Researchers Find Spent Coffee Grounds Could Make Concrete 30% Stronger

Spent coffee grounds are about as ordinary as waste gets, and at the scale the world drinks coffee, they pile up fast. Researchers at RMIT University in Australia now report that grounds, once charred, can be mixed into concrete to make it as much as 30 percent stronger.

The starting problem is the volume. The world produces about 10 billion kilograms of coffee waste every year, and most of it ends up in landfill. RMIT engineer Rajeev Roychand points out that organic waste breaking down there releases large amounts of greenhouse gases, including methane and carbon dioxide, which feed climate change.

Concrete has a problem of its own. Global construction keeps growing, and so does demand for resource-heavy concrete. Natural sand, usually taken from river beds and banks, is a key ingredient, and RMIT engineer Jie Li says that extracting it to keep up with demand has a major effect on the environment. Sand is a finite resource, she notes, and mining it makes a sustainable supply hard to maintain. Her argument for a circular-economy approach is that it could keep organic waste out of landfill while also preserving natural resources like sand.

The grounds cannot simply be stirred into the mix. Organic material such as coffee leaches chemicals that weaken concrete. So the team heated the waste to more than 350°C, around 660°F, while depriving it of oxygen, using relatively low energy. This process is called pyrolysis. It breaks down the organic molecules and leaves a porous, carbon-rich charcoal known as biochar. That material can bond with the cement matrix and become part of it.

Temperature mattered. Roychand and colleagues also tried pyrolyzing the grounds at 500°C, but the biochar particles that resulted were not as strong as those made at the lower temperature.

The researchers are clear that this is early work. They still need to assess the long-term durability of the coffee-cement hybrid, and they are now testing how it performs under freeze/thaw cycles, water absorption, abrasion and a range of other stresses. In parallel, the team is working on biochars made from other organic waste, including wood, food waste and agricultural waste.

RMIT engineer Shannon Kilmartin-Lynch describes the findings as exciting but preliminary, saying they offer an innovative way to greatly cut the amount of organic waste going to landfill. She also says her own motivation comes from an Indigenous perspective of Caring for Country: making sure materials have a sustainable life cycle and avoiding landfill to limit environmental harm.

The research was published in the Journal of Cleaner Production. The next results to watch are the durability tests, since a stronger concrete only matters if it holds up over decades of weather and wear.