Each year the world piles up roughly ten billion kilograms of spent coffee, most of it bound for landfill, where it quietly releases greenhouse gases. Meanwhile, the construction boom keeps drawing down river sand, a resource that cannot be renewed on human timescales. Researchers at RMIT University in Australia have found a way to let these two problems answer each other: by slowly charring used grounds without oxygen into a porous biochar, they report that the grounds can make concrete roughly 30 percent stronger. Whether that strength endures through freezing, soaking, and wear is the quest
Charred Coffee Grounds Could Make Concrete 30% Stronger, RMIT Researchers Report
Most coffee waste ends up in landfill. RMIT thinks it belongs in concrete.
Mark
So what's the headline claim here?
Mimi
That adding charred coffee grounds to concrete could make it about 30 percent stronger, according to the RMIT team in Australia.
Mark
Why char them first? Why not just mix in the grounds?
Mimi
Raw organic material leaks chemicals that weaken concrete. Heating the grounds above 350 °C without oxygen turns them into biochar, which bonds into the cement matrix.
Luke
Before we get carried away, that 30 percent is the researchers' own figure, and it comes from lab work. The article doesn't say what the baseline concrete was or how it was tested.
Mark
Fair. What else is unproven?
Mimi
Durability. The team says they still need to test freeze-thaw cycles, water absorption, abrasion, and other stressors.
Luke
Right, and that's the big one. Strength on day one isn't the same as holding up for decades. Nothing here tells us how long the material lasts.
Mark
And the sand angle?
Mimi
Li says sand mining from river beds and banks has a big environmental impact. The idea is that biochar could help ease that pressure.
Luke
Notice the wording, though: could. The piece doesn't say how much sand it would replace, or whether that would matter at scale.
Mimi
True. What's clear is the temperature finding: 350 °C worked better than 500 °C, and the team is now trying wood, food, and agricultural waste too.
Der Puls
- Roughly ten billion kilograms of coffee waste a year is rotting in landfills, releasing methane and carbon dioxide with every passing season.
- River sand is being stripped from riverbeds and banks faster than it can recover, and a construction boom keeps raising the pressure on what remains.
- Raw coffee grounds leak chemicals that weaken concrete, so the team must first pyrolyze them below 350 °C without oxygen into bonding biochar.
- Early lab results point to a 30 percent strength gain, but freeze-thaw, water absorption, and abrasion tests have yet to confirm that the material will last.
- The team is already widening the search to wood, food, and agricultural waste, aiming to turn more of the world's discards into building material.
Each year the world piles up roughly ten billion kilograms of spent coffee, most of it bound for landfill, where it quietly releases greenhouse gases. Meanwhile, the construction boom keeps drawing down river sand, a resource that cannot be renewed on human timescales. Researchers at RMIT University in Australia have found a way to let these two problems answer each other: by slowly charring used grounds without oxygen into a porous biochar, they report that the grounds can make concrete roughly 30 percent stronger. Whether that strength endures through freezing, soaking, and wear is the question that will decide if this remains a promising experiment or becomes a practice.
Every year the world produces around ten billion kilograms of coffee waste, and most of it ends up in landfill, where it slowly gives off greenhouse gases. Researchers at RMIT University in Australia see an opportunity in that pile: mixed into concrete, charred coffee grounds may make the material about 30 percent stronger.
The idea answers two pressures at once. Construction is booming, and with it the appetite for sand, which is dug from riverbeds and banks faster than nature can replace it. RMIT engineers Rajeev Roychand and Jie Li argue that a circular approach, one that keeps organic waste out of landfill while sparing natural sand, could ease both burdens together.
Raw grounds cannot simply be stirred in, because they leak chemicals that weaken concrete. The team heats them below 350 °C in an oxygen-free environment, a process called pyrolysis, which leaves a porous, carbon-rich biochar that bonds with the cement. Their earlier tests found that heating to 500 °C produced weaker particles, so temperature turned out to matter.
The researchers are candid about what remains unknown. Durability under freeze-thaw cycles, water absorption, and abrasion is still under test. The group is also developing biochars from wood, food waste, and farm residue. Shannon Kilmartin-Lynch, who describes the work as early-stage, links his motivation to the Indigenous idea of Caring for Country, which asks that every material be given a life cycle that avoids needless waste. The strength gain is encouraging, but concrete is judged over decades, and the coming durability results will show whether coffee-cement can leave the laboratory.
Every year, the world produces about 10 billion kilograms of coffee waste, roughly 22 billion pounds, and most of it ends up in landfill. Researchers at RMIT University in Australia think a share of it belongs somewhere else: inside concrete, where charred coffee grounds may make the material about 30 percent stronger.
The idea addresses two problems at once. The first is the waste itself. Rajeev Roychand, an RMIT engineer, explained that organic waste in landfill emits large amounts of greenhouse gases, including methane and carbon dioxide, which contribute to climate change.
The second problem is sand. Construction is booming worldwide, and concrete demand rises with it. Jie Li, another RMIT engineer, pointed out that natural sand, typically taken from river beds and banks, is being extracted at a rapid pace to feed the industry, with a significant environmental impact. Sand is a finite resource, he said, and sand mining creates long-lasting challenges for keeping supply sustainable. His argument is that a circular-economy approach could keep organic waste out of landfill while better preserving natural resources like sand.
The grounds cannot simply be stirred into the mix. Raw organic material leaks chemicals that weaken concrete. So the team first heated the coffee waste to over 350 °C (about 660 °F) in an environment without oxygen, using low energy levels. The process is called pyrolysis. It breaks down the organic molecules and leaves a porous, carbon-rich charcoal known as biochar, which 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. The work, including scanning electron microscope images of the surface structures of the pyrolyzed grounds, was published in the Journal of Cleaner Production in 2023.
The researchers are careful about what they have not yet shown. They still need to assess the long-term durability of the coffee-cement product. Testing is underway on how the hybrid material performs under freeze-thaw cycles, water absorption, abrasion, and other stressors.
The team is also looking beyond coffee, working on biochars made from other organic waste sources, including wood, food waste, and agricultural waste.
Shannon Kilmartin-Lynch, an RMIT engineer, described the research as early-stage but said the findings offer an innovative way to greatly reduce the amount of organic waste going to landfill. He said his inspiration comes from an Indigenous perspective of Caring for Country: ensuring a sustainable life cycle for all materials and avoiding landfill to minimize environmental impact.
What to watch is the durability data. A 30 percent strength gain is promising, but concrete is judged over decades, and the freeze-thaw, water, and abrasion results will show whether coffee-cement can leave the lab.
Bemerkenswerte Zitate
Our research is in the early stages, but these findings offer an innovative way to greatly reduce the amount of organic waste going to landfill.— Shannon Kilmartin-Lynch, RMIT engineer (paraphrased)
Organic waste in landfill emits large amounts of greenhouse gases, including methane and carbon dioxide.— Rajeev Roychand, RMIT engineer (paraphrased)
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