Coffee waste made concrete stronger. Not weaker, not merely good enough as a filler, but measurably stronger. That is the finding from a team at RMIT University in Melbourne, and the numbers are specific enough to be worth walking through.

The engineers took used coffee grounds and charred them at 350 degrees Celsius in an oxygen-free process, turning them into a charcoal-like material called biochar. They then used the biochar to replace 15 percent of the natural sand in a concrete mix. The best mix gained 29.3 percent in strength under load compared with an ordinary mix that used plain sand. The work was published in the Journal of Cleaner Production in 2023.

Two problems are being aimed at here at once. One is a mountain of organic waste. The other is sand, which turns out to be far more scarce and contested than most people assume.

What the researchers actually did

You cannot tip used coffee grounds straight into a concrete mixer and expect anything good. Raw grounds leak organic compounds that interfere with how cement sets, and the result is a weaker mix. The RMIT team’s answer was to bake the grounds to 350 degrees Celsius without oxygen. Without oxygen the material chars instead of burning, leaving a porous, carbon-rich biochar that gets along with cement.

Part of the appeal is what it keeps out of landfill. As lead author Dr Rajeev Roychand put it, “The disposal of organic waste poses an environmental challenge as it emits large amounts of greenhouse gases including methane and carbon dioxide, which contribute to climate change.” Australia alone produces roughly 75 million kilograms of ground coffee waste a year, most of it headed to landfill. Globally the figure is around 10 billion kilograms.

With biochar in hand, the team used it to stand in for some of the sand in the mix. The best result came when biochar replaced 15 percent of the sand by volume, and at that level the concrete tested 29.3 percent stronger under compression than the plain mix. Compressive strength is the property that matters most for concrete in a structure: how much squeezing load the material can take before it fails.

The temperature mattered, too. Biochar made at 350 degrees performed much better than biochar made at 500 degrees. The researchers think the difference is structural: microscopy showed that the hotter biochar was more porous and had extensive micro-cracking, which they suggest came from its skeleton becoming fragile under greater thermal breakdown. In other words, heating the coffee grounds more did not make the resulting concrete stronger.

Why sand is the part worth replacing

Replacing sand may sound like an odd thing to celebrate. Sand feels endless. It is not, at least not the kind construction needs. Desert sand is too smooth and rounded to bind well in concrete, so the industry leans on angular river sand instead. That is where the pressure lands.

The scale is large. RMIT cites a figure of 50 billion tonnes of natural sand used in construction each year.  Professor Jie Li, the corresponding author, notes that “the ongoing extraction of natural sand around the world – typically taken from river beds and banks – to meet the rapidly growing demands of the construction industry has a big impact on the environment.” He frames it as a supply problem as much as an environmental one, pointing to “critical and long-lasting challenges in maintaining a sustainable supply of sand due to the finite nature of resources and the environmental impacts of sand mining.”

The pitch, then, is to solve two problems by connecting them. Li describes it as a circular-economy move: “With a circular-economy approach, we could keep organic waste out of landfill and also better preserve our natural resources like sand.”

What this doesn’t tell us yet

Some caution is warranted. This is one study, done at lab scale, and the researchers are the first to say so. Joint lead author Dr Shannon Kilmartin-Lynch put it plainly: “Our research is in the early stages, but these exciting findings offer an innovative way to greatly reduce the amount of organic waste that goes to landfill.”

The obvious open question is durability. Concrete that tests strong after 28 days in a lab is not the same as concrete that holds up over decades of wetting, drying, freezing, and load. Compressive strength is one property among several that engineers care about, and the study measured that one. How coffee biochar concrete behaves over the long run, and outside controlled conditions, is exactly what still needs to be established.

To their credit, the team has been chasing those questions rather than resting on the headline number. They have taken the material into a world-first coffee concrete footpath trial, moving it out of the lab and into real building conditions. Separately, a full life-cycle study reported carbon cuts of 15, 23 and 26 percent when biochar replaced 5, 10 and 15 percent of the sand, along with lower fossil-fuel use.

What we’d want to see before any of this leaves the lab in earnest: durability testing across years rather than days, proof that the strength gain survives real-world variation in coffee grounds and mixing, and an accounting of whether charring millions of tonnes of grounds makes energetic sense at scale.

None of that undoes the finding. It just marks the distance between a striking lab result and a material you would actually pour into a foundation. The coffee-into-concrete idea has cleared the first hurdle, which is more than most waste-reuse ideas manage. The harder tests are still ahead.