The incorporation of biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More into concrete presents a paradox: while it offers substantial environmental benefits by reducing cement consumption and enabling carbon storage, its ‘general’ high porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More (see Figure 1) is detrimental for the fresh concrete properties, which are essential for practical construction applications.

Biochar can be engineered to have high porosity when the thermo-chemical conversion happens at higher temperature (ca. > 500 ℃ and a long retention time, ca. > 1h). This high porosity can result in a water absorption capacity of 2.20  g/g [1].  When such biochar is added to concrete mix, at replacement levels as low as 5 wt.%, it can reduce concrete slump from ca. 95% to 73%, necessitating a 62% increase in superplasticiser dosage to restore the workability [2]. Furthermore, biochar is considerably lighter than cement, with a bulk density roughly one-fourth that of cement [2].

Consequently, assuming biochar and cement have similar particle shapes and sizes, a mass-based replacement introduces a substantially larger number of biochar particles into the cementitious matrix than the cement particles they displace. This excess of highly porous particles absorbs significant water from the paste, thus, severely diminishing concrete workability.

To address this issue, Ferreira et al. [3] emphasised that a volumetric replacement approach should be adopted to maintain acceptable rheological performance for porous supplementary cementitious materials. Both apparent particle density and bulk density of porous supplementary cementitious materials can be used to determine the appropriate method for replacing cement. However, it must be recognised that biochar contains numerous non-connected pores [4], broadly categorised as water-accessible and water-inaccessible. When fully saturated, water fills all water-accessible surface pores on biochar. However, concrete mixtures have limited free water due to low water-to-cement ratios, preventing biochar from reaching full absorption and leaving many pores only partially filled. This behaviour mirrors that of coral aggregate [5], which requires partial pre-wetting.

Therefore, using apparent particle density leads to an increased slurry volume due to additional voids initially disregarded in the assumption. In contrast, the bulk density method effectively reduces paste volume [2]. Importantly, bulk density introduces fewer biochar particles compared to particle-density-based replacement. This reduction lowers water absorption from the paste and minimises inter-particle friction, helping to preserve workability. Moreover, since porous biochar inherently creates weak zones within the concrete, a lower particle count results in fewer weak zones, ultimately enhancing both strength and durability. Therefore, volumetric replacements based on bulk density of biochar is recommended for use in concrete.

References:

[1] R.A. Mensaha, D. Wang, V. Shanmugama, G. Sasa, M. Förstha, O. Dasa, Fire Behaviour of Biochar-based Cementitious Composites, JCOMC  (2024). https://doi.org/10.1016/j.jcomc.2024.100471.

[2] D. Wang, G. Sas, O. Das, The importance of volumetric w/c for porous supplementary cementitious materials in concrete, J. Build. Eng. 111 (2025) 113290. https://doi.org/10.1016/j.jobe.2025.113290.

[3] R.L.S. Ferreira, L. Pinto, A.F. Nóbrega, A.M.P. Carneiro, Diatomaceous earth: A review of its characteristics and effects on the properties of mortars, Constr. Build. Mater. 421 (2024) 135711. https://doi.org/10.1016/j.conbuildmat.2024.135711.

[4] Y. Li, B.H. Ding, X. Geng, Effect of biochar on microplastics penetration treatment within soil porous medium under the wetting-drying cycles and optimisation of soil-biochar mixing format, Sci. Total Environ. 935 (2024) 173194. https://doi.org/10.1016/j.scitotenv.2024.173194.

[5] Wang, B. Luo, Q. Feng, W. Zhang, M. Elchalakani, F. Xu, Development of Preplaced Alkali-Activated Coral Concrete for a Marine Environment, J. Mater. Civ. Eng. 36 (2024). https://doi.org/10.1061/JMCEE7.MTENG-16226.