This study focuses on the enhancement of the technical and environmental performance of MgO cements through the inclusion of brucite, GGBS, talc and serpentine as partial MgO substitutes in concrete blocks. The influence of these additives on the microstructure, hydration, carbonation, and mechanical performance of blocks cured under natural and elevated CO2 conditions is presented. An optimum replacement level was determined for each composition via unconfined compressive strength (UCS), scanning electron microscopy (SEM), x-ray diffraction (XRD), thermogravimetric analysis (TGA) and acid digestion. Mixes subjected to accelerated carbonation indicated up to 100% carbonation in 7 days. Results highlight the suitability of each additive in replacing MgO without compromising performance.