The incorporation of biochar as an additive in cementitious materials presents a promising pathway toward climate-neutral concrete. Biochar, a carbon-rich solid obtained from biomass pyrolysis, can act as a micro-filler, internal curing agent and long-term carbon sequestration medium. This study examines the influence of biochar particle size and cement type on the fresh and hardened properties of cement pastes and micro-concretes. Two biochar types with median particle sizes of D₅₀ = 128 μm and D₅₀ = 51 μm were incorporated with CEM I 42.5 N, CEM II/B-M 42.5 N, and CEM III/B 32.5 N cements. Flow spread, water demand, density, and compressive strength were evaluated. The results show that fine biochar particles significantly increase water demand and accelerate workability loss, whereas coarse biochar promotes better strength development. Among the binders, CEM III/B 32.5 N demonstrated the best long-term strength performance, suggesting synergistic hydration between slag and biochar-induced internal curing. These findings confirm that properly optimized biochar addition can improve mechanical stability while contributing to CO₂ sequestration in concrete systems.