Scientists Turn Human Waste Into Concrete, And One Mix Got 42% Stronger

Human waste usually has one destination: the sewer. Now, researchers in India have found a bizarre but potentially useful second life for it by turning processed fecal sludge into biochar and using that material as part of a concrete mixture.

The results were enough to make the unusual experiment stand out. After 91 days of curing, concrete containing 10% fecal-sludge biochar recorded a 42% increase in flexural strength, while other tests also showed improvements in several properties of the material.

Scientists Are Testing A Very Unusual Cement Substitute

Concrete is so common that most people barely think about what goes into it. Roads, bridges, houses, tunnels and huge buildings all rely on the material, which can be poured into different shapes when wet and becomes strong enough to withstand enormous loads once it has cured.

The problem is that cement, one of concrete’s main ingredients, carries a significant environmental cost. Producing it requires limestone to be heated and processed at high temperatures, a process responsible for substantial carbon dioxide emissions and one reason researchers are searching for alternatives.

A team led by civil engineer Raghuvesh Tiwari of Manipal University Jaipur in India decided to investigate whether one particularly abundant waste material could replace a portion of the cement used in concrete.

That waste material was human fecal sludge.

The researchers obtained fecal-sludge biochar from a treatment plant in Warangal, India, then processed it into a fine powder that could be incorporated into concrete. Their research found that the resulting material could produce “significant improvements in concrete properties.”

The experiment was not about dumping untreated sewage into construction materials. Instead, the researchers used a carefully processed product created through a controlled heating process.

Human Waste Was Turned Into Biochar First

Before the material could become part of the concrete, the fecal sludge had to undergo several stages of processing. The sludge was first dried and then heated in an oxygen-poor environment at temperatures between 350°C and 450°C.

That process, known as pyrolysis, produces a carbon-rich material called biochar. Once the biochar was created, the researchers ground and sieved it into a fine powder so it could be incorporated into the concrete mixture.

Biochar is already being studied as an alternative material for construction because it can be produced from many different types of organic matter. Researchers have investigated biochar made from sources including wood, sawdust and rice husks.

The Indian team wanted to know whether fecal sludge could produce a material with similar useful characteristics. To test the idea, they created several concrete mixtures in which different portions of the cement were replaced with fecal-sludge biochar.

The replacement levels were 5%, 10% and 15%. Researchers then subjected the samples to a series of tests designed to measure how the unusual concrete performed.

Those tests included compressive strength, flexural strength, water absorption, porosity and drying shrinkage.

One Concrete Mix Became 42% Stronger

The results showed that the amount of biochar made a major difference to how the concrete performed. The strongest overall mixture was generally the one containing 5% fecal-sludge biochar, although the 10% mixture produced the largest reported improvement in one important measure.

After 91 days of curing, the 5% mixture recorded an average 20% increase in compressive strength and a 36% increase in flexural strength compared with conventional concrete.

The 10% mixture produced an even larger increase in flexural strength. Researchers recorded a 21% increase in compressive strength and a 42% increase in flexural strength after 91 days of curing.

That 42% figure is the statistic that makes the experiment especially eye-catching, but it needs some context. It does not mean all concrete containing human-waste biochar becomes 42% stronger, because the result depended on the specific mixture and testing conditions.

The researchers also discovered that increasing the amount of biochar did not continue improving the material indefinitely. When 15% of the cement was replaced, the concrete still gained strength during curing, but its overall performance fell behind the 5% and 10% mixtures.

That suggests the unusual ingredient works best within a relatively narrow range.

Tiny Pores Could Be Helping The Concrete Cure

So why would a material made from processed human waste make concrete stronger in the first place? The researchers believe several physical and chemical effects could be working together inside the mixture.

One important characteristic is the highly porous structure of the biochar. Its tiny cavities can absorb water and gradually release it as the concrete cures, effectively creating microscopic reservoirs that keep moisture available for the chemical reactions responsible for hardening.

That process could help explain why the concrete continued developing strength over the longer curing period. Instead of the water simply disappearing from the mixture, some of it could remain stored inside the porous biochar particles and become available later.

The researchers also found that the biochar was rich in silica, which introduces another possible explanation for the strength improvements. Silica can react with compounds created as cement cures and contribute to the formation of calcium silicates associated with concrete strength.

This type of reaction is known as pozzolanic activity. Similar chemistry has been used in construction materials for centuries, including in ancient Roman concrete, although the Romans obviously were not using processed human fecal sludge as their source.

The Biochar Can Also Fill Tiny Gaps

The physical structure of the particles may provide another advantage. Because the biochar was ground into a fine powder, its particles could occupy small spaces between other components in the concrete mixture.

A more tightly packed mixture can reduce unwanted gaps and improve the way different ingredients bond together. The researchers were able to observe differences in the concrete’s internal structure under a microscope.

The 5% biochar mixture showed a denser and more tightly bonded structure than conventional concrete. That observation fits with the strength measurements recorded during the experiment.

