Concrete made with sewage-derived biochar gets stronger over time, study finds

Concrete used in buildings and bridges is made by mixing cement with sand, gravel and water. Producing cement is resource-intensive, and each ton manufactured releases roughly one ton of carbon dioxide. Cement production accounts for 5 to 8 percent of global CO2 emissions.

To reduce cement use, the construction industry has long substituted a portion of cement with waste materials such as coal ash and rice husk ash. These materials react with byproducts formed when cement hardens with water, generating compounds that strengthen the concrete.

Now, a new study has found that concrete in which 5 to 10 percent of cement was replaced by biochar — a charcoal powder made by roasting sewage sludge from septic tanks — became stronger than standard concrete after three months.

Biochar is a fine charcoal produced by heating organic matter with little or no oxygen. It has traditionally been used to enrich soil or to remediate land contaminated with heavy metals.

A research team led by Jaipur Raghuvanshi Tiwari and Priyansa Mehra of Manipal University in India published their findings in the 2026 issue of the international journal Scientific Reports, measuring the strength and water absorption of concrete in which biochar derived from sewage sludge was substituted for cement.

The research team dried sediment collected from a sewage treatment facility in India, then roasted it in a low-oxygen kiln at 350 to 450 degrees Celsius for two hours to produce the biochar.

The team prepared 20 concrete mixes by varying the water-to-cement ratio across five levels and replacing 0, 5, 10 or 15 percent of the cement by weight with biochar. Compressive strength — resistance to crushing — and flexural strength — resistance to bending — were measured at four, eight and 13 weeks as the concrete cured.

Concrete with 5 percent of its cement replaced by biochar recorded the highest compressive and flexural strength at all three measurement points. Concrete with 10 percent of its cement replaced by biochar showed lower compressive and flexural strength than standard concrete at the four-week mark.

Concrete strength typically plateaus after four weeks of curing, making that point the standard benchmark. Biochar-infused concrete, however, continued to harden beyond that mark. The mix with 10 percent biochar showed average compressive strength 21 percent higher at 13 weeks than at four weeks, and flexural strength 42 percent higher, ultimately surpassing most standard concrete mixes by a small margin.

The research team said silicon compounds in the biochar reacted with calcium hydroxide produced as cement hardened, generating additional strengthening material, and that biochar particles also filled microscopic gaps between cement grains, further boosting strength.

Concrete with 15 percent of its cement replaced by biochar was generally weaker than standard concrete, as the amount of cement binding the mix together had been reduced too significantly.

Given that the material originated from sewage, the team also measured heavy metal concentrations in the concrete. Levels of mercury, copper, zinc and other metals in the biochar-infused concrete were lower than in the raw biochar itself. However, heavy metal concentrations in the concrete rose as the biochar substitution rate increased from 5 to 15 percent.

The team acknowledged several limitations: the study used sewage sludge from only one treatment facility, and all testing was conducted under controlled laboratory conditions.

The researchers also said they were unable to assess how the concrete would hold up in extreme environments such as harsh weather or high-salinity coastal settings. They noted that whether heavy metals could leach out of the concrete into water was also left untested.

DOI: 10.1038/s41598-026-66956-6

Tiwari, R., Mehra, P., Hussain, S. et al. Mechanical, durability, and microstructural performance of biochar-modified concrete using faecal sludge-derived biochar. Sci Rep (2026).