In a study published in the journal Materials, Gintautas Tamošaitis and his team from Kaunas University of Technology explored a dual-purpose approach for using biochar waste (BW) in alkali-activated slag binders. The researchers produced biochar by heating wood waste at 600 °C for six hours in an oxygen-free environment. They then tested its impact on “geopolymer” concrete made primarily from industrial metallurgical slag and a zeolitic by-product. The study distinguished between using tiny amounts of biochar (up to 0.5%) as a chemical additive and larger amounts (1% to 25%) as a bulk filler.

The results revealed a delicate balance between strength and insulation. When used as an additive at the optimal level of 0.25%, the biochar particles distributed themselves evenly throughout the mixture, acting as “crystallization centers” that helped form a denser, more robust internal structure. This specialized concrete achieved a 28-day compressive strength of 44.4 MPa, outperforming reference samples that contained no biochar. High-resolution imaging confirmed that the biochar adhered perfectly to the surrounding cement-like matrix, with healthy hydration products growing directly inside the biochar’s tiny pores and channels.

However, the material’s properties changed dramatically when biochar was added in larger quantities. As a filler, biochar introduced more air and 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, which caused the concrete’s density to drop significantly. While this reduced the overall strength, it turned the concrete into an exceptional insulator. The addition of just 5% biochar filler reduced the material’s thermal conductivity from 0.58 W/(m·K) to a much lower 0.29 W/(m·K)—effectively doubling its ability to trap heat. For builders, this means walls made with biochar-slag composites could keep homes warmer while using less energy.

The study also highlighted the material’s acoustic benefits. Urban noise pollution is a growing concern, and porous biochar proved to be an effective tool for blocking unwanted sound. In high-frequency ranges—where human ears are most sensitive—the samples containing the highest biochar content (25%) reduced noise levels by approximately 30% compared to a standard environment. This noise reduction, combined with low weight and superior insulation, positions biochar-slag composites as a versatile solution for modern sustainable construction. By turning industrial by-products into high-value building blocks, this research offers a practical path toward carbon-neutral and energy-efficient architecture.

Source: Tamošaitis, G., Vaičiukynienė, D., Kantautas, A., Villalón Fornés, I., Borg, R. P., & Vitola, L. (2026). Lightweight, Heat-Insulating, Alkali-Activated Slag Composites with Carbon-Based Biochar Additive and Filler. Materials, 19(2), 277. https://doi.org/10.3390/ma19020277