Effect of soaking time on the physicochemical properties and mineralization efficiency of low-calcium fly ash–CO₂

The global greenhouse effect is escalating, leading to the progressive deterioration of ecosystems and climate worldwide. As the primary greenhouse gas, reducing CO₂ emissions is crucial for effectively mitigating this effect. Fly ash–CO₂ mineralization and sequestration technology represents a promising approach for carbon fixation and emission reduction. However, the low carbonation efficiency of fly ash–CO₂ remains the central constraint hindering effective CO₂ mineralization and sequestration. Thus, elucidating the microscopic mechanisms and key influencing factors of fly ash–CO₂ carbonation is essential toward addressing this limitation.

To investigate these issues, a custom-designed setup was used to conduct experiments, including conventional immersion, immersion–carbonation, and microstructural characterization tests to examine the effects of immersion time on alkaline metal ion leaching and diffusion, as well as the impact of the microstructures of fly ash and CO₂ on the carbonation efficiency.

Results show that the pH increase rate exhibited a negative exponential decay relative to immersion time. After 24 hours of immersion, the carbonation efficiency and sequestration capacity reached peak values of 12.855% and 12.91 kg/t, respectively, representing a 0.25-fold increase over non-immersed fly ash. Raw, unmineralized fly ash contains amorphous silica (SiO₂), mullite (Al₂SiO₅), and amorphous silica hydrate (SiO₂·xH₂O). No diffraction peaks were detected for calcium carbonate, calcium hydroxide, or magnesium hydroxide, confirming the absence of calcium carbonate in the original sample. Conversely, the mineralized sample contained phases such as amorphous silica hydrate (SiO₂·xH₂O), quartz (SiO₂), mullite (Al₂SiO₅), calcium carbonate (CaCO₃), and hydroxides (Ca(OH)₂ and Mg(OH)₂). The absence of the magnesite diffraction peak indicated that magnesium did not participate in the mineralization reaction. Furthermore, the observed low-intensity diffraction peaks were broad, indicating low sample purity and small crystal size, confirming the predominantly amorphous composition of the matrix. The diffraction-peak intensity of calcium carbonate initially increased and then decreased with increasing immersion time, with the maximum mass fraction (1.10%) observed at the 24-hour mark. Raw fly ash particles were spherical and dispersed. By comparison, the carbonated samples exhibited agglomeration and cementation, peaking at the 24-hour mark, with amorphous calcium carbonate deposited on the particle surface. Additionally, the carbon content of the carbonated fly ash increased with immersion time (within 24 hours), indicating a higher carbonation degree. The frequencies of larger and smaller particles increased and decreased, respectively, with the immersion time (≤ 24 h), suggesting positive and negative correlations between larger and smaller particles and the carbonation degree, respectively.

This study investigated the leaching and diffusion characteristics of fly ash–derived alkaline metal ions and their impacts on the microphysicochemical properties of fly ash, as well as their relationship with CO₂ mineralization efficiency. Through a series of experiments involving fly ash immersion across different durations, followed by mineralization tests and microstructural characterization of the mineralized fly ash, the influences of soaking time on the leaching and diffusion behavior of alkaline metal ions were evaluated. Additionally, the effects of leaching on the microstructure of fly ash, as well as the efficiency of CO₂ mineralization, were examined. Overall, these findings provide theoretical guidance for optimizing reaction parameters and enhancing mineralization efficiency in fly ash–CO₂ mineralization processes.