Biochar as green additives in cement-based composites with carbon dioxide curing

Biochar is a green material obtained from the thermochemical conversion of waste biomass under an oxygen-limited condition. This study demonstrated a novel and sustainable method for incorporating waste wood-derived biochar in cement-based composites with CO₂ curing. The X-ray diffraction (XRD) and thermogravimetric analyses (TGA) illustrated that the incorporation of biochar promoted the generation of additional cement hydrates owing to moisture regulation effect, but did not accelerate or delay the hydration process as indicated by the isothermal calorimetry results. Specifically, the addition of 1 wt% biochar increased the compressive strength of composites by 8.9%. However, the incorporation of 5 wt% biochar reduced the compressive strength due to the porous and brittle structure of biochar. CO₂ curing was employed to mitigate these adverse effects. The CO₂ curing approach accelerated carbonation more effectively than the method of air curing for composites production. After carbonation, the abundant cement hydrates improved the bonding strength and the carbonates densified the microstructure, which substantially enhanced the mechanical strength and carbon sequestration. Therefore, the combined use of waste biochar and CO₂ curing could be a green technology for enhancing properties of cement-based composites and promoting waste recycling and CO₂ utilisation.