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Engineers close a circular economy loop by making stronger concrete with old coffee grounds

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Engineers close a circular economy loop by making stronger concrete with old coffee grounds

Their technique not only keep coffee grounds out of landfills, it also works as a replacement for dwindling sand resources.
September 1, 2023

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Researchers have given tons of waste coffee grounds a “double shot” at life, by infusing it into concrete—where it not only increases material strength by 30%, but also could be a significantly more sustainable alternative to the mined sand on which the concrete industry depends. 

“This waste-to-resource approach creates a closed-loop circular economy,” says Rajeev Roychand, materials scientist at RMIT University, Australia, and the study’s lead author.

In Australia alone, where the study was based, shops and households produce around 75,000 tons of coffee grounds every year. The majority of this ends up in landfills where, like all organic matter, it produces greenhouse gasses, including the especially potent methane. 

Leaving it to fester in landfills wastes this high-carbon resource. In fact, researchers have also looked at how abundant coffee grounds could be used to make concrete and other products. But until this study, no one had settled on a successful method to infuse coffee into concrete without also compromising its functionality.

This study’s secret was that it pyrolyzed the coffee grounds to make a carbon-rich biochar, heating them experimentally at different temperatures, before placing the grounds into the concrete mix. This enabled the researchers to pinpoint the precise temperature that gave the coffee the right material properties to reinforce the concrete without weakening it.

 

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When they tested grounds that had been heated to 350 °C and 500 °C, and infused them into the mix at ratios of 5, 10, 15, and 20%, the researchers discovered that a 15% share of grounds, heated at the lower temperature of 350 °C, performed the best. In fact, that increased the concrete’s strength under pressure by a striking 30%.

A close analysis of the concrete followed, using x-rays, scanning microscopy, and an assessment of its carbon, hydrogen, nitrogen, and sulfur content, to draw up a profile of the interior. This helped reveal that at this temperature and ratio, the grounds create a helpful porous structure in the concrete. “The pores within biochar absorb and retain water, serving as micro reservoirs, which can then be released slowly into the surrounding concrete matrix, aiding in the hydration of cement particles,” Roychand explains. “This can lead to improved strength development, reduced micro-cracking, and enhanced overall durability.”

Increased strength is obviously a boon for the construction industry. But there are other reasons to bring this waste material into buildings. Not only would it keep coffee grounds out of landfills, it could also replace a share of the required sand, “which is a finite natural resource and continuously needs to be mined to meet the growing demands of the construction industry,” Roychand says.

What’s more, Roychand and colleagues calculated that replacing 15% of Australia’s concrete mix with coffee grounds would generate enough demand to use up all the available coffee waste in the country and divert it from the landfill.

This would be a rare double-win for the environment, meanwhile for industry there is a dangling carrot in the form of reduced production costs from the cheap, available waste. “There is keen interest from local councils and the construction industry,” says Roychand of Melbourne where he works. “We are in consultation with a few councils who are keen to translate this research into the field applications for their upcoming projects.”

If the idea went global, it could make use of the 1 million tons of coffee grounds that are ditched in landfills every year—and just like the reinvigorating power of caffeine, inject new energy into old ideas. 

Roychand et. al. “Transforming spent coffee grounds into a valuable resource for the enhancement of concrete strength.” Journal of Cleaner Production. 2023.

Image: Montaag

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