By Tess Macallan, Journalist, Infrastructure Magazine

As a country known for its thriving coffee culture, it’s no surprise that Australia generates 75 million kilograms of ground coffee waste every year. Most of this ends up in landfill, contributing to greenhouse gas emissions and exacerbating environmental degradation. To combat this, a team of RMIT University researchers are on a mission to transform discarded coffee grounds into a valuable resource that can support a more sustainable construction sector.

The construction industry is one of the largest consumers of natural resources and a significant contributor to carbon emissions, making circular practices crucial for mitigating the impacts of resource depletion and climate change. From the repurposing of end-of-life work apparel to freeway noise walls made of plastic waste, circular initiatives are becoming increasingly prevalent across the sector.

Due to this need for more sustainable options in the sector, a team of engineers from RMIT University have published a study, Transforming spent coffee grounds into a valuable resource for the enhancement of concrete strength, that offers a new way to utilise coffee grounds.

Lead author, Dr Rajeev Roychand, said the motivation behind the research was reducing some of the 6.87 million tonnes of organic waste that ends up in Australia’s landfills per year, which contributes to three per cent of the country’s greenhouse gas emissions.

“Our research group discussed shifting our focus to diverting this waste from landfills and developing innovative solutions to transform waste into a valuable resource for different applications,” Dr Roychand said.

“Incidentally, this conversation happened over a cup of coffee in our team meeting. That’s why we decided to start with coffee grounds.”

The developed technique uses a low-energy process without oxygen at 350 degrees Celsius to turn used coffee grounds into biochar, which can then be used to make concrete 30 per cent stronger.

Converting waste for concrete

As the first study to prove that waste coffee grounds can be used to improve concrete, it has not been without its challenges. “Organic waste in its raw form is detrimental to the concrete strength due to the organic compounds hindering the hydration reaction of cement,” Dr Roychand said.

“They need to be converted into biochar to make it compatible with concrete. But even to make biochar the process parameters are highly critical to deliver the best performance in its concrete applications.

“We started with raw coffee waste, which saw a significant reduction in concrete strength. This is because coffee waste leaches out organic compounds that hinder cement reaction, thereby negatively impacting its strength development. The team then worked on a method to break these organic compounds while preserving the carbon content, for that we used the pyrolysis process.”

Typically, pyrolysis – heating organic waste in the absence of oxygen – is carried out at higher temperature for the efficient thermal decomposition of organic waste. However, a key focus of the study was minimising energy use.

“We started with pyrolysing at a temperature of 500°C,” joint lead author Dr Shannon Kilmartin-Lynch said.

“This saw significant enhancement in concrete strength compared to that of the raw coffee grounds in concrete, but it was still lower than concrete that does not contain any coffee waste.

“Since our focus was to minimise the energy consumption, we went for a lower pyrolysis temperature (i.e., 350 °C) and hoped that the lower decomposition could potentially help in improving the properties. It was a blessing in disguise. To our excitement, we noticed an improvement in concrete strength by almost 30 per cent.”

Conserving the earth’s natural resources

Incorporating biochar into concrete offers several benefits. By significantly increasing the strength of concrete, the amount of cement content required can be reduced if needed. Furthermore, this waste to resource transformation creates the potential for diverting this waste from going to landfills in concrete applications, promoting sustainability and a closed-loop circular economy.

50 billion tonnes of natural sand is used in construction projects globally each year, with a considerable portion of this used in making concrete. This level of sand extraction has a significant impact on the environment, including habitat destruction, erosion, and disruption of river ecosystems.

Coffee biochar could replace a portion of the sand that is used in concrete production, reducing that environmental impact. Group Leader, Professor Jie Li, said, “The construction industry relies on continuous mining of finite natural resources to meet the ever-growing demand for the infrastructure development for the growing population.

“To keep this industry sustainable, we need to grab all innovative solutions that promote circular economy without compromising the mechanical and durability properties of the structural elements.”

Watch this space

While the team’s research is still in the early stages, coauthor Dr Mohammad Saberian said the study is receiving plenty of attention from those who want to collaborate and translate the research into field applications.

“We are starting with field trials in collaboration with a local council and a few construction companies and we are hoping to expand these trials internationally.

RMIT University researchers Dr Rajeev Roychand, Dr Mohammad Saberian and Dr Shannon Kilmartin-Lynch with Jordan Carter, Co-founder of the Indigenous-owned Talwali Coffee Roasters (pictured left to right). Image courtesy of Carelle Mulawa-Richards, RMIT University

“We would be very happy to collaborate with local, state, and federal governments and construction companies willing to translate this research into field applications in different parts of the world.”

The construction industry has a long way to go until sustainability is fully integrated into everyday processes and projects.

However, initiatives such as the recycling of organic waste, can support the sector to reduce its environmental footprint and drive sustainability forward. The concrete industry is particularly well-positioned to contribute significantly to increasing the recycling of organic waste.

Dr Roychand said the research team will continue working on a range of waste materials and developing optimum process parameters for all forms of organic waste.

“There’s more groundbreaking research in the pipeline, so stay tuned.”

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