<p>Reactive magnesium oxide cement (RMC) is emerging as a sustainable binder in construction applications due to its ability to sequester CO₂ through carbonation, forming stable carbonates. However, the efficiency of RMC carbonation relies heavily on maintaining sufficient humidity and CO₂ concentration during curing. Various additives—including hydration agents, carbonate species, and seeds—have demonstrated effectiveness in enhancing both hydration and carbonation of RMC, thereby improving its mechanical performance. This study explores the use of biochar—a highly porous, carbon-based by-product of biomass pyrolysis—as a sustainable and cost-effective carbonation aid by evaluating its impact on the physical, rheological, mechanical, and microstructural properties of RMC composites. The results showed that the incorporation of 2 wt% biochar significantly improved early-age mechanical performance, with compressive strength increasing from 37.8 to 45.8 MPa at 7-days under CO₂ curing, and promoted the formation of hydrated magnesium carbonates (HMCs), raising total HMCs content from 5.4 to 13.9 wt% at 7-days under CO₂ curing. This improvement is attributed to biochar’s micro-filler effect, internal curing capability and its ability to facilitate CO₂ diffusion. Moreover, the inclusion of biochar effectively shortened the curing time, further enhancing the sustainability of CO₂ curing by reducing energy consumption. In conclusion, this study highlights the potential of biochar as a bio-renewable additive in RMC-based composites, enhancing brucite and HMCs formation, shortening CO₂-curing time and contributing to development of sustainable, carbon-efficient construction materials.</p>