Biochar demonstrates strong decarbonization potential in concrete but significantly affects workability. To address this and to better understand the effects of biochar on concrete, as well as its role in decarbonizing it, this study applied a volumetric water-to-binder ratio (w/b), replacing cement with biochar powder at 5%, 10%, and 20% by volume, and compared the results with a 5% weight-based replacement. Additionally, prewetted biochar aggregate was used to replace sand at 30%, 60%, and 100% by volume. Various properties were assessed, including rheology, mechanical strength, hydration products, fire resistance, and carbon emissions of the concrete.

For biochar powder used as a cement replacement, the results indicate that the volumetric w/b ratio effectively improved workability and enhanced internal curing, leading to increased hydration products and improved mechanical performance compared to weight-based replacement. Despite a lower cement content, the 10% volumetric biochar sample achieved higher strength than the 5% weight-based sample. Biochar used as a sand replacement in concrete can better enhance internal curing and increase the degree of hydration due to improved moisture retention from the larger replacement volumes compared with its use as a cement replacement. Furthermore, replacing sand with biochar aggregate effectively reduces total shrinkage due to improved internal curing. Total shrinkage at 28 days was reduced by 41%, 55%, and 65% for 30%, 60%, and 100% biochar aggregate replacement levels, respectively.

Regarding fire resistance, both types of biochar increased the temperature gradients in concrete during heating. However, biochar aggregate contributed to a greater increase due to its higher volume and sand replacement ratio than biochar powder, which intensified thermal damage and, at 400 °C, even offset the benefits of accelerated cement hydration.

Due to energy recovery and carbon sequestration, wood biochar replacing 20% of cement reduced concrete carbon emissions by 42%, while fruit biochar replacing 100% of crushed sand reduced emissions by 167%, indicating that the concrete can become an effective carbon sink. However, carbon emissions should not be the sole consideration. Wood biochar used as a cement replacement reduced 56-day compressive strength by 3%, 6%, and 13% at 5 vol%, 10 vol%, and 20 vol%, respectively. Similarly, when fruit biochar replaced sand, the 56-day compressive strength decreased by 7%, 21%, and 47.4% at 30 vol%, 60 vol%, and 100 vol%, respectively.