Accelerating construction decarbonization: biochar-Limestone Calcined Clay Cement (LC3) synergy for carbon mitigation and performance enhancement.

The integration of biochar into cementitious materials has emerged as a groundbreaking approach to carbon sequestration, yet its high-dosage utilization and synergistic effects with sustainable cement formulations remain largely unexplored. This study proposes a novel design for low-carbon cement composites by incorporating high levels of biochar up to 30 wt% into both ordinary Portland cement (OPC) and Limestone Calcined Clay Cement (LC3) systems, fundamentally altering their hydration kinetics, microstructural evolution, and mechanical performance. The findings reveal that biochar enhances the flowability of OPC pastes but slightly reduces that of LC3 due to its high surface area and interparticle friction. It promotes the hydration process of OPC through internal curing and intensifies the early-stage reaction of LC3 via nucleation effects. Additionally, biochar's well-developed pore network may facilitate the redistribution of calcium species and the diffusion of CO2, dramatically promoting carbonation and inducing the crystallization of vaterite and calcite, while simultaneously increasing the polymerization degree of C-(A)-S-H and reducing unhydrated clinker content for both OPC and LC3 pastes. Despite porosity-induced reductions in the compressive strength of biochar-modified cement, plain LC3 and LC3 with 15% biochar achieved an 18% and 4% enhancement in flexural strength over plain OPC pastes, attributed to the formation of nanostructured rod-like ettringite. Critically, the LC3 mix with 30% biochar exhibited remarkable carbon reduction efficiency, achieving a mitigation potential of up to 88.2 kg CO2 eq./m3/MPa for flexural strength and 1.1 kg CO2 eq./m3/MPa for compressive strength, indicating its potential to contribute to the cement industry's decarbonization efforts. Overall, this work demonstrates the feasibility of incorporating high biochar contents into LC3 composites, highlighting their carbon mitigation potential for sustainable construction.