Image Courtesy: Scientific Reports

Concrete is one of the world’s most widely used construction materials, but its key ingredient, cement, comes with a major environmental cost. Researchers in India may have found an unusual way to reduce the amount of cement needed while improving certain properties of concrete: biochar made from processed human waste.

A team led by civil engineer Raghuvesh Tiwari of Manipal University Jaipur tested fecal-sludge biochar as a partial replacement for cement. Their results showed that some mixes became stronger and less porous during curing, with the 10 percent replacement producing a 42 percent improvement in flexural strength after 91 days.

The researchers obtained the biochar from a fecal sludge treatment plant in Warangal, India. Human waste collected at such facilities can be dried and heated through pyrolysis, a process that uses temperatures of roughly 350 to 450 degrees Celsius in a low-oxygen environment. The resulting carbon-rich material was then ground and sieved into a fine powder.

The team replaced 5, 10, and 15 percent of the cement in conventional concrete with the processed biochar. They then tested each mixture for compressive and flexural strength, water absorption, porosity, and drying shrinkage.

The 5 percent mixture generally delivered the strongest overall performance. After 91 days of curing, it showed average increases of about 20 percent in compressive strength and 36 percent in flexural strength. The 10 percent mixture recorded a 21 percent increase in compressive strength and a 42 percent increase in flexural strength.

The researchers believe the biochar’s porous structure plays an important role. Its tiny cavities can absorb water and gradually release it during curing, effectively providing additional moisture for the chemical reactions that strengthen concrete.

The material may also contribute silica that reacts with compounds produced during cement hydration, creating additional calcium silicates. Fine biochar particles can further fill microscopic gaps and improve how tightly the concrete’s components bond.

However, more biochar is not necessarily better. At 15 percent cement replacement, the concrete’s strength fell behind the lower replacement levels, while microscopic analysis revealed more pores, cracks, and poorly bonded areas.

The researchers say additional testing is needed before the material could be considered for widespread construction. Its performance under freeze-thaw cycles, high salinity, and extreme temperatures remains unclear, while potential heavy-metal leaching from sewage-derived material also requires investigation. The study did not assess its effect on carbon emissions.

If those concerns can be addressed, turning human waste into a useful construction ingredient could offer a way to reduce waste while using less cement, potentially tackling two environ