimal concentration of 0.06wt% achieving a balanced enhancement of both properties. Microstructural and spectroscopic analyses (SEM-EDS, XRD, FTIR) revealed that Al-NPs promote the formation of a denser calcium aluminosilicate hydrate (C–A–S–H) gel, facilitate CaCO₃ precipitation, and release reactive Al(OH)₄⁻ ions that drive pozzolanic reactions. However, excessive Al-NPs led to agglomeration and microstructural heterogeneity, impairing performance. This study establishes the dual role of Al-NPs in advancing the multifunctionality of backfill materials. The ANCB system thus presents a scalable, carbon-negative strategy for underground mining, effectively bridging the gap between waste valorization and in-situ CO₂ sequestration.