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Recycled coffee grounds an unlikely binder in road base

It is a commodity that, like aggregate, is metaphorically one of modern society’s “building blocks”. Over 2.25 billion cups of coffee are consumed globally every day, and according to a Wikipedia list of 143 countries by consumption per capita, Australia ranked 42nd, downing three kilograms per person.

This was well behind Norway (12kg) at number one and other nations such as Canada (6.5kg), the United States (4.2kg) and even New Zealand (3.7kg). Nevertheless, despite this modest consumption, Australians still pride themselves as prolific coffee drinkers and some cities – Melbourne, Sydney, Brisbane – proudly consider themselves “coffee capitals”.

Now, if a Melbourne-based university engineering team is successful, Australia could lead the way in the appropriate diversion of coffee grounds from landfill and as a suitable binding agent in road construction.

The Geotechnical Engineering group in the Centre for Sustainable Infrastructure at Swinburne University of Technology is pioneering this research. The team is led by Professor Arul Arulrajah, who has been a chief investigator in 22 research projects worth $4.3 million (inclusive of Australian Research

Council, state government and industry grants) since joining Swinburne in 2006. His research over the best part of 25 years in areas such as recycled aggregates (including the blending of recycled glass and crushed rock in pavements, and crushed brick blends for sub-base applications), land reclamation, dredging, ground improvement, pavement geotechnics, in-situ testing and field instrumentation has also been recognised by Australian and international awards.

As part of a team comprising PhD student Teck-Ang Kua, Arulrajah had a paper published in the American Society of Engineers’ Journal of Materials in Civil Engineering in May this year that discussed the viability of a coffee grounds/fly ash geopolymer as an embankment structural fill material. A second paper that reported on the engineering and environmental evaluation of spent coffee grounds stabilised with industrial by-products as a road subgrade material is currently in press.

Both papers outlined how Arulrajah and Kua had collected coffee grounds from cafés on the Swinburne campus and around Melbourne to create in the laboratory a new geopolymer combining the grounds with alkaline liquid slag and fly ash that could be used as a road subgrade material.

Niche passion

Arulrajah described the coffee grounds as typical of his “niche research” interests in sustainable materials. “The coffee grounds are one of my passions,” he said. “I could not help but notice that baristas would throw coffee grounds into a bin that traditionally goes into a municipal waste collection. I thought, ‘Why not look at this sort of material from a road construction angle?’ I guess that was the inspiration for this particular research on coffee grounds as a road subgrade material.”

Arulrajah explained that he and Kua approached the project with two aims: one, to prove sustainable, useful coffee grounds could be used in road construction; and two, to create an alternative low carbon binder to Portland cement.

“My recent research has been into the use of alkaline activation of industrial by-products. It’s a process called geopolymerisation,” Arulrajah said. “I’ve also done some projects with construction and demolition materials using geopolymers instead of Portland cement. Instead of using cement, we use a geopolymer, which means you need an industrial by-product as a precursor. Typically it’s a fly ash or slag and we also need to combine it with the alkaline to create an alternative binder.

“So that’s the approach we used here with the coffee grounds. We used a liquid alkaline and the precursor – your fly ash or slag – to create this binder for the coffee grounds. There were various mix designs that we needed to go through and various temperatures we needed to evaluate to come up with the optimum mixed design which was 70 per cent coffee grounds and 30 per cent fly ash/slag.”

Like most quarried aggregates that go through the lab, the binder was oven-dried and subjected to moisture and dry density testing to determine the optimum moisture content. “Typically, with quarry materials, you go for a 100 degree temperature,” Arulrajah said. “With the coffee grounds, because there is a high organic content, we went for a low temperature of 50 degrees, so that the organics wouldn’t burn off. What we did with the coffee grounds is the same as you do with earth fill material in a laboratory environment. We dry it out and then add moisture to determine the maximum dry density and optimum moisture content in which you need to apply compaction in the field.”

