Australian engineers say they can make concrete nearly 30% stronger by incorporating processed grounds into the material

In an idea that fittingly arose over a cup of coffee, researchers have devised a technique to recycle used coffee grounds to make stronger concrete.

Engineers at RMIT University say they have developed a way to make concrete nearly 30% stronger by incorporating processed coffee grounds into the material.

The researchers have converted waste coffee grounds into biochar, a lightweight residue similar to charcoal, and used that biochar to replace a portion of the sand required to make concrete.

The idea arose from a desire to minimise coffee waste within the workplace, said study co-lead Dr Shannon Kilmartin-Lynch, a vice-chancellor’s Indigenous postdoctoral research fellow at RMIT.

“There was a lot of ground coffee and coffee pods being discarded,” he said. “[We wanted] to see if we could transform those spent coffee grounds into a more valuable sort of material.”

The researchers are now collaborating with local councils on future infrastructure projects such as the construction of walkways and pavements.

The technique could be environmentally beneficial if it can reduce the amount of coffee waste going to landfill, as well as the demand for natural sand used in the construction industry, the engineers say.

Food waste accounts for about 3% of Australia’s annual greenhouse emissions, according to the National Food Waste Strategy Feasibility Study. Australia produces an estimated 75,000 tonnes of coffee waste per year.

Creating biochar involves roasting used coffee grounds in the same way unused beans are roasted to enhance their taste, said study co-lead Dr Rajeev Roychand of RMIT.

“We do the same thing, but in the absence of oxygen [to prevent carbon dioxide from being produced],” Roychand said. “We don’t want carbon to get into the atmosphere and add to greenhouse gas emissions.”

The process, called pyrolysis, involves heating the coffee waste to about 350C. The team says their technique is more energy efficient because it requires lower than usual temperatures.

“Typically pyrolysis has a high energy [input] because you need to raise temperatures to somewhere between 700 to 900C,” Kilmartin-Lynch said.

By replacing 15% of the sand typically used in concrete with coffee biochar, the researchers found that the addition enhanced strength by 29.3%.

“Structurally, the coffee biochar itself is finer than a sand … but it’s also a porous material, so it allows the cement to bind within the porous structure of the biochar itself,” Kilmartin-Lynch said.

“It definitely still is in its initial phase – there are further tests to be done on the durability and things like that.”

If all waste coffee grounds produced in Australia each year were converted into biochar, it would amount to roughly 22,500 tonnes, the researchers estimate.

However, about 28.8 million tonnes of sand are required each year to produce the approximately 72 million tonnes of cement concrete made in Australia.

The research was published in the journal Journal of Cleaner Production.