Effect of sea salt coupled with reaction temperature on aqueous carbonation of recycled concrete powder: Insights into CO2 sequestration and CaCO3 polymorphism

Water-mediated carbonation has emerged as a promising approach for CO 2 mineralization using recycled concrete powder (RCP), enabling permanent carbon sequestration. However, to ensure practical implementation of aqueous carbonation, the environmental concerns and energy consumption associated with its high-water demand must be minimized. Seawater, an abundant and Ca/Mg-rich resource, presents a potentially sustainable alternative to freshwater. This study aims to provide a comprehensive evaluation of the feasibility of using sea salt solution (as simulated seawater) for carbonating RCP. The carbonation efficiency, phase assemblage, and microstructural evolution of calcium carbonate (CaCO 3 ) polymorphs were systematically investigated at different reaction temperatures (20 °C, 45 °C, and 60 °C). The results revealed that temperature and the mobility of Na + and Mg 2+ ions in the solution were the key parameters governing the precipitation rate and polymorphic transformation of CaCO 3 in sea salt-RCP suspensions. At lower temperatures, amorphous CaCO 3 and calcite were preferentially formed, whereas elevated temperatures (> 45 °C) promoted the nucleation and crystallization of aragonite due to enhanced surface energy of carbonate crystals. At 60 °C, fibriform aragonite transformed into needle-like aragonite, accompanied by a 1.21-3.28 µm increase in the crystal length and an 11.9% rise in crystallinity over 10-180 min. Overall, the proposed approach demonstrates a sustainable carbonation pathway that minimizes freshwater consumption, sequesters 164-173 g CO 2 (per kg of RCP), and yields low-carbon cementitious materials enriched with silica gel and aragonite.