How scientists from RMIT University in Australia are using spent coffee to resolve one of the building industry’s biggest shortages.
While there has long been a focus on the impact of single-use coffee cups on the environment, the issue of spent coffee grounds often goes under the radar. Globally, it is estimated that more than two billion cups of coffee are consumed each day, resulting in around 10 million tonnes of used coffee grounds each year.
While some coffee drinkers make the effort to compost their spent coffee, most waste grounds end up in landfill, where they generate methane, a substance more than 28 times as potent as carbon dioxide at trapping heat in the atmosphere.
Eager to explore alternative final destinations for the huge volume of used grounds produced each year, researchers from RMIT University in Melbourne, Australia, have created a solution that solves the waste issue and a global sand shortage that’s plaguing the building industry.
Coffee is often credited with stirring inspiration, and for RMIT researcher Dr Rajeev Roychand, a cup of coffee was the spark that ignited his interest in coffee waste.
“As a researcher, we are constantly trying to solve challenges the community face,” says Roychand.
“In this research, our main focus was the issue of organic waste ending up in landfill. As a team, we discussed the potential options we could explore to transform these waste materials and coffee seemed like the perfect starting point.”
Roychand and his engineering team began studying spent coffee, looking for novel ways it could be used. After exploring many avenues, they turned to the construction industry and started to examine whether spent coffee could be used to produce stronger concrete.
In their initial research, the team found out the hard way that the process wasn’t as simple as adding coffee grounds to a cement mix and giving it a good stir.
“We realised spent coffee releases organic compounds, which were hindering the reaction of cement. Normally, cement reacts with water, which gives concrete its binding properties. The coffee was hindering that reaction,” says Roychand.
To compensate, the team roasted the spent grounds to break down the organic component released by coffee.
“In simple terms, this is the same roasting process used for raw coffee beans, which brings out their aroma and taste,” he says.
“The only difference is that the spent grounds are burned in the absence of oxygen to avoid creating carbon dioxide emissions – a process known as pyrolysis.”
The team experimented with roasting the spent coffee at 350°C and 500°C, and eventually found the former produced better results. Through this process they produced biochar: a black, carbon-rich, and porous product similar to charcoal.
“We replaced 15 per cent of the sand component in concrete with the coffee biochar. We tested strength and other properties and found it was 30 per cent stronger than concrete without coffee biochar,” he says.
“The coffee concrete is stronger because concrete requires water to harden and gain strength, but as cement hydration progresses, it consumes this water, reducing the concrete’s internal relative humidity and causing self-desiccation. This process can lead to microscale shrinkage cracking, weakening the concrete.
“Biochar offers an optimal combination of carbon content and pore structure, enabling it to act as microscale water reservoirs within the concrete mix. As the concrete hardens, these biochar particles gradually release stored water, ensuring continuous hydration of cement particles even as internal humidity decreases. This internal curing mechanism mitigates self-desiccation, reduces shrinkage cracking, and promotes more complete cement hydration. The result is a denser, less permeable concrete microstructure with improved strength, addressing a key challenge in concrete technology.”
Roychand’s research also found the new concrete can reduce required cement content by as much as 10 per cent due to its strength. As such, because coffee waste is denser than sand, it has the potential to replace up to 655,000 tonnes of sand in concrete, which could help alleviate the global shortage of sand suitable for construction needs.
Roychand says this new innovation has the potential to replace up to 90 million tonnes of sand in concrete – more than the weight of the Great Wall of China.
Saudi Arabia, a country known for its abundant deserts, actively imports sand from Australia to use for construction. Sand weathered by wind is too smooth and rounded to lock together to form stable concrete. On the other hand, sand weathered by water, typically found in riverbeds and banks, is coarse enough to be part of the concrete process. However, the extraction of sand from rivers can be a destructive process, often leading to long-term environmental damage.
What is reassuring about coffee biochar, according to the RMIT team, is that it has the potential to significantly reduce carbon emissions in cement production because the biochar only requires heating to about 350°C. In the typical cement-making process, temperatures of around 1500°C are required.
Although the product is still in the trial stage, Roychand and team are confident in biochar’s potential.
“Research is basically venturing into the unknown. You keep trying, something doesn’t work, you see what is lacking, and then you build upon your previous work. It’s a typical component of research,” says Roychand.
What may be more difficult to navigate are the potential commercial problems. With the project being relatively cost-intensive, Roychand says the construction industry has been reluctant to get on board, especially since sand has been a vital component of concrete since time immemorial.
However, there are signs the tide is starting to turn. Support from the Australian government as well as RMIT has helped the team trial their product on a larger scale.
In Victoria, RMIT researchers and Macedon Ranges Shire Council have partnered with Australian-owned BildGroup to construct the world’s first coffee concrete footpath. The project has even received interest from Major Road Projects Victoria.
“They want to see its durability and how it performs, because when you invest in large infrastructure, you can’t simply try things out. We are doing the durability studies now, and will open up more applications for the concrete in the future,” says Roychand.
“Our focus going forward is to cover all forms of organic waste so we can divert it from landfill to high-value applications, like concrete. We are broadening the horizon of our research.”
This article was first published in the September/October 2024 edition of Global Coffee Report. Read more HERE.
