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 CO₃^2–, 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.