The cement industry contributes around 5-7% of man-made CO2 emissions globally because of the Portland Cement (PC) production. Therefore, innovative reactive magnesia cement, with significant sustainable and technical advantages, has been proposed by blending reactive MgO and hydraulic binders in various proportions. MgO is currently produced from the calcination of magnesite (MgCO3), emitting more CO2 than the production of PC, or from seawater/brine which is also extremely energy intensive. Hence this research aims to investigate an innovative method to produce MgO from reject brine, a waste Mg source, through carbon sequestration, by its reaction with CO2, to provide a comparable low carbon manufacturing process due to the recycling of CO2. The produced deposits are then calcined to oxides with potential usage in construction industry. The entire system is a closed loop to achieve both environmental optimisation and good productivity. This research focuses on the chemical manufacturing process, integrated with material science knowledge and advancements, instead of concentrating purely on chemistry evaluations. Six series of studies were conducted, utilising MgCl2, CaCl2, MgCl2-CaCl2, MgCl2-CaCl2-NaCl, and MgCl2-CaCl2-NaCl-KCl to react with CO2 under alkaline conditions. The precipitates include hydrated magnesium carbonates, calcium carbonates and magnesian calcite. Generated carbonates were then calcined in a furnace to obtain MgO, CaO or dolime (CaO•MgO). All six series of carbonation processes were carried out under a controlled pH level, to study the constant pH’s effect on the process and resulting precipitates. Other controllable factors include pH, temperature, initial concentration, stirring speed, and CO2 flux rate. In conclusion, the optimum parameters for the production of the carbonated precipitates are: 0.25MgCl2 + 0.05CaCl2 + 2.35NaCl + 0.05KCl, 700rpm stirring speed, 25oC room temperature, pH=10.5, and 500cm3/min CO2 infusion rate. Reaction time is within a day. These parameters are chosen based on the sequestration level, particle performance morphology and the operational convenience. The optimum calcination parameters are at 800oC heating temperature with a 4h retention time.