High-temperature properties of biochar-concrete

• Biochar increases mass loss of concrete after high-temperature exposure • Biochar replacing cement mitigates concrete strength loss at high temperatures • Replacing sand with biochar aggregate lowers thermal conductivity and causes thermal damage This study investigates the high-temperature performance of concrete (representing fire exposure) where cement and sand were partially replaced with biochar powder and biochar aggregate, respectively. The sand replacement levels were 30 vol.% and 60 vol.%, and the cement replacement ratios were 10 vol.% and 20 vol.%. The target temperatures were 400°C and 800°C. The results indicate that biochar powder as the cement replacement had a limited effect on the temperature gradients in concrete. In contrast, biochar aggregate significantly increased the temperature gradients in concrete due to its substantial volume, which leads to destructive thermal damage. Biochar increases the mass loss of concrete after exposure to high temperatures, and its use as an aggregate further enhances this effect. Similar to the control samples, concrete with biochar powder exhibited a compressive strength increase after 400°C exposure due to accelerated hydration. However, concrete with biochar aggregate showed no such strength gain, as the beneficial effect was negated by destructive thermal damage.