Indian scientists develop a new concrete using human waste to make it significantly stronger than conventional cement

Scientists in India have found a surprising way to turn a sanitation problem into a potential construction material: biochar made from treated human waste can replace part of the cement used in concrete and, at the right concentration, make the material stronger.

Researchers from Manipal University Jaipur and Louisiana Tech University produced biochar from faecal sludge collected at a faecal sludge treatment plant in Warangal, Telangana. They then used the processed material to replace 5%, 10% and 15% of cement in concrete and tested how the mixes performed over different curing periods.

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The results, published in Scientific Reports, suggest that the sweet spot may be between 5% and 10% replacement. At these levels, the concrete showed improvements in strength at longer curing ages, while replacing 15% of cement reduced performance.

Human waste does not go directly into the concrete

The idea sounds unusual, but the process is more controlled than it may first appear. The researchers obtained faecal sludge from the Site Sanitation Resource Park in Warangal. The sludge was first air-dried and then placed in an oven at about 105°C for 24 hours.

It was then subjected to controlled pyrolysis inside a muffle furnace under limited oxygen. The material was heated to between 350°C and 450°C and held at the peak temperature for two hours. This converted the dried sludge into a carbon-rich material known as biochar. The biochar was subsequently ground and sieved before being mixed into concrete.

So, the final construction material does not contain untreated faecal sludge. Instead, it contains a processed, carbon-rich product derived from the sludge.

Concrete became stronger as it cured

The researchers prepared conventional concrete mixes and compared them with concrete in which some of the cement was replaced by the sludge-derived biochar. The strongest results appeared at later curing ages.

For concrete containing 10% biochar, the researchers recorded an average increase of about 12% in compressive strength after 56 days and 21% after 91 days compared with the corresponding conventional concrete mixes.

The 5% biochar mixes also performed well. They recorded average compressive-strength increases of about 10% at 56 days and 20% at 91 days. One of the 5% mixes showed a gain of about 22%.

The improvement was even more noticeable in flexural strength, which measures how well concrete resists bending. Concrete containing 10% biochar showed an average increase of 29% after 56 days and 42% after 91 days. The 5% mixes recorded average increases of 25% and 36% at the same curing ages.

But more biochar was not necessarily better. At a 15% replacement level, the researchers observed a decline in strength. The study suggests that too much biochar creates a dilution effect, reducing the amount of cementitious material available to form strength-giving hydration products.

Why does the waste-derived material strengthen concrete?

One of the main reasons appears to be the unusual structure of the biochar. Under a microscope, the material showed a rough and highly porous surface, with numerous tiny cavities. The researchers found pore sizes in the micrometre range.

That structure allows the biochar to absorb and retain considerable amounts of water. The biochar used in the experiment had a water absorption capacity of about 61%. This matters because concrete needs water for cement hydration. Instead of all the water being immediately available in the mix, some of it is held inside the biochar and can be released gradually as the concrete cures.

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The researchers describe this as an internal curing effect. The biochar can therefore act somewhat like a tiny water reservoir inside the cement matrix. As curing continues, the retained water can support further hydration, potentially helping the concrete continue gaining strength beyond the period when conventional concrete is normally assessed.

There is another factor. The biochar contains substantial amounts of silica and calcium oxide. The researchers found that the combined proportion of silicon dioxide, aluminium oxide and ferric oxide was about 52%, meeting the cited ASTM threshold used for pozzolanic materials.

Silica can react with calcium hydroxide produced during cement hydration to form additional cementitious compounds, including calcium-silicate-hydrate, or C-S-H. This material plays an important role in concrete strength.

The researchers also found evidence of a filler effect. Fine biochar particles can occupy spaces within the cement matrix, helping reduce voids and improving particle packing.

The concrete also absorbed less water at lower replacement levels

Strength was not the only property that improved. Concrete containing 5% biochar showed lower water absorption than the control concrete across the curing periods examined. The researchers linked this to the filler effect and the internal curing behaviour of the biochar.

The 10% mixes initially performed less impressively but became comparable with conventional concrete at longer curing ages.

The researchers also observed lower porosity in several of the biochar mixes, which they associated with refinement of the cement matrix and the interfacial transition zone between the cement paste and aggregates.

However, the 15% mixes again stood out for the wrong reason. Their higher biochar content was associated with greater water absorption, possibly because the larger number of biochar pores became connected with capillary pores in the cement matrix.

It can also lock away some heavy metals

There is another potentially important finding. Because the starting material comes from faecal sludge, the researchers specifically examined heavy metals in the biochar and concrete.

They found that concentrations of several metals fell when the biochar was incorporated into the concrete. For example, mercury in the raw biochar was measured at 45.4 ppb. In the tested concrete mixes containing 5%, 10% and 15% biochar, the measured values were 5.09, 24.9 and 35.2 ppb respectively.

Zinc and manganese concentrations also fell substantially in the concrete compared with the raw biochar.

The researchers suggest that the cement-biochar matrix can immobilise or trap heavy metals through processes such as adsorption, precipitation and incorporation into hydration products.

That could be important because safe handling of treated faecal sludge remains a major challenge for sanitation systems.

There is a catch

The findings are promising, but this is not yet a case of replacing large quantities of conventional cement with human-waste-derived material in buildings. The researchers found that 15% replacement reduced strength, while the best mechanical results generally came from 5% and 10% replacement.

The study also found that increasing the amount of biochar reduced the workability of fresh concrete. Its high water absorption means the material can consume some of the freely available water, making the mix stiffer. There are also important questions that the researchers say still need to be answered.

The study was conducted using faecal sludge from a specific treatment plant, meaning the findings cannot automatically be applied to sludge from every location. The researchers say future work should test material from a wider range of treatment plants and geographical regions.

They also want more extensive long-term durability testing, including behaviour under extreme environmental conditions. Importantly, a Toxicity Characteristic Leaching Procedure, or TCLP, analysis is planned in future research to examine the leaching behaviour of potentially toxic contaminants.

Scaling up could also be difficult because pyrolysis requires equipment, energy and investment.

From sanitation waste to construction material

The bigger idea behind the research is not simply stronger concrete. Cement production is responsible for significant carbon emissions, and replacing even a portion of cement with an alternative material could reduce demand for conventional cement. At the same time, converting faecal sludge into biochar could create another route for dealing with waste generated by sanitation systems.

The researchers say their results show that faecal-sludge-derived biochar can serve as a partial cement replacement while delivering comparable or improved mechanical and durability characteristics under the conditions tested.

For now, the most promising result is fairly specific: 5% to 10% biochar replacement produced the strongest overall performance in the experiments, while 15% was too much.

It is an unusual recycling idea, but the science behind it is less strange than the headline suggests. Human waste is not being mixed straight into concrete. It is being processed into a carbon-rich material, carefully characterised and then used in small quantities to modify the cement matrix.

The next question is whether that laboratory success can survive the much messier reality of large-scale construction.