The other challenge was to devise a mix design with the optimum moisture content that conformed to the specifications of various Australian road authorities. Arulrajah said the geopolymer binder met road authority specifications in terms of California Bearing Ratio (CBR) values and unconfined compressive strength (UCS) testing, and two subgrade products were developed that were the equivalent of regular road subgrade materials.

Commercial potential

With the initial laboratory phase complete, the new subgrade is now ready for durability testing. Arulrajah said he and Kua were keen to explore the long-term biodegradation of the new subgrade, particularly how it reacted to wet and dry cycles, as Australian roads are often subjected to both drought and floods.

“This current phase of research is looking more into the long term durability or biodegradation of the material,” Arulrajah said. “From previous work we’ve done with geopolymer binders, we know that the geopolymer tends to prevent the breakdown of organics over time, so we’re fairly confident that we will get some positive results from this current phase of long-term testing.”

If the current phase is successful, then the third phase will be to test the coffee ground/fly ash/slag geopolymer binder in a road construction application and the fourth phase (and long-term goal) will be to partner with an entity with a view to commercialising the end product.

“We are currently not sure who would be keen to partner us,” Arulrajah said. “We would need an entity to partner with us, to essentially provide a demonstration space as well as some research funding in order to take it into the field phase.”

Arulrajah expressed a preference for a commercial partner to be Australian, particularly if it’s based in Melbourne. “My personal preference is to do something in Melbourne because it’s my hometown and the coffee capital of the world,” he said. “I would want Australia to be at the forefront of this particular research and given that the idea for this particular use of coffee grounds started here in Melbourne, it would make a nice story for it to be finished off in Melbourne as well.”

However, Arulrajah does not rule out partnering with overseas interests. There has been Australian and international media interest in Arulrajah and Kua’s research since it was announced in May. “One or two media companies in the US and one in the Netherlands contacted Swinburne and asked me to talk about our research,” Arulrajah said. “Our work has generated quite a bit of interest and I guess it’s something that all of us can relate to, as a lot of us are coffee drinkers.”

Although he is uncertain just how many cafés operate across Australia, Arulrajah has estimated that each café would generate a “ballpark figure” of 150kg of coffee grounds per week. He has reasoned that if you multiply this random figure by the number of cafés nationwide, then between 50,000 and 100,000 tonnes of coffee grounds is going to landfill each year.

Grounds to KM

From his team’s research, Arulrajah has concluded that 50,000 tonnes of coffee grounds could produce up to five kilometres of road each year. “I think that even 5km of road is fairly significant because if you were to build any sort of road, that would cover a fair amount of coffee grounds,” he said. “And I guess the whole goal is to divert coffee grounds going to landfill. In Melbourne, we probably have the strongest coffee culture in the world. This would become a fairly significant quantity in order for everybody to look for the reuse options for coffee, and I think, ‘What better way than roads?’ To get just five or 10 or 15km of roads out of coffee grounds is essentially just one project and you will be able to eliminate all the coffee grounds from that fill.”

Arulrajah added that there are other environmentally friendly benefits to using coffee grounds in road subgrades. “The coffee grounds are an end of life material, so if it’s discarded to landfill, there’s no carbon footprint at all. Fly ash produced in Australia accounts for 12 million tonnes per annum and slag may be somewhere around three million tonnes per annum. So together with the coffee grounds, these are all traditional waste materials that we are looking to divert from landfill. So you get a lot of green savings there, thanks to the use of alkali activation, instead of Portland cement, which emits one tonne of CO2 per tonne of Portland cement that we manufacture.”

Arulrajah concluded that this focus on non-traditional waste materials offers quarries the opportunity to diversify their operations. “There’s always new ideas and new products that will continuously come into the market from recycled waste. There’s always waste, regardless of what kind of spoil is coming out of a virgin material production process. Quarries should be looking at the mix of spoils they are producing and consider the reuse options for these waste materials as well.” 

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