Production of aragonite whiskers by carbonation of fine recycled concrete wastes: An alternative pathway for efficient CO₂ sequestration

In this study, a wet carbonation method targeting high carbonation rate was developed to prepare aragonite whisker using fine recycled concrete waste (FRCW), aiming to effectively capture CO₂ and convert FRCW into high-value products. The effect of operational factors, including MgCl₂ concentration, temperature, CO₂ concentration and duration on the formation of aragonite was systemically investigated. The results indicated this carbonation process can not only produce needle-like aragonite whisker-rich materials but also capture a large amount of CO₂ (0.19 g CO₂ per g FRCW) within an hour. The MgCl₂ concentration and temperature were key parameters governing the nucleation of aragonite, while the formation of needle-like aragonite was favored in a MgCl₂-FRCW suspension with a minimum Mg²⁺/Ca²⁺ molar ratio >0.16 at a temperature >60 °C. A lower CO₂ concentration of <50% only slightly decreased the carbonation rate without affecting the types of carbonation products formed, indicating the potential to sequestrate CO₂ from industrial flue gas directly. In addition, amorphous carbonation phases including silica gel, decalcified C–S–H and amorphous calcium carbonate were produced apart from the dominant reaction product-aragonite. Based on the results, the formation of aragonite could be divided into two steps: 1. The FRCW reacted with MgCl₂ to form a new FRCW-MgCl₂-Mg(OH)₂–CaCl₂ system. 2. The Ca²⁺ reacted with CO₃²⁻ to form aragonite and brucite was solubilized back to MgCl₂, resulting in possible recycling and reusing MgCl₂ for another carbonation cycle. The proposed approach exhibits a novel direction of sequestering CO₂.