CO ₂ Mineralization-Driven Phase Reconstruction and Cementitious Activity Enhancement in Fly Ash

Fly ash, a byproduct of coal-fired power plants, is an emerging material for CO 2 capture and storage through CO 2 mineralization. However, the mechanism of modification of mineralization on the cementitious activity of fly ash remains unclear. In this study, a self-designed mineralization device coupled with a multimethod characterization methodology was applied to investigate the evolution of physical properties, crystal phases, and microchemical structures of fly ash during mineralization. Results indicated that physical changes contributed only marginally to the cementitious activity, whereas phase transformation played a pivotal role in its enhancement. The mechanism is driven by a transient acidic environment created by CO 2 dissolution: (1) the proton-assisted erosion disrupts the highly polymerized glassy aluminosilicate networks, facilitating the release of active Si and Al units; (2) concurrently, a dual-pathway phase reorganization occurs where Ca 2+ precipitates as nano-CaCO 3 serving as nucleation sites, while the released Si/Al units repolymerize into low-polymerization calcium aluminosilicate hydrate (C–A–S–H) gels acting as reactive precursors. These transformed products synergistically lower the reaction barrier for cement hydration. Consequently, the initial setting time was shortened by 30% and the 3 day compressive strength was increased by 10%. This work elucidates the acid-driven reorganization mechanism, providing a theoretical foundation for the high-value utilization of mineralized fly ash in building materials and carbon-capture fields.