Experimental Study on the Effect of Hydrothermal Activation on the Physicochemical Properties and Mineralization Efficiency of Fly Ash-Supercritical CO2

To address the challenge of low mineralization efficiency of fly ash-supercritical carbon dioxide (CO2), a hydrothermal activation method was proposed to enhance fly ash-supercritical CO2 mineralization. The study systematically investigated the mechanism through which hydrothermal activation parameters influenced the efficiency of fly ash-supercritical CO2 mineralization through a series of experimental reactions. The effects of hydrothermal activation temperature and time on mineralization efficiency were investigated macroscopically. The results indicated that the mineralization efficiency increased by 202.93 % when hydrothermally activated at 220 °C for 30 min. Furthermore, when the activation time was ≤10 min, 220 °C exhibited a significant effect on the mineralization efficiency. For activation time ≥20 min, mineralization efficiency exhibited high stability with increasing temperature. Subsequently, under 220 °C conditions, mineralization efficiency reached 18.79 % for 5 min and 21.69 % for 30 min, representing only a 15.43 % increase despite a six-fold increase in time. Microscopic analysis of the pore structure of hydrothermally activated fly ash indicated that pore parameters (volume, specific surface area, and volume change rate) exceeded those of original fly ash for pore diameters ≥9 nm, and the opposite was observed for pore diameters <9 nm. Additionally, compared with the pristine fly ash, the pore size and volume fraction of the mineralized fly ash macropores increased, while mesopores decreased, and small pores exhibited a slight decrease in volume fraction and size. This indicated that pores ≥9 nm were primarily affected by hydration expansion, while pores <9 nm were primarily affected by mineralization precipitation. The hydrothermal activation mineralization reaction predominantly affected medium and large pores ≥9 nm, exhibiting less effect on smaller pores.