Reactive MgO cement derived from brine could be a low-carbon alternative to Portland cement, as it avoids the need to burn carbon-containing raw materials and it can absorb CO2 gas to form high-strength mineral carbonates. Additionally, the incorporation of a high concentration of aqueous carbonate species can further contribute to its negative CO2 footprint. This paper aims to study the influence of aqueous carbonate species on the hydration and carbonation behavior of reactive MgO cement derived from brine. Although the hydrated MgO exhibited much higher compressive strength in the presence of bicarbonate solution, it still did not exhibit compressive strength sufficient for structural applications (> 10 MPa) under ambient conditions. TGA and XRD results revealed that the carbonate and/or bicarbonate solution altered the hydration degree and morphology of formed Mg(OH)2 but did not form new hydration products. In contrast, when MgO paste was exposed to concentrated CO2 gas (20 % CO2) it produced magnesium carbonates and reached dramatically higher compressive strength, nearly 60 MPa by 28 days. TGA and XRD results suggest that an amorphous phase forms in sodium bicarbonate and sodium sesquicarbonate systems, which plays a key role in the enhancement of compressive strength.