Waste rice straw biochar recycled concrete: Carbon sequestration, durability and microstructure

Adding biochar to recycled concrete (RC) is considered an effective method to enhance carbon sequestration efficiency. This study introduces a sustainable and low-carbon green building material that replaces a portion of cement with waste rice straw biochar (WRSB) in RC and undergoes CO₂ curing. XRD and FTIR indicate that an appropriate amount of biochar can promote CO₂ absorption and accelerate the hydration process. Specifically, adding 5 % biochar by weight (WRSB-RC5) results in a carbon sequestration capacity of 24.66 g per kilogram of concrete and a compressive strength of up to 37.74 MPa. Furthermore, due to the rapid accumulation of carbonation products in the pores of biochar and recycled aggregate (RA), the rate of SO₄²⁻ attack on hydration products is mitigated, and the sulfate resistance coefficient of WRSB-RC5 reaches as high as 90.7 %. Additionally, the carbonation products reduce the total volume of capillary pores accessible to water, resulting in a freeze-thaw strength loss rate of only 8.68 %. In the early reaction stage, biochar stores additional moisture and adsorbs CO₂, while in the later stage, it releases moisture to accelerate the internal hydration reaction and generate more carbonation products, significantly enhancing carbon sequestration and durability. Therefore, the combination of WRSB and mineral carbonation can serve as an environmentally friendly building material to improve the performance of RC, promote the utilization of construction waste, and enhance carbon sequestration.