Fly ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More, a waste product of coal combustion, is a major environmental concern due to its leachable heavy metals. But new research shows that by combining it with 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, a solid byproduct of biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More, it can be transformed into sustainable building materials with a significantly reduced risk of contamination. In a study published in the journal Biochar, authors Sai Praneeth, Sanandam Bordoloi, and Ajit K. Sarmah found that adding just 6% biochar to fly ash-cement building blocks reduced the leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More of heavy metals like aluminum, selenium, barium, and chromium by 72%, 48%, 58%, and 53%, respectively.

The research addressed the dual challenges of reducing carbon emissions from cement production and mitigating environmental risks from waste materials. The team created eco-friendly mortar bricks by replacing a portion of the fly ash with biochar, using only 2% Portland cement in the mix. They tested various mix designs and found that blocks with 2% biochar achieved the highest compressive strength, increasing the strength of the control samples (without biochar) by 10%. This increased strength is attributed to biochar’s ability to act as a pore-filling agent, improving water retention and allowing for a more complete internal curing process. Even at higher concentrations, all the biochar-amended blocks achieved a compressive strength of 8–12 MPa, meeting Eurocode 6 standards for use in harsh environmental conditions.

To assess the environmental risks of using these blocks, the researchers conducted a series of leaching tests that simulated real-world conditions, such as acid rain and varying liquid-to-solid ratios. The results showed that while raw fly ash is not classified as hazardous waste for landfills, its aluminum (1.84 mg/L), selenium (1.04 mg/L), and chromium (1.90 mg/L) concentrations still exceeded drinking water limits. However, the inclusion of biochar significantly mitigated these risks. At a 6% biochar amendment rate, concentrations of aluminum, selenium, barium, and chromium were reduced by more than half, with chromium levels specifically dropping below USEPA drinking water limits when biochar exceeded 4%.

The study also investigated how pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More affects metal leaching. Both aluminum and chromium leaching were highest under extremely acidic (pH 2-3) and strongly alkaline (pH 12-13) conditions. Aluminum exhibited a “U-shaped” amphoteric leaching trend, while chromium followed an oxyanionic pattern, with minimal leaching for both elements at a neutral pH. The authors found that pH was the dominant factor controlling metal leaching, with biochar having a negligible influence on this behavior.

Overall, this research demonstrates that incorporating a small amount of biochar into fly ash-cement blocks not only creates a stronger, more sustainable building material but also significantly reduces the potential for heavy metal contamination at the end of its life cycle. These findings provide a valuable framework for developing new sustainable construction materials and for informing the management of demolition waste.

Source: Praneeth, S., Bordoloi, S., & Sarmah, A. K. (2025). Low carbon biochar amended fly ash-cement building blocks: Assessment of metal leaching scenarios. Biochar, 7(1), 80.