What can spent coffee grounds do beyond where they grow coffee beans?

Researchers at RMIT University in Australia found that spent coffee grounds can replace part of the sand in concrete and raise compressive strength. Grounds pyrolysed at 350°C and blended into cement at a 15% ratio produced concrete 29.3% stronger than a standard mix. The world produces up to 60 million tonnes of spent grounds a year, most of it landfilled.

Landfilled grounds release carbon dioxide and methane as they decompose, and methane has 21 times the greenhouse warming potential of carbon dioxide. Untreated grounds weakened the concrete, so the team tested pyrolysis at 350°C and 500°C and replacement rates of 5%, 10%, 15% and 20%. The porous organic carbon locks in moisture and limits micro-cracks. The work is still early, and the public is divided on its feasibility.

The world produces up to 60 million tonnes of spent coffee grounds every year, most of which end up in landfill, where they release carbon dioxide and methane as they decompose — and methane has 21 times the greenhouse warming potential of carbon dioxide. A research team at RMIT University in Australia tried pyrolysing coffee grounds and using them to replace part of the sand in concrete, and found that grounds treated at 350°C and blended into cement at a ratio of 15% produced concrete with a compressive strength 29.3% higher than a standard mix. The technique not only gives coffee grounds a new outlet, but can also reduce river sand mining and lower greenhouse gas emissions. The research is still at an early stage, however, and netizens are divided on its feasibility.

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On 22 August, foreign media reported that a research team had found that replacing a certain proportion of the sand in concrete with spent coffee grounds can increase the strength of the concrete by 30%, meaning coffee grounds have the potential to become a valuable construction material.

It is estimated that the world generates as much as 60 million tonnes of spent coffee grounds each year. Normally this kind of plant waste is sent to landfill, where it releases carbon dioxide and methane during decomposition, and methane has 21 times the greenhouse warming potential of carbon dioxide.

To tackle this problem, a research team at RMIT University in Australia collected coffee grounds from local cafés to explore the feasibility of using them to replace part of the sand in concrete — sand is usually added to concrete as a filler. The results showed that, in its untreated state, replacing sand with spent coffee grounds actually reduces the strength of the concrete.

To help the coffee grounds blend better into the concrete, the team tried pyrolysing them at 350°C and 500°C respectively, then replacing 5%, 10%, 15% and 20% of the sand in the concrete. Pyrolysis generally produces organic carbon, a substance with a porous structure that can lock in moisture and prevent micro-cracks from forming inside the concrete as it dries — micro-cracks weaken the strength of the structure. The team's analysis found that coffee grounds pyrolysed at 350°C and mixed into cement at a ratio of 15% showed a 29.3% increase in compressive strength compared with a standard mix.

Image source: New Scientist

This approach can reduce both the amount of sand mined and the amount of coffee grounds sent to landfill, while also lowering greenhouse gas emissions and improving the quality of concrete, giving a further push to a greener, circular-economy model of construction. Although the research is still at an early stage, the researchers say the results are encouraging, and given how widespread coffee consumption is, the technique could potentially be applied in buildings around the world.

Besides saving landfill space, coffee concrete also eases another environmental problem — protecting limited natural resources. To meet the fast-growing demands of the construction industry, the world mines roughly 40 to 50 billion tonnes of river sand and gravel each year, putting enormous pressure on the environment.

Image source: New Atlas

Next, the researchers plan to carry out long-term mechanical and durability tests on the potential use of organic carbon in construction, and to further explore how different pyrolysis temperatures affect the material's performance.

On social media, some netizens said they normally only use coffee grounds as fertiliser and remain sceptical about the idea of using them to build houses as proposed in this research, but that if it really can be done, it would be a good thing nonetheless.

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