New eco-friendly building blocks cut carbon emissions and reduce toxic metal leaching

A team of scientists has created low-carbon building blocks that not only reduce greenhouse gas emissions but also minimize the leaching of toxic metals when construction waste reaches the end of its life cycle. The findings, published in Biochar, highlight the potential of biochar-amended fly ash cement blocks as a sustainable alternative for the construction industry.

Cement production is responsible for about 8% of global carbon dioxide (CO₂) emissions, making it one of the most carbon-intensive industries worldwide. Fly ash, a byproduct of coal combustion, is widely used as a supplementary cementitious material, but it can leach harmful elements such as aluminum, selenium, and chromium into the environment. Addressing both issues, the researchers replaced part of the fly ash with rice husk biochar, a porous carbon-rich material produced from biomass pyrolysis.

The study tested building blocks containing 2%, 4%, and 6% biochar by weight. After 28 days of curing, the blocks achieved compressive strengths of 8–12 megapascals, meeting Eurocode 6 standards for use under severe conditions. Importantly, leaching tests under simulated acid rain showed that with 6% biochar, aluminum, selenium, barium, and chromium concentrations dropped by 72%, 48%, 58%, and 53%, respectively. While aluminum levels remained above drinking water limits, chromium concentrations fell below regulatory thresholds when biochar content exceeded 4%.

“Biochar serves a dual role—enhancing the pore structure of cement blocks to improve strength, and binding metals to reduce environmental risks,” said corresponding author Professor Ajit K. Sarmah of the University of Auckland. “This makes it a promising material for sustainable construction and safer waste management.”

Leaching experiments across varying pH levels revealed typical patterns: aluminum followed an amphoteric trend, while chromium showed an oxyanionic trend, with the lowest leaching occurring near neutral pH. These insights are valuable for geochemical modeling and for guiding policymakers and landfill operators in assessing risks from demolition waste.

Beyond environmental benefits, biochar also acts as a carbon sink, permanently locking away CO₂. The research team suggests that further work, such as large-scale field studies and life-cycle assessments, could strengthen the case for deploying biochar-based construction materials globally.

“This study offers a blueprint for developing low-carbon building materials that perform well mechanically while addressing long-term environmental concerns,” said lead author Sai Praneeth.

The findings point toward a future where construction practices contribute not only to structural safety but also to climate change mitigation and pollution control. 

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Reference: Praneeth, S., Bordoloi, S. & Sarmah, A.K. Low carbon biochar amended fly ash–cement building blocks: Assessment of metal leaching scenarios. Biochar 7, 80 (2025). https://doi.org/10.1007/s42773-025-00472-0 

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Media Contact:
Wushuang Li
liwushuang@vip.126.com

About Biochar

Biochar 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. 

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