Scientists Have a Secret Ingredient for Better Concrete: Your Poop

3 min readHere’s what you’ll learn when you read this story:Cement is responsible for 88 percent of the concrete industry’s total greenhouse gas emissions, which account for a staggering 8 percent of overall emissions worldwide.A new study focuses on a way to incorporate biochar, derived from human feces, to not only lower cement’s emissions but also improve its compression and strength. The study found that introducing between 5 to 10 percent of this porous, poop-based biochar created tiny reservoirs that provided the necessary water to form stable calcium silicates.Today’s world is built with concrete. Every year, the world produces roughly four billion tonnes of the moldable, durable slurry of sand, gravel, water, and cement. While concrete is necessary for the homes, office buildings, roads, and bridges to make life possible, its creation takes an environmental toll in more ways than one. A study earlier this year, for example, warned that countries are exceeding the rates at which Earth can naturally replenish river sand, the go-to aggregate for most concrete. But the most worrisome item on concrete’s ingredient list is cement.Although it makes up only seven to 15 percent of most concrete mixes, cement accounts for roughly 88 percent of the carbon dioxide emissions associated with concrete production, mainly due to the extreme heat required to break down limestone. In total, the concrete industry accounts for roughly 8 percent of our global greenhouse gas emissions.This environmental cost is one of the big reasons scientists and engineers have searched for various ways to amend concrete’s list of ingredients to not only replace cement, but maybe even stumble across a better, stronger, and more durable material. Last year, scientists from Northwestern University experimented with the shell-building attributes of mollusks and coral to create carbon-negative concrete, and in 2023, a study in Australia explored the possibility of using spent coffee grinds as a kind of biochar, a proposed replacement for cement that creates organic matter via pyrolysis.Now, a team of researchers from Manipal University Jaipur and Louisiana Tech University are considering a possible cement substitute that’s a bit more of a head-scratcher—human poop. While the initial idea might seem a bit off-putting, in many ways, utilizing human waste makes a lot of sense. For one, there’s no shortage of it. The average human produces roughly 100 to 250 grams of the stuff every day—multiply that by billions and, well, you see where this is going. The results of the study were published in the journal Scientific Reports.The authors relied on fecal sludge treatment plants (FSTPs) to safely manage human waste so it could be turned into biochar. To do that, the fecal sludge is first air-dried and then heated in a low-oxygen environment starting at around 100 degrees Celsius for 24 hours. The heat then rises roughly 10 degrees Celsius per minute before reaching a max temperature of 450 degrees Celsius to complete the carbonization of the sludge. Once cooled in the furnace to prevent oxidation, the raw sludge biochar was then processed through grinding and sieving and prepared for the concrete mixture.The study analyzed four types of mixes: 0, 5, 10, and 15 percent of biochar. What they found is that concrete containing 5 to 10 percent of the biochar showed “improved compression and flexural strengths” after curing for up to 91 days. The 15 percent mix instead showed a decline in strength. So why does a little bit of poop—but not too much—make the concrete mix stronger?“The increase in the strength with curing duration is due to the honeycomb-like pore structure of biochar, making it an excellent water absorbent material that can sustain water and redistribute it to enable internal curing,” the authors write in the study. “This biochar cement concrete composite acts as a water reservoir, providing a perpetual supply of water internally and increasing the degree of hydration due to significant levels of silica in biochar that bind with the calcium hydroxide to form stable calcium silicates.”In other words, to borrow a well-worn response to anything that initially sounds unpleasant: “Don’t knock it until you try it.”Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.