The carbonation of chrysotile [Mg₃Si₄O₁₀(OH)₄] under subcritical conditions was experimentally investigated in alkali solution, and the chemical composition as well as the morphological and structural changes were discussed. The starting material was hydrothermally treated by aqueous direct carbonation at a temperature of 100°C and a CO₂ partial pressure range of 0.5 MPa∼4 MPa at pH 13. Highly crystalline magnesite was synthesized under a CO₂ partial pressure of 3 MPa. The carbonation rate increased at the proportional rate according to the applied CO₂ pressure to approximately 57%. The surface morphology of chrysotile changed from the fibrous form to a round or oval shape at the initial stage and subsequently to magnesite with well-faceted rhombohedral planes.
The dissolution rate of Mg was higher than Si, such that the Mg:Si ratio of chrysotile decreased from 1.56 to 0.4∼0.6 as the reaction time increased. The resultant silica-rich layer of the reaction product ultimately changed through the Mg-depleted skeletal phase to the amorphous silica phase. The experimental results suggest that carbonation in alkali solution under subcritical conditions is of great significance because an excessive amount of acid and alkali reagents can be eliminated in the carbonation process.