Cement manufacturing and concrete constructions are associated with high carbon dioxide (CO2) emission. This can be potentially reduced by sequestering CO2 through calcium carbonate mineralization using the technique of accelerated carbonation curing (ACC). This research investigates and compares the role of biochar – as-produced (BC) and CO2-saturated (SatBC), and silica fume (SF) respectively on carbon sequestration, strength and hydration of cement composites. In case of SatBC, the CO2 is pre-adsorbed in biochar pores a priori before deployment in cement matrix. The study found that addition of BC can densify the cement matrix through accelerating hydration and improving the total hydration. This leads to reduction in carbon dioxide diffusion and carbonate mineralization initially (7-day age) than control. However, the net carbonate mineralization in biochar-cement paste is similar to control and silica fume-paste after 28 days of carbonation. Accounting for the carbonate mineralization and carbon sequestered in the biochar structure, ACC of BC-CP and internal carbonation in SatBC-CP can lead to 5–6% reduction in carbon dioxide-equivalent (CO2-eq) than control. Addition of biochar yields higher carbonate mineralization from carbonation of calcium hydroxide compared to silica fume-cement (SF-CP), where more carbonate mineralization is contributed by carbonation of binder gel. This leads to 8–10% higher compressive strength and lower loss in strength (4–5% loss) in BC-CP compared to SF-CP (15% loss) at 28-day age. Application of SatBC as admixture substantially improves compressive strength compared to uncarbonated control and SF-CP at 7-day age and demonstrates similar strength as carbonated SF-CP after 28 days. The results suggest that spent biochar, previously used as industrial CO2 adsorbent, can be valorized as “green admixture” in cement-based materials. In summary, this research illustrates the potential of biochar to manufacture low carbon cementitious building materials, that can turn future cities into “carbon sinks”.