Effects of Mg ions on the structural transformation of calcium carbonate and their implication for the tailor-synthesized carbon mineralization process

Carbon mineralization with alkaline earth metal-bearing industrial waste is recognized as a promising CO2 removal technology because it can stably store CO2 at a large scale and simultaneously produce solid carbonates (e.g., CaCO3) that can be utilized in various industries, such as paper, plastic filler and additives, food and pharmaceuticals, and construction materials. In order to maximize the value of the solid carbonates, their purity and properties need to be controllable. However, the crystalline structures and polymorphisms of the solid carbonates, particularly CaCO3, largely depend on various factors: pH, temperature and pressure, reaction time, types of ions and their ratios, degree of saturation, stirring rate, feeding order, etc. In general, industrial waste includes calcium with a small amount of magnesium. We investigated the effects of Mg2+ on the structural transformation of CaCO3 particularly focusing on the Mg2+/Ca2+ ratios in the range of 0.2 - 0.33. Because the ex situ carbon mineralization can be conducted under the aqueous phase including Ca2+ and Mg2+ via pH swing processes, it is worth investigating the effects of Mg2+ on the precipitation of CaCO3. Other controlling factors of the formation of crystalline CaCO3, such as the concentrations of Ca2+ and CO32-, and the reaction time were also considered. The results revealed that not only the ratio of Mg2+/Ca2+ but the change of the ratio of CO32-/Ca2+ by stepwise synthesis control the structural transformation of CaCO3.