However, the microscopic images also showed why more biochar was not necessarily better. At the 15% replacement level, the structure began to deteriorate, with more pores, cracks and poorly bonded regions appearing within the material.

That could explain why the highest replacement level did not outperform the lower concentrations. The researchers appear to have found a point where adding more of the unusual material begins to undermine the structure it was supposed to improve.

It Could Give Human Waste A Second Life

The appeal of the research goes beyond the bizarre headline. Human fecal sludge already has to be collected, treated and managed, particularly in rapidly growing urban areas where sanitation systems deal with enormous quantities of waste.

If a portion of that waste could be converted into a useful construction material, it could potentially create another destination for a waste stream that otherwise requires treatment and disposal.

At the same time, reducing the amount of cement required in some concrete mixtures could potentially reduce demand for an ingredient associated with significant carbon dioxide emissions. The researchers are therefore investigating an approach that could connect two very different environmental challenges.

Several potential benefits stand out:

• Less cement could be required: Replacing a portion of cement could reduce the quantity needed in certain concrete mixtures.
• Waste could become a resource: Treated fecal sludge could potentially be converted into a construction material rather than being viewed only as something requiring disposal.
• Concrete properties could improve: The 5% and 10% mixtures produced meaningful strength improvements under the laboratory conditions tested.
• Existing treatment plants could supply the material: The researchers obtained their fecal-sludge biochar from a treatment facility in Warangal, India.

The concept sounds almost too convenient, but there are still major questions that researchers need to answer before this material could be considered for widespread construction.

The Researchers Say More Testing Is Needed

Laboratory concrete samples are tested under controlled conditions, while buildings and infrastructure have to survive years of changing weather and environmental stress. A material that performs well in a controlled experiment still needs extensive testing before engineers could safely use it in major structures.

The researchers specifically noted the need to examine how the biochar concrete performs under conditions such as freeze-thaw cycles, salinity and extreme temperatures. Those tests could reveal problems that are impossible to identify from short-term laboratory experiments alone.

There is also an important environmental question that the current research does not answer. The study did not assess the overall impact of the method on carbon emissions, meaning the experiment cannot yet establish whether the entire process produces a meaningful reduction in greenhouse gases.

Replacing cement could potentially lower emissions associated with cement production, but producing, heating, processing and transporting biochar also require energy and resources.

A proper lifecycle assessment would therefore be needed before the environmental benefits could be measured accurately.

Heavy Metals Could Become Another Problem

Sewage sludge can contain heavy metals, creating a separate concern when researchers consider turning treated waste into construction materials. The presence of those substances does not automatically make the concrete unsafe, but scientists need to understand exactly what happens to them after the biochar becomes part of the finished material.

One possibility is that concrete could effectively lock potentially harmful substances inside its structure. That could potentially reduce their exposure to the surrounding environment, but researchers do not yet know how stable that containment would remain over long periods.

They also need to investigate whether heavy metals could eventually leach out of the concrete under different environmental conditions. That question would become particularly important if fecal-sludge biochar were ever used on a much larger scale.

For now, those uncertainties mean the material remains a research subject rather than a ready-made replacement for conventional concrete.

More Poop Did Not Make Better Concrete

Perhaps the most revealing result from the experiment is that the researchers did not simply find that more fecal biochar produced stronger concrete. The results instead suggest that there is a useful balance between adding enough biochar to improve the mixture and adding so much that the concrete’s structure begins to suffer.

At 5%, the biochar concrete generally absorbed less water and had lower porosity than conventional concrete. Its drying shrinkage was also lower, giving the mixture several advantages beyond its measured strength.

At 10%, the concrete continued to perform well and produced the striking 42% increase in flexural strength after 91 days. At 15%, however, the internal structure became less favorable and overall strength lagged behind the lower replacement levels.

That finding could prove useful as researchers continue experimenting with waste-derived construction materials. The objective is not simply to put as much waste as possible into concrete, but to identify a mixture that delivers useful performance without creating new problems.

The Weirdest Ingredient Could Have A Practical Future

The idea of using human waste in concrete sounds like something designed purely to grab attention, but the science behind the experiment is considerably more practical.

Fecal sludge can be processed through pyrolysis to create biochar. That biochar has a porous structure, contains silica and can help fill microscopic spaces within concrete, giving researchers several possible explanations for the improvements they observed.

The research does not prove that buildings will soon be constructed with poop-derived concrete. It does show that a material humans produce every day and normally treat as waste can be processed into something with potentially useful engineering properties.

The next step is testing whether those laboratory results survive real-world conditions, while researchers also investigate durability, heavy-metal behavior and the method’s overall carbon footprint.

For now, one of the world’s most unavoidable waste products has passed a surprisingly successful test: after being processed, a little human waste helped make concrete stronger.

Sources:

• Tiwari, R., Mehra, P., Hussain, S., & Anand, S. (2026). Mechanical, durability, and microstructural performance of biochar-modified concrete using faecal sludge-derived biochar. Scientific Reports. https://doi.org/10.1038/s41598-026-66956-6