Researchers Turn Spent Coffee Grounds into Biochar That Could Make Concrete 30% Stronger

In a laboratory at RMIT University in Australia, researchers have been heating used coffee grounds to temperatures above 350 °C, roughly 660 °F, in a space with no oxygen. What comes out is a porous, carbon-rich charcoal called biochar. Mixed into cement, it made the resulting concrete about 30 percent stronger.

The work addresses two problems at once. The world produces around 10 billion kilograms of coffee waste each year, about 22 billion pounds, and most of it goes to landfill. RMIT engineer Rajeev Roychand explained that organic waste in landfill gives off large amounts of greenhouse gases, including methane and carbon dioxide, which contribute to climate change.

The second problem is concrete itself. A booming global construction market keeps demand high for a resource-intensive material, and a key ingredient is natural sand, typically taken from river beds and banks. RMIT engineer Jie Li said that this extraction has a major environmental impact. Sand is a finite resource, she noted, and sand mining brings its own long-lasting damage, which makes a sustainable supply hard to maintain. In her view, a circular-economy approach could keep organic waste out of landfill while better preserving natural resources like sand.

The obstacle is that raw coffee grounds cannot simply be stirred into concrete. Organic material leaches chemicals that weaken the finished product. The team's answer was pyrolysis, the oxygen-free heating process that breaks down the organic molecules. The resulting biochar can form bonds with the cement matrix and become part of it, rather than undermining it.

Temperature mattered. Roychand and his colleagues also pyrolyzed the grounds at 500 °C, but the biochar particles from that batch were not as strong. The lower-temperature approach, above 350 °C, also uses comparatively little energy.

The researchers are careful about what has not yet been shown. They still need to assess the long-term durability of the coffee-cement hybrid, and they are now testing how it holds up under freeze/thaw cycles, water absorption, abrasion and a range of other stresses. They are also exploring biochars made from other organic waste, including wood, food waste and agricultural waste.

RMIT engineer Shannon Kilmartin-Lynch described the research as early-stage, though she called the findings exciting and an innovative way to greatly reduce the organic waste going to landfill. She also spoke of the motivation behind her work, which comes from an Indigenous perspective of Caring for Country: ensuring a sustainable life cycle for all materials and avoiding landfill to limit environmental impact.

The research was published in the Journal of Cleaner Production. The durability results will show whether the 30 percent strength gain holds up outside the lab, and whether the approach can extend to other waste streams.