Instantaneous Solid–Liquid–Gas Carbonation of Ca(OH)2 and Chameleonic Phase Transformation in CO2‑Expanded Solution

In this study, a CO2-expanded ethanol–water solution system was applied for the solid–liquid–gas carbonation of Ca(OH)2 to CaCO3. The system revealed, for the first time, instantaneous (within 1 min) conversions of Ca(OH)2 (95.1–100.0%) with the volume ratios of ethanol between 10% and 80%, indicating that both the water content and the expansion of the system take effect in the carbonation process. The instantaneous carbonation with 80% ethanol ratio led to production of 12.8% aragonite polymorph. Further examination of this carbonation case under different reaction times found that, interestingly and importantly, pure calcite phase was stabilized within a short time (210 min) as compared to those reported in the literature; however, in this period, the system exhibited chameleonic phase transformation between calcite, aragonite, and vaterite, normal to abnormal and vice versa.