Aqueous mineral carbonation of industrial wastes, such as fly ash, is a promising sequestration technology to reduce CO2 emissions in small-/medium-sized plants. In this paper, the carbonation capacity of a leachate rich in Mg2+ and Ca2+ contents was examined to clarify the competition between the carbonation of these two cations and the speciation of the resulting carbonate precipitate, under the mild carbonation conditions using 20−80 °C and atmospheric pressure. As confirmed, the carbonation precipitation of the two cations was completed in 30−40 min. At room temperature, increasing the Mg2+/Ca2+ molar ratio was in favor of the carbonation rate of Mg2+, which is maximized at the Mg2+/Ca2+ molar ratio of 2. In contrast, the carbonation rate of Ca2+ was decreased monotonically as a result of the competition from Mg2+. For both cations, their carbonation rate was maximized at 60 °C. In comparison to the formation of predominant calcite and vaterite in the presence of sole Ca2+ in the leachate, the coexistence of two cations resulted in the preferential formation of amorphous species, aragonite and magnesian calcite. The quantity of the amorphous phase was increased remarkably upon increasing the Mg2+/Ca2+ molar ratio at room temperature. An increase in the carbonation temperature further deteriorated the crystallization of the carbonation precipitate, resulting in the increase of the amount of amorphous species and the phase change of calcium carbonate from calcite to aragonite.