Controllable synthesis of all the anhydrous CaCO3 polymorphs with various morphologies in CaCl2-NH3-CO2 aqueous system

Polymorphism- or morphology-controllable synthesis of CaCO3 is of significance. Usually only calcite CaCO3 can be obtained from the ordinary CO2-Ca(OH)2 solution route; the very low solubility of Ca(OH)2 also causes many defects with this route. The route by CaCl2 and Na2CO3 exhibits some advantages, but the consumption of Na2CO3 is a deficiency. Therefore, this work studies the precipitation of CaCO3 with CaCl2 in NH3-CO2 aqueous solution, which will make the overall reaction “CaCO3 → CaCO3” if tracing the origin of the reactants. A main focus of the present work is to elucidate how to obtain various CaCO3 polymorphs with different morphologies through this synthesis system and the corresponding crystal growth mechanism. The results show that, just by simply regulating the reaction conditions, all three anhydrous CaCO3 polymorphs with various morphologies (e.g., cubic, lamellar, spherical, needle-like and branched) can be obtained in the absence of any additives. The crystal growth process can be well explained based on either Ostwald ripening or dissolution-recrystallization mechanism, depending on the reaction conditions. In addition, the rare phase transition of metastable vaterite to metastable aragonite can also be obtained from the present synthesis system, besides the usual phase transition of vaterite to the most thermodynamically stable calcite. This work provides the essential theoretical support for further developing a green process for CaCO3 production by using various calcium-containing carbonate minerals, such as limestone, calcite and dolomite.