To streamline the salt manufacturing process, a novel method is required to recover and upgrade Ca and Mg from the concentrated seawater discharged by salt manufacturers in Japan. The synthesis of carbonate by reactive crystallization between the dissolved Ca2+ and Mg2+ in concentrated seawater and CO2, which is based on the relative solubility of the salts, is an effective separation/recovery method. Dolomite (CaMg(CO3)(2)), which is a double salt, is used in various industrial fields. To improve the functionality of CaMg(CO3)(2), it is necessary to achieve a high Mg/Ca ratio. Generally, high concentrations of Ca2+, Mg2+, and CO32- are necessary for the synthesis of CaMg(CO3)(2) with a high Mg/Ca ratio. Therefore, the fine bubble formation technique, which generates regions of high ion concentrations near the minute gas-liquid interfaces of the bubbles, was applied in the reactive crystallization of CaMg(CO3)(2). In this study, we evaluated several concentrated seawater samples to determine the most suitable one for the synthesis of CaMg(CO3)(2) with a high Mg/Ca ratio during the reactive crystallization with CO2 fine bubble injection. Three concentrated seawater samples: ED brine, concentrated brine, and removed-K brine, were used. These samples have different concentration products of Ca(2+)and Mg2+([Ca2+][Mg2+]), as well as various concentrations of coexisting ions, such as Na+, K+, Cl-, Br-, and SO42-. When the removed-K brine, which had a high [Ca2+][Mg2+], was subjected to a high CO2 flow rate, CaMg(CO3)(2) with a higher Mg/ Ca ratio was produced in a shorter crystallization time. Moreover, the crystallization phenomena of CaMg(CO3)(2) in the concentrated seawater samples were compared with those of the CaCl2/MgCl2 mixed solutions, and it was found that the effects of the [Ca2+][Mg2+] on the production of CaMg(CO3)(2) with a high Mg/Ca ratio were more significant than those of the coexisting ions.