Every morning, the world brews and then discards an enormous quantity of coffee, most of it carried to buried heaps where it slowly rots and releases gases that warm the air. In a laboratory in Australia, researchers at RMIT University have offered those spent grounds a second life: charred into biochar and folded into concrete, they may lend the material up to 30 percent more strength. It is a modest act of reuse, but it touches two of the larger pressures of our age, the hunger of construction for sand and the quiet cost of what we throw away. Whether the promise endures the long test of wea
Charred Coffee Grounds Could Make Concrete Up to 30% Stronger, Australian Researchers Find
Spent coffee grounds usually end their lives in a landfill.
Mark
So the headline is that coffee makes concrete stronger. Is it really that simple?
Mimi
Not quite. The raw grounds actually hurt concrete, because they leach chemicals that weaken it. The trick is charring them first into biochar.
Mark
How do they do that?
Mimi
They heat the grounds above 350 °C with no oxygen, which is pyrolysis. What's left is a porous, carbon-rich charcoal that bonds with the cement matrix.
Luke
Before we get carried away, "up to 30 percent" is a ceiling, not an average. The reporting doesn't say what mix or conditions produced it.
Mark
Fair. What else is unknown?
Luke
Durability. The team says it hasn't yet assessed long-term performance. Freeze/thaw, water absorption and abrasion tests are still running. Strength on day one isn't the same as strength over decades.
Mimi
And the temperature detail is telling. At 500 °C the biochar particles came out weaker, so the process is sensitive to how it's done.
Mark
Why does it matter beyond the lab?
Mimi
Two problems at once. About 10 billion kilograms of coffee waste a year, mostly landfilled, and sand being mined from river beds for concrete.
Luke
Though nothing here quantifies how much sand would actually be replaced. That's the part the story leaves open.
Mimi
Right, and the team is already trying wood, food and agricultural waste, so coffee may be just the first example.
Der Puls
- Roughly 10 billion kilograms of coffee waste is generated each year, and most of it rots in landfill, releasing methane and carbon dioxide.
- Concrete demand is climbing with the global construction boom, pulling ever harder on finite river sand.
- Raw grounds would weaken concrete, so the team heats them above 350 °C in an oxygen-free chamber to turn them into biochar.
- Hotter is not better: biochar made at 500 °C produced weaker particles, so the window of success is narrow.
- The up-to-30-percent strength gain is promising, but freeze/thaw, water absorption and abrasion tests are still underway.
- The team is extending the method to wood, food and agricultural waste, with durability results the next thing to watch.
Every morning, the world brews and then discards an enormous quantity of coffee, most of it carried to buried heaps where it slowly rots and releases gases that warm the air. In a laboratory in Australia, researchers at RMIT University have offered those spent grounds a second life: charred into biochar and folded into concrete, they may lend the material up to 30 percent more strength. It is a modest act of reuse, but it touches two of the larger pressures of our age, the hunger of construction for sand and the quiet cost of what we throw away. Whether the promise endures the long test of weather and wear remains an open question.
Spent coffee grounds almost always end their days in a landfill. At RMIT University in Australia, researchers have found them a different destination: inside concrete. Once charred, the grounds can make the material up to 30 percent stronger.
The scale of the waste is what makes the idea worth attention. The world produces about 10 billion kilograms of coffee waste a year, and most of it is buried, where rotting organic matter gives off methane and carbon dioxide. RMIT engineer Rajeev Roychand pointed to those emissions when the work first appeared in 2023.
Concrete, meanwhile, has its own hunger. The booming construction market demands more of it, and that means more sand, usually drawn from riverbeds and banks at heavy environmental cost. Jie Li, another RMIT engineer, argues that a circular approach could keep organic waste out of landfill while easing the pressure on this finite resource.
Raw grounds cannot simply be stirred into a mix, because their organic compounds leach out and weaken the concrete. The team heats the grounds above 350 °C in an oxygen-free environment, a process called pyrolysis, leaving a porous, carbon-rich biochar that bonds with the cement. Temperature proved decisive: at 500 °C the resulting particles were weaker.
The researchers are careful not to overstate the result. Durability under freeze and thaw, water absorption and abrasion is still being tested, and the team is trying wood, food and farm waste as well. Shannon Kilmartin-Lynch, also of RMIT, called the work early but promising, and linked it to an Indigenous idea of Caring for Country: keeping materials in a sustainable cycle rather than sending them to waste. The findings, published in the Journal of Cleaner Production, will ultimately be judged by how the material holds up in the real world.
Spent coffee grounds usually end their lives in a landfill. Researchers at RMIT University in Australia have found a different destination for them: inside concrete, where, once charred, they can make the material up to 30 percent stronger.
The scale of the waste is what makes the idea worth a look. The world produces about 10 billion kilograms of coffee waste a year, roughly 22 billion pounds, and most of it is buried. RMIT engineer Rajeev Roychand pointed out when the work was first published in 2023 that rotting organic waste gives off large amounts of greenhouse gases, including methane and carbon dioxide.
Concrete has its own appetite. A booming global construction market keeps pushing up demand for it, and that means sand. Jie Li, another RMIT engineer on the team, noted that natural sand is typically pulled from river beds and banks, and that the extraction has a heavy environmental impact. Sand is a finite resource, he said, so keeping the supply sustainable is a long-term difficulty. His argument for a circular-economy approach is that it could keep organic waste out of landfill while better preserving resources like sand.
The obstacle is that raw coffee grounds can't simply be stirred into a mix. Organic material leaches chemicals that weaken concrete. So the team treated the grounds first, heating them to above 350 °C (about 660 °F) in an oxygen-free environment, using relatively low energy. The process is called pyrolysis. It breaks down the organic molecules and leaves behind biochar, a porous, carbon-rich charcoal that can bond with the cement matrix and become part of it.
Temperature mattered. Roychand and his colleagues also tried pyrolyzing the grounds at 500 °C, but the biochar particles that came out of that batch were not as strong.
The researchers are careful about how far the result can be pushed. They say they still need to assess the long-term durability of the coffee-cement product. Testing is under way on how the hybrid material handles freeze/thaw cycles, water absorption, abrasion and a range of other stresses. The team is also working on making biochar from other organic waste, including wood, food and agricultural sources.
Shannon Kilmartin-Lynch, also of RMIT, described the work as early-stage but promising, calling the findings an innovative way to greatly cut the organic waste that goes to landfill. He also described the thinking behind his own part of the research, which comes from an Indigenous perspective of Caring for Country: making sure there is a sustainable life cycle for all materials and avoiding sending things to landfill to limit the impact on the environment.
The research appeared in the Journal of Cleaner Production. The next results to watch are the durability tests, since the 30 percent figure matters only if the material holds up under the conditions real structures face.
Bemerkenswerte Zitate
Natural sand extraction, typically from river beds and banks, has a big environmental impact; a circular-economy approach could keep organic waste from landfill and preserve resources like sand.— Jie Li, RMIT University engineer
The research is in its early stages, but the findings offer an innovative way to greatly reduce the organic waste that goes to landfill.— Shannon Kilmartin-Lynch, RMIT University engineer
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