Biochar made from human waste kept concrete stable and may cut metal leaching

In laboratory tests, replacing some cement with faecal-sludge biochar also increased concrete strength at selected doses.

Editorial illustration — not from the study.

Researchers tested biochar produced from faecal sludge collected at treatment plants as a partial replacement for cement in concrete. They measured strength, water absorption, porosity, drying shrinkage and changes in the material’s internal structure.

At a 10% replacement level, concrete had higher compressive and flexural strength after 91 days. A 10% biochar mortar had drying shrinkage similar to ordinary Portland cement mortar after 120 days, while concrete containing 15% biochar had water absorption and porosity comparable to control concrete after 28 days.

What the biochar did

The strongest reported results came at selected replacement levels. After 91 days of curing, concrete with 10% biochar had a 21% increase in compressive strength and a 42% increase in flexural strength compared with the control mix. After 28 days, concrete containing 15% biochar had water absorption and porosity comparable to conventional concrete. After 120 days, drying shrinkage in mortar with 10% biochar was similar to that of ordinary Portland cement mortar. The researchers also reported lower heavy-metal concentrations in concrete specimens as biochar content increased.

Why waste-derived concrete matters

The findings indicate that faecal-sludge-derived biochar could partly replace cement without necessarily compromising the tested strength, porosity or dimensional-stability measures at the reported doses. Using this material could also help manage a waste stream while potentially lowering the release of harmful heavy metals from concrete, although the study does not establish how the material would perform in all construction settings.

Evidence and open questions

This was a materials study based on laboratory measurements of concrete and mortar after specified curing periods. The abstract reports results for selected biochar replacement levels, but it does not provide the full mix designs, testing conditions, statistical details or a detailed account of the heavy-metal leaching tests. The results therefore show performance under the study’s test conditions, not long-term field performance or the safety of every faecal-sludge-derived biochar product.

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