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

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.