AbstractFormulation of highly workable reactive magnesium oxide (MgO) cement (RMC)–based composites is critical to their wider adoption in the construction industry. Yet, the performance of the binder with the presence of chemical admixtures is not well understood. This study assessed the influence of polycarboxylate ether (PCE)–based superplasticizers on the hydration and carbonation kinetics of RMC. Its application was also demonstrated. RMC-based composites were prepared with conventional aggregates, superplasticizers to aid workability, and fibers to control cracking induced by volume changes under both hydration and carbonation. Two carbonation methods were utilized: 20% CO2 in a carbonation chamber and CO2 in an enclosed plastic bag to emulate a low-technology carbonation process. Highly workable RMC-based mortar mixes were achieved with a superplasticizer content of 2% by weight, despite the presence of 0.2% by volume polyethylene microfibers. The admixtures slowed down the hydration rate and altered the morphology of the hydration product. Compressive strength of 32.7 MPa was attained after 7 days under 20% CO2 in a carbonation chamber compared with 15.6 MPa under carbonation inside the plastic bag condition. The crystalline carbonation products within the exterior regions were dominated by nesquehonite. A significantly reduced carbonation level was observed within the interior regions due to the formation of the carbonation products and the resulting pore refinement within the exterior regions. Also, the PE fibers provided a crack control mechanism, and their failure mode was characterized by pullout. The finalized mix was successfully applied for the construction of a large-scale architectural structure.