Insights into the influence of biochar powder on the fresh state properties and strength development of concrete

Incorporating biochar into concrete can significantly reduce its carbon footprint; however, many studies have reported reduced workability and compressive strength at higher biochar dosages, particularly when using biochar with high carbon content. This study investigates the fresh-state properties of biochar-amended concrete, with the dual aim of optimizing flowability and assessing how enhancements in fresh behavior influence mechanical performance. Concrete mixes containing biochar at 5 %, 10 %, and 15 % by weight of cement were prepared. To maintain a constant cement content across all mixes, biochar partially replaced silica sand on a volumetric basis. Scaled-down experiments were performed to evaluate the compatibility of three superplasticizers (SPs) with biochar concretes, focusing on biochars with moderate (∼50 %) and high (∼90 %) carbon content. The results demonstrate that optimized SP dosages can significantly improve dispersion and sustain fluidity across biochar contents of 5–15 %. The study further identifies specific surface area (SSA) as a critical property influencing rheological behavior, enabling flow optimization data from moderate-carbon-content biochar to be extrapolated to high-carbon-content variants using SSA ratios. With the most compatible SP at its optimized dosage, biochar concretes achieved slump values of 15–17 cm, porosities as low as 8 %, and compressive strengths exceeding 40 MPa across all tested biochar dosages, irrespective of biochar type. These findings highlight the potential of biochar-amended concrete to deliver high mechanical performance while supporting sustainable construction practices at elevated biochar incorporation levels. • Flow optimization achieves low porosity and high compressive strength in biochar concrete at high biochar dosage. • Multiple types of biochar can be incorporated into concrete at up to 15 wt% cement while maintaining slump and strength. • SSA ratios of biochars guide flow optimization across biochars with moderate to high carbon content.