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Concrete can be made 29 per cent stronger by incorporating recycled coffee grounds.

An estimated of spent coffee grounds are produced globally each year, with most ending up in landfill. Their decomposition in landfill releases methane, which has a global warming effect 21 times stronger than that of carbon dioxide.

at RMIT University in Melbourne, Australia, and his colleagues wondered if they could keep coffee grounds out of landfill by finding new uses for them in building materials.

They collected used coffee grounds from several local cafes and investigated whether they could be used to replace some of the sand that is typically incorporated into concrete as filler.

In an unmodified state, the spent coffee grounds were found to weaken concrete when they replaced the sand component.

However, they became more useful when the researchers heated them in a 350°C furnace for 2 hours in the absence of oxygen to create a charcoal-like substance called biochar.

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Replacing 15 per cent of the sand in concrete with this biochar resulted in concrete blocks that were 29 per cent stronger than conventional blocks.

The addition of biochar may make concrete stronger because it has a porous structure that traps moisture, says Roychand. This may stop the concrete from drying out on the inside and developing micro-cracks that can weaken its structure, he says.

The researchers are now hoping to collaborate with councils and industry groups to set up field trials of their coffee biochar-enhanced concrete. “Several councils that are battling with the disposal of organic waste have shown interest in our work,” says Roychand.