Study on alkali modification of fly ash to enhance carbon sequestration efficiency and microstructural evolution

• Ca(OH)₂ modification achieves a breakthrough carbon sequestration efficiency of 144.46% in fly ash, far exceeding theoretical limits and outperforming NaOH and Na₂CO₃. • A novel "dynamic calcium cycle" mechanism is proposed to explain the sustained reactivity and prevention of passivation. • Multi-scale characterization directly correlates microstructural evolution (size reduction, surface area increase) with macroscopic performance. • Synergistic effects between the modifier and fly ash are unlocked, not merely additive effects. • The study provides a theoretical foundation for designing high-performance fly ash-based carbon sequestration materials for industrial CCUS applications. To address the low carbon sequestration efficiency of fly ash, this study employs alkali modification (using NaOH, Na₂CO₃, and Ca(OH)₂) to enhance its mineralization performance. The physicochemical properties and carbon sequestration efficiency of modified fly ash were systematically investigated. Experimental results demonstrate that alkali modification significantly optimizes the microstructure of fly ash: the average particle size decreases from 9.50 μm to 3.70 μm, resulting in an increased specific surface area, which provides more active sites for CO₂ adsorption and mineralization. The surface morphology transitions from smooth glassy phases to rough, porous structures with dispersed CaCO₃ deposits, improving mass transfer and reaction efficiency. Among the modifiers, Ca(OH)₂ exhibits the highest carbon sequestration efficiency (144.46%), far surpassing NaOH (15.71%) and Na₂CO₃ (26.25%). Alkali modification induces mineral phase transformations, enhancing surface reactivity and active sites. Mechanistically, reduced particle size and increased specific surface area accelerate CO₂ diffusion and reaction within fly ash. Additionally, the release of active components and mineral phase transitions promote thorough CO₂ mineralization. This study provides a theoretical foundation for optimizing fly ash-based carbon sequestration processes and advancing CCUS technologies in industrial applications.