Role of Aspartic Acid in the Synthesis of Spherical Vaterite by the Ca(OH)2–CO2 Reaction

Vaterite crystals with a spherical shape were successfully obtained by the addition of aspartic acid (Asp) in the calcium hydroxide (Ca(OH)2)–carbon dioxide reaction system. Crystalline products were characterized by X-ray diffraction, field emission scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and differential scanning calorimetry (TG-DSC). The experimental results indicate that the addition of Asp can inhibit the growth of calcite but promote the formation of vaterite, and the spherical vaterite CaCO3 is gradually dominant along with the increase of Asp. Meanwhile the content of the vaterite phase is prominently influenced by the reaction temperature, and the optimal temperature for vaterite is at 40 °C. It is proposed that vaterite is induced through the intermediate chelated by Asp and Ca(OH)2, and then Asp is adsorbed on the surface of vaterite by chelation, preventing the metastable vaterite from transforming to calcite via a dissolution–recrystallization process. Further, the decrease of pH in the solution could reduce the chelation between Asp and Ca2+, leading to the transformation from vaterite to calcite. Hence, the study on the formation and transformation of vaterite will provide additional insight into biomineralization mechanism and industrial production.