Indian Study Shows Sewage Sludge Biochar Boosts Concrete Flexural Strength by 42%

Concrete is among the most widely used construction materials in the world, and efforts to make it more sustainable have gained momentum in recent years. Now, Indian researchers have reported that incorporating biochar made from treated sewage sludge into concrete can significantly improve its strength, offering a potential way to reduce the environmental footprint of construction while managing sanitation waste.

The study, led by civil engineer Raghuvesh Tiwari of Manipal University Jaipur, was accepted for publication in the peer-reviewed journal Scientific Reports. The research team used sewage sludge collected from a treatment plant in Warangal, Telangana. The sludge was dried and heated to temperatures between 350 and 450 degrees Celsius in a low-oxygen environment, a process known as pyrolysis. The resulting carbon-rich biochar was ground into a fine powder.

In the experiments, the researchers replaced 5%, 10% and 15% of the cement in concrete mixes with this biochar. They then tested the cured concrete for compressive strength, flexural strength, water absorption, shrinkage and porosity. Compressive strength is a material's ability to withstand axial loads, while flexural strength measures resistance to bending and is critical for structures such as beams and bridges.

The results showed that moderate replacement levels produced notable improvements. Concrete in which 5% of the cement was replaced by biochar gained 20% in compressive strength and 36% in flexural strength compared with conventional concrete after 91 days of curing. When the replacement level was raised to 10%, flexural strength increased by 42%, while compressive strength rose by 21%.

The researchers attributed these gains to biochar's porous structure, which allows it to absorb water and release it slowly during the curing process. This prolonged moisture availability may help the hydration reactions that underlie concrete's hardening and strength development. In addition, the biochar contains silica, which can react with cement compounds to form substances that further enhance mechanical properties.

However, the relationship was not linear. Raising the biochar content to 15% led to more pores and cracks within the concrete, causing its overall strength to decline. This indicates that there is an optimal dosage, likely in the lower range, before the material begins to disrupt the concrete matrix.

The potential benefits extend beyond strength. Cement production accounts for roughly 8% of global carbon dioxide emissions, according to international agencies. Substituting a fraction of cement with sewage-derived biochar could help lower industrial emissions and reduce the burden of municipal waste, which is a growing challenge in rapidly urbanizing countries.

Nevertheless, the researchers caution that the material is not ready for real-world construction. Further investigations are needed to assess its behaviour under extreme temperatures, salt exposure and freeze-thaw cycles, which could affect durability. A critical safety concern is the possible release of heavy metals from sewage-based biochar into the environment. Rigorous testing for leaching and long-term stability will be necessary to ensure the material is safe for public use.

The findings add to a growing body of research exploring the use of agricultural and municipal waste-derived biochars as supplements in cement-based materials. If proven viable, this approach could contribute both to greener concrete and to scientific sanitation practices.