Engineers at RMIT University have developed an innovative approach to concrete that could yield major performance boosts in construction.
As the construction industry faces increasing challenges from material shortages and a growing demand for sustainable practices, engineers at RMIT University have come up with an innovative solution.
By replacing a portion of the sand used in concrete with coffee biochar, the team has developed a construction material that has already made the transition from the lab to real world applications, including as part of a footpath trial and in a road upgrade for Victoria’s Big Build program of works.
Australia generates 75 million kg of ground coffee waste each year, most of which ends up in landfill. It was an issue that prompted engineers to look for ways to recycle the waste, according to lead innovator Dr Rajeev Roychand of the RMIT School of Engineering.
The resulting material has the potential to replace up to 655 million kg of sand in concrete, addressing the issue of coffee waste as well as providing an environmentally friendly solution for the construction industry.
“Coffee biochar reduces reliance on dwindling sand supplies, cuts landfill waste, and acts as a carbon sink to lower concrete’s carbon footprint – and it also enhances its mechanical properties, boosting compressive strength,” Roychand said.
Watch more: This modelling tool can enhance the durability of concrete structures
From the lab to the field
Biochar not only reduces the environmental impact of concrete, it also alters the material’s properties due to its significantly lower density. While sand has a density of around 2700 kg/m³, biochar’s density is much lighter, at approximately 350 kg/m³.
“For the same volume, biochar weighs much less than sand, which can reduce the overall weight of concrete and improve its performance, especially in applications where weight is a critical factor,” Roychand said.
Additionally, the lower density of biochar allows for easier transport and handling, offering environmental and logistical benefits over traditional sand.
During laboratory trials, coffee biochar was derived by pyrolysing spent coffee grounds at 350°C, Roychand said.
“The lab results indicated that when used as a partial replacement for sand, coffee biochar could enhance concrete strength by up to 30 per cent.”
However, there were some challenges when the process was applied in the field.
“During the first field trial, the commercial-scale pyrolysis equipment used could not operate at the optimal 350°C due to operational constraints, which affected the quality of the coffee biochar,” he said.
Instead, it was produced at 450°C and was wet when it was added to the concrete, introducing excess moisture into the concrete mix, which can disrupt the water-to-cement ratio.
“This can lead to altered workability, reduced concrete strength and longer curing times, all of which can affect the final quality and timeline of the construction.”
In the second field trial with BildGroup and Major Road Projects Victoria, they managed to achieve the pyrolysing temperature of 400°C, still not optimal but an improvement on the first trial, and the biochar was dried before using it in concrete.
“The results that were supplied to us by the concrete testing company organised by BildGroup showed a 52 per cent increase in compressive strength at seven days, and a 21 per cent increase at 28 days for biochar-blended concrete,” Roychand said.
Read how Victoria University has used granules made of disposable coffee cups as a substitute for sand in concrete.
Environmental benefits
Cement is the largest contributor to carbon emissions in concrete, so reducing its usage through the significant strength enhancement achieved by using biochar is a powerful way to cut down on the carbon footprint of construction.
“This strength improvement means that less cement is needed to achieve the same performance, further amplifying the environmental benefits.
“Unlike traditional high-temperature methods that burn materials and release carbon dioxide, pyrolysis limits oxygen to preserve carbon in the biochar, which is essentially carbon sequestration.”
It’s an attractive proposition to the construction industry, with the RMIT project gaining significant international interest, including a collaboration with Swedish biochar company C-Green and Australia’s Ambioblock, which specialises in PFAS remediation, including the reuse of PFAS-impacted concrete.
“Ambioblock has collaborated with us to assess the performance of biochar and other materials, such as PFAS-affected soils. Our goal is to enhance sustainability while maximising the environmental benefits of incorporating their product.”
As the project continues to expand, the team has broadened its focus beyond coffee grounds to explore a variety of organic waste materials, with the goal of transforming all forms of organic wastes into a valuable resource.
“What began as research into coffee grounds has evolved into a comprehensive program. We are now working with a variety of organic waste materials, each delivering significant performance benefits and contributing to a more sustainable future for construction.”
This EA OnDemand webinar explores the maximum tolerance of carbon emissions for procured concrete.
