Biochar could help build stronger, lower-carbon cement
Study identifies the biochar traits that matter most for making cement mortars stronger while reducing carbon footprint
image: Relevant biochar characteristics influencing compressive strength of biochar-cement mortars
Credit: Julia Hylton, Aaron Hugen, Steven M. Rowland, Michael Griffin & Lori E. Tunstall
Concrete is everywhere, from roads and bridges to homes, hospitals, and high-rise buildings. But the cement that holds concrete together comes with a major climate cost. Cement production is responsible for a significant share of global carbon dioxide emissions, largely because making cement requires heating limestone to very high temperatures.
A study published in Biochar suggests that biochar, a carbon-rich material made by heating biomass in low-oxygen conditions, could help reduce the carbon footprint of cement-based materials without weakening them. In some cases, it may even make them stronger.
Researchers from Colorado School of Mines and the National Renewable Energy Laboratory tested 16 different commercially produced biochars made from softwood, hardwood, and agricultural feedstocks. Each biochar was milled into fine particles and used to replace 10% of the cement mass in mortar mixtures. After 28 days of curing, every biochar-cement mortar showed comparable or improved compressive strength compared with the control mortar. One biochar mixture increased strength by just over 48%.
“Biochar has often been viewed mainly as a carbon storage material, but our results show that the right biochar can also contribute to the engineering performance of cement-based materials,” said corresponding author Lori E. Tunstall of Colorado School of Mines. “The key is understanding which biochar properties matter, rather than treating all biochars as the same.”
Previous studies have often reported strength losses when biochar replaces more than 2% to 5% of cement. This has limited confidence in using higher biochar dosages in concrete. The new study addresses this challenge by comparing a broad range of biochars under the same mix design and by carefully measuring their physical and chemical characteristics.
The researchers then used statistical and machine learning methods to identify which biochar traits best explained differences in mortar strength. Three characteristics stood out: initial saturation percentage, oxygen-to-carbon ratio, and water-soluble silicon concentration.
Initial saturation percentage describes how much of a biochar’s water-holding capacity is already filled before it enters the cement mix. Biochars with lower initial saturation performed better, likely because they had more available capacity to absorb mixing water and later release it during curing. This supports the idea that biochar can act as an internal curing agent, helping hydration products form within the cement matrix.
Water-soluble silicon had a positive relationship with strength. In cement chemistry, silicon can help form calcium silicate hydrate, often called C-S-H, the main binding phase responsible for concrete strength. Biochars that released more soluble silicon may therefore help strengthen the surrounding cement matrix.
In contrast, a higher oxygen-to-carbon ratio was linked with lower strength. The authors suggest that this ratio may reflect surface functionalization and polarity, which could affect how biochar interacts with the highly alkaline cement environment.
The study also highlights the importance of preparation. All biochars were milled to an average particle size of about 10 to 15 micrometers, helping them integrate more effectively into the mortar. The authors note that biochar is highly variable, depending on feedstock and pyrolysis conditions, so reporting and controlling biochar properties will be essential for future concrete applications.
If optimized, biochar-cement materials could offer a dual benefit: storing stable carbon in long-lived building materials while reducing the amount of cement needed. In the study’s mix design, replacing 10% of cement mass with biochar was estimated to offset about 32% of the mortar’s carbon footprint compared with the same mix without biochar.
“Our findings provide a practical path forward,” Tunstall said. “By selecting or producing biochars with low initial saturation, low oxygen-to-carbon ratios, and useful soluble silicon content, we may be able to increase biochar use in cement without sacrificing strength.”
The authors conclude that biochar has strong potential as a pathway toward high-strength, lower-carbon, and eventually carbon-neutral concrete, but further validation across additional biochars and concrete systems will be needed before large-scale adoption.
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Journal Reference: Hylton, J., Hugen, A., Rowland, S.M. et al. Relevant biochar characteristics influencing compressive strength of biochar-cement mortars. Biochar 6, 87 (2024).
https://doi.org/10.1007/s42773-024-00375-6
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.