Scientists use coffee waste to make concrete 30% stronger

The new tech sees wasted coffee grounds transformed into biochar, using a sustainable low-energy process without oxygen at 350 degrees Celsius.

A 'double shot' at life

“The inspiration for our work was to find an innovative way of using the large amounts of coffee waste in construction projects rather than going to landfills – to give coffee a ‘double shot’ at life,” said Roychand, a Postdoctoral Research Fellow at RMIT.

“Several councils that are battling with the disposal of organic waste have shown interest in our work.  

“They have already engaged us for their upcoming infrastructure projects incorporating pyrolysed forms of different organic wastes.”

Pyrolysis is a key technology for use in converting biomass to an intermediate liquid product that can be refined into petrochemical replacements. It can now be used to turn harmful coffee waste into a useful resource for the construction industry.

“Inspiration for my research, from an Indigenous perspective, involves Caring for Country, ensuring there’s a sustainable life cycle for all materials and avoiding things going into landfill to minimize the impact on the environment,” said joint lead author, Dr Shannon Kilmartin-Lynch, a Vice-Chancellor’s Indigenous Postdoctoral Research Fellow at RMIT.

“The concrete industry has the potential to contribute significantly to increasing the recycling of organic waste such as used coffee.

“Our research is in the early stages, but these exciting findings offer an innovative way to greatly reduce the amount of organic waste that goes to landfill.”

Using coffee biochar instead of sand

The coffee biochar may now be used instead of sand in the production of concrete which happens to be in low quantities worldwide creating a serious lack of resources. Sand is notably the most-extracted solid material in the world and the second-most used global resource after water.

“The ongoing extraction of natural sand around the world – typically taken from river beds and banks – to meet the rapidly growing demands of the construction industry has a big impact on the environment,” said corresponding author and research team leader professor Jie Li.

“There are critical and long-lasting challenges in maintaining a sustainable supply of sand due to the finite nature of resources and the environmental impacts of sand mining,” he added.

“With a circular-economy approach, we could keep organic waste out of landfill and also better preserve our natural resources like sand.”

Coffee biochar provides an alternative source of raw materials compared to sand that is sustainable, reducing the environmental impact of the construction industry. However, this is not the only waste-derived biochar developed at RMIT. The institution has explored all kinds of organic waste options.

“Our research team has gained extensive experience in developing highly optimized biochars from different organic wastes, including wood biochar, food-waste biochar, agricultural waste biochar, and municipal solid-waste biochar, for concrete applications,” said co-researcher Dr Mohammad Saberian.

The study is published in the Journal of Cleaner Production. 

Study abstract:

The decomposition of organic waste going to landfills produces methane gas, which is 21 times worse than CO2 in its global warming potential. Spent coffee grounds (SCG) are one type of organic waste that makes up a significant proportion of the organic waste going to landfills. Therefore, it becomes imperative to look for a recycling solution to transform this waste into a valuable resource. The concrete industry has the potential to contribute significantly to increasing the recycling rate of this waste material. However, due to its high organic content, it is unsuitable to be used directly in structural concrete. Therefore, this experimental project looks at pyrolysing this waste at different temperatures (350 and 500 °C) to identify its suitability in improving the physicochemical and mechanical properties of concrete. The raw and pyrolysed forms of SCG were used as a replacement of fine aggregates (FA; sand) at 5, 10, 15 and 20% volume replacement levels. X-ray fluorescence (XRF), Carbon, Hydrogen, Nitrogen, and Sulfur (CHNS) analysis, laser diffraction particle size analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM) and compressive strength tests were undertaken to investigate the properties of the raw material and their performance in the blended concrete composites. The results show that the leaching of organic compounds from the SCG hinders the hydration reaction of cement particles, thereby significantly hampering the compressive strength of SCG-blended concrete. However, pyrolysing the SCG at 350 °C led to a significant improvement in its material properties, which resulted in a 29.3% enhancement in the compressive strength of the composite concrete blended with coffee biochar.