Researchers have found that concrete can be made 30% stronger by replacing a percentage of sand with spent coffee grounds, an organic waste product produced in huge amounts that usually ends up in landfill. The method also reduces the use of natural resources like sand, further contributing to a greener circular economy approach to construction.It’s estimated that, worldwide, the total amount of spent coffee grounds (SCG) produced annually is 60 million tons (54 million tonnes), making it the most abundant waste generated in coffee preparation. Traditionally, most of those coffee grounds end up in landfill.
Now, researchers from RMIT University have found a practical, first-of-its-kind use for this particular kind of waste: incorporating it into concrete.
“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 Rajeev Roychand, the study’s lead author.
Due to their fine particle size, SCG have been proposed as a useful component in civil and construction applications. So, the researchers decided to put this to the test. They first collected SCG from various cafés around Melbourne, Australia, and dried them to remove moisture. Then the dried organic material was heated at two different temperatures – either 350 °C (662 °F) or 500 °C (932 °F) – using a low-energy, oxygen-free process called pyrolysis to create biochar.
Twelve mix designs were used to compare the effects of SCG in the form of untreated SCG, 350-degree-heated SCG and 500-degree-heated SCG, on the mechanical and microstructural behavior of concrete. The different SCGs were incorporated into ordinary Portland cement at 0%, 5%, 10%, 15% and 20% volume as a replacement for fine aggregate. Here, the fine aggregate used was natural sand.
Fresh concrete was poured into molds and vibrated to remove air pockets. It was then cured at room temperature for 24 hours, demolded and cured in a water tank until it was tested for compressive strength and analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Compressive strength is the maximum compressive stress a solid material can sustain without fracturing.
Among the various concrete composites examined by the researchers, one particular mix stood out. By substituting 15% of the sand with SCG that had been pyrolyzed at 350 °C (662 °F), the material exhibited remarkable improvements in its properties, boasting a significant 29.3% increase in compressive strength.
Even though the study is still in its early stages, the researchers have great optimism regarding the results and their potential influence on global construction, considering the abundant availability of coffee.
Co-lead author Shannon Kilmartin-Lynch emphasized the potential contribution of the concrete industry in recycling organic waste, such as used coffee. "Although our research is still in its infancy, these exciting findings present an innovative solution to significantly reduce the volume of organic waste that ends up in landfills."
In addition to its waste-reducing benefits, this concrete production technique also addresses another pressing environmental concern: the conservation of finite natural resources. Each year, we extract approximately 40 to 50 billion tons of sand and gravel for construction purposes.
Jie Li, co-corresponding author of the study, pointed out the detrimental impact of extracting natural sand from river beds and banks worldwide to meet the soaring demands of the construction sector. "By adopting a circular economy approach, we can divert organic waste from landfills and better preserve our valuable resources, such as sand."
Moving forward, the researchers have plans to conduct long-term mechanical and durability tests on the 350-degree-heated coffee biochar, exploring its potential applications in the construction industry. Additionally, they aim to investigate the effects of different pyrolyzing temperatures on the material's performance.
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