Human Faeces Biochar Could Make Concrete Stronger

Concrete is, in the most literal sense, the bedrock of the modern world.

Virtually every building project on the planet relies on the material in one way or another. However, there is one enormous drawback: producing cement, one of concrete's essential components, is exceptionally damaging to the environment.

Heating and processing limestone to create cement ranks among the world's major sources of carbon dioxide emissions. It is therefore unsurprising that scientists have been seeking ways to reduce the problem.

Researchers led by civil engineer Raghuvesh Tiwari at Manipal University Jaipur in India may have found a clever answer that addresses two environmental issues at once.

Their paper, accepted for publication in Scientific Reports, says that incorporating biochar produced from human faecal-sludge slurry processed at faecal sludge treatment plants "yields significant improvements in concrete properties."

Why concrete and cement matter

Concrete's remarkable utility comes from its combination of characteristics. While wet, it is workable and can be cast into almost any form; once cured, it becomes strong enough to carry immense loads and sufficiently durable to endure for decades, or even millennia.

It is normally produced from several ingredients in carefully defined proportions: sand, gravel, water and cement, which is mainly made from limestone.

Unfortunately, the recipe is sensitive. Changing the balance or replacing part of the cement with another material can result in concrete that lacks the strength or longevity of the genuine article. A construction material is of little use if it quite literally cracks under pressure.

Biochar is one increasingly promising replacement for cement. This carbon-rich substance is created by heating organic material in an environment with little oxygen.

One advantage of biochar is that it can be made from a wide range of organic materials.

Humans, meanwhile, generate a considerable volume of organic material: up to roughly 400 grams of faeces per person each day. That may not sound like much for one individual, but multiplied across millions of people, it is remarkable that sewer systems work as effectively as they do.

In any case, those daily tonnes of faeces must go somewhere, and this is where the connections begin to emerge.

In India, faecal sludge treatment plants have been established to manage human waste safely. Their contents can be converted into biochar using pyrolysis, so the researchers asked: if biochar from sawdust, wood, rice husks and other sources can help make concrete, why not biochar from human faeces?

To test the idea, Tiwari and his colleagues collected faecal-sludge biochar from a treatment facility in Warangal, India.

Producing the biochar involves first drying the faecal sludge, then heating it in a low-oxygen setting at 350 to 450 degrees Celsius to create biochar. It is subsequently ground and sieved into a fine powder.

The team used this powder to substitute 5, 10 and 15 percent of the cement in conventional concrete. They then put the resulting poopcrete through a range of tests to determine how well it performed.

Faecal-sludge biochar makes stronger poopcrete

Remarkably, adding a small amount of faeces improved the concrete.

The researchers assessed shrinkage, compressive strength, flexural strength, water absorption and porosity across the various mixtures, with particularly interesting results.

In general, the most robust concrete was produced when poop biochar replaced 5 percent of the cement.

Poopcrete also had another notable quality: it continued to gain considerable strength as it cured. After 91 days, the researchers found that the 5 percent blend achieved average increases of 20 percent in compressive strength and 36 percent in flexural strength.

With 10 percent of the cement replaced, the gains were greater still: 21 percent for compressive strength and an impressive 42 percent for flexural strength.

The increased strength of poopcrete did not appear to come at the expense of several other properties examined by the team.

At 5 percent replacement, it typically took in less water and showed lower porosity than standard concrete, while also experiencing less drying shrinkage. At 10 percent, its overall performance remained broadly similar to that of ordinary concrete.

However, as is so often the case, there can be too much of a good thing - or, in this instance, too much poop.

When 15 percent of the cement was replaced, the concrete still became stronger during curing, but its overall strength was lower than that of the 5- and 10-percent mixtures.

How poop biochar strengthens concrete

Why would a small addition of poop make concrete stronger?

The researchers believe that several mechanisms operate together. Poop biochar is highly porous, with numerous tiny cavities that absorb water before slowly releasing it as the concrete cures.

In effect, the biochar particles act as miniature internal reservoirs, ensuring water remains available for the chemical reactions that harden and reinforce the cement.

Yet the poop does more than simply retain water.

The biochar contains abundant silica, which can react with compounds generated as the cement cures, producing additional calcium silicates that contribute to concrete's strength.

Put another way, it is pozzolanic: part of the same broad category of chemical reactions famously used in ancient Roman concrete, except that human faeces take the place of the volcanic material used by the Romans.

The small biochar particles can also fill spaces within the concrete, improving how tightly its ingredients are packed and bonded.

The researchers were able to observe this effect through a microscope. Concrete containing 5 percent biochar displayed a denser structure with more closely bonded material than conventional concrete.

At 15 percent, however, that structure started to worsen, with additional pores, cracks and weakly bonded areas becoming visible.

This does not mean skyscrapers built from poopcrete will be appearing tomorrow. The researchers stress that further work is required to evaluate its performance in real-world conditions, including freeze-thaw cycles, salinity and extreme temperatures.

There is also the issue of heavy-metal build-up, a recognised concern with sewage sludge. Encapsulating potentially harmful heavy metals in concrete could be beneficial, but it remains unknown how securely they would stay contained or whether they could leach out over time.

The study did not evaluate the effect of this approach on carbon emissions either.

Even so, replacing a portion of concrete's cement with a material derived from a waste stream that humans produce in inexhaustible quantities could potentially address two challenges at once.

And if there is one thing humans can be counted on to generate reliably and in substantial amounts, it is certainly poop.

The research has been accepted for publication in Scientific Reports.

This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. Although we take pride in our process, we are only human. If you notice an error, please let us know.