Researchers took biochar made from human waste at a treatment plant in Warangal, India, ground it into a fine black powder and mixed it into concrete in place of a tenth of the cement. After 91 days the concrete was 21 percent harder to crush.

Concrete is the second most-used substance on Earth after water, and the cement that holds it together is one of the biggest headaches in climate policy.

Making cement means heating limestone until it breaks down, a process that accounts for around 8 percent of global CO2 emissions. And we are not slowing down. Concrete production is projected to climb from roughly 14 billion cubic metres a year toward 20 billion by mid-century. So anything that lets you use less cement per wall, without the wall getting weaker, is worth a close look.

That is the claim in a new paper out of India, and it comes with an unusual ingredient: human waste. A team turned treated sewage sludge into a fine black powder, swapped it in for a tenth of the cement, and got concrete that ended up harder to crush, not softer.

It’s one study, and an early-access version still subject to edits, so treat what follows as a promising result, not a settled fact.

Why cement is the part worth fixing

The problem with concrete is not the gravel or the sand or the water. It is the cement paste that binds them together. Estimates of cement’s climate footprint vary a bit depending on how you count, landing somewhere between 5 and 8 percent of global human-caused CO2 emissions each year. Most of that comes from the chemistry of making cement, not the fuel in the furnace. That is why switching to cleaner energy alone will not fix it.

One line of research has spent years asking whether you can replace some of that cement with biochar. Biochar is a carbon-rich char made by heating organic matter with very little oxygen. The appeal is twofold: you use less cement, and the carbon in the char, which the original plant or waste pulled out of the air, gets locked inside the building instead of released. The open question has always been whether the concrete survives the swap.

What the Warangal team actually did

The material here started at a sewage-sludge treatment plant in Warangal, in the Indian state of Telangana, a facility set up as a public-private partnership to tackle the city’s sewage problem. Human solid waste that would otherwise be dumped or burned gets processed there.

V Srinivas Chary, whose institution is a partner on the project, praised the operation, saying that “From the toilet to the final conversion of the biochar, the process is scientific and comprehensive.” 

The research team, led by civil engineer Raghuvesh Tiwari of Manipal University Jaipur with colleagues at Louisiana Tech University, took that sludge, dried it, and heated it to between 350 and 450 degrees Celsius with very little oxygen. That heating step drives off gases and leaves char behind. They ground the char into a powder, then replaced 5, 10, and 15 percent of the cement in test concrete with it and ran the usual checks: how well it resisted crushing and bending, how much it shrank, how much water it soaked up, how many gaps it had inside.

Dumped waste versus a carbon sink in the wall

The study sets up a pointed contrast. Normally a city’s sewage sludge is a liability to be disposed of, and a builder’s cement is a material whose production loads the atmosphere. Both are problems flowing toward waste and emissions. The first is bigger than you might think: a single person produces up to around 400 grams of faeces a day, which across a city becomes a constant stream that has to go somewhere.

The idea here is to route that stream into the wall. The waste replaces some of the cement, and the carbon in the char stays put instead of being burned off. It is a tidy story on paper: two problems that might cancel out a slice of each other. The caveat is that this study did not actually measure the carbon side of that ledger, so the climate benefit is still an argument rather than a number the paper delivered.

Why it got stronger, not weaker

The surprise is that adding a char to concrete made it tougher. The study reports this clearly for the middle dose. As the authors put it, “After a 91-day curing period, concrete with a 10% biochar substitution demonstrated a 21% increase in compressive strength and a 42% increase in flexural strength.” The 5 percent mix was close behind, gaining about 20 percent in crushing strength and 36 percent in bending strength over the same period. Those gains are measured against this study’s own plain concrete, not a universal benchmark.

There is a limit, though, and the study found it. The 15 percent mix did worse. Past a certain point, the char stops helping and simply dilutes the cement, a useful reminder that the good result here depends on getting the dose right. The authors’ own summary is careful: the results indicate that biochar at the 5 and 10 percent levels “yields significant improvements in concrete properties.”

The catch, and what still has to be proven

A strong lab result is not a paving slab. Several things stand between this and a real building site, and the paper is upfront about the biggest one. Sewage sludge can carry heavy metals, and whether those slowly leak out of the finished concrete over time is an open question. That is not something you can wave away when the material might end up in a school foundation or a water channel.

How long it lasts is the other unknown. The tests ran to 91 days in controlled conditions. Real structures face decades of freezing and thawing, wetting and drying, weight and vibration, and nobody has shown yet how this concrete holds up over that span. The researchers say themselves that more work is needed to assess how it performs under real-world conditions such as freeze-thaw cycles and extreme temperatures. 

What the study earns, we think, is attention rather than adoption. The explanation is plausible, the numbers are real for the mix that was tested, and the idea that a waste stream and a carbon-heavy material might each solve part of the other’s problem is worth chasing.