Boral has undertaken trials to prove crushed recycled concrete can effectively absorb captured CO2.
The push to decarbonise Australia’s construction and infrastructure sectors is accelerating, but one material sits at the centre of the challenge: concrete.
Concrete is one of the most widely used construction materials in the world, but it comes with a major carbon footprint, one that is increasingly difficult to abate.
Boral head of sustainability and innovation, Dr Ali Nezhad, said this is exactly why the company is investing so heavily in lower-carbon materials and new technologies that change how concrete is made, used, and recycled.
“Concrete emissions come mostly from cement, which is the main binder,” he said.
“Almost 65 per cent of those emissions are hard-to-abate emissions incurred during cement manufacturing. So, carbon capture becomes an important decarbonisation lever to address that.”
“One challenge is the utilisation or the storage of that carbon,” Nezhad said.
“You’re looking for storage in places or in materials that you can reutilise somewhere. That’s more important from the environmental point of view and the financial point of view.”
Nezhad said that the team at Boral began searching for a material which can absorb CO2 and hold it permanently.
The answer lay in concrete itself.
The process of re-carbonation, according to Boral, is where carbon dioxide is absorbed by the concrete used in buildings and infrastructure through their service life and end of life phases.
“Concrete loves to absorb carbon dioxide,” Nezhad said.
“Studies have shown that concrete could absorb anywhere between 20 and 55 per cent of the process emissions from cement manufacturing during the life of the building or infrastructure.”
For Nezhad and the team, that question became: what if this process could be engineered and accelerated?
“With that concept, we knew that concrete is good at absorbing carbon, and we knew we needed to store carbon in a material,” he said.
“Then the idea was, can we just use concrete to store the carbon emissions from our cement manufacturing?”
The breakthrough came through Boral’s recycling operations.
Construction and demolition waste already provides the company with significant volumes of crushed concrete, and crushing increases surface area, which in turn boosts CO2 absorption.
“If the concrete is crushed into a smaller body, that gives it a larger surface area and better absorption,” Nezhad said.
“Luckily, the crushed concrete we are looking for is the product of our recycling business.”
Boral demonstrated the system at larger scale, proving that crushed recycled concrete can effectively absorb captured CO2. But absorption was only half the battle; the material also needed to be useful.
The team then developed concrete mixes where natural aggregates were replaced with these re-carbonated aggregates.
“We had to demonstrate that it can be reused and utilised,” Nezhad said.
“You get two advantages. One is a reduction in the embodied carbon of concrete because of the carbon that’s going into those aggregates, and the other is the environmental benefit of reusing natural aggregates through recycling.”
According to Nezhad, concrete and asphalt together account for anywhere between 20 and 70 per cent of the embodied carbon of buildings and infrastructure.
“Our buildings and infrastructure can’t be decarbonised unless these materials are decarbonised,” he said.
Boral’s plan does not limit itself to carbon-absorbing aggregates.
Nezhad said that more than 90 per cent of concrete’s embodied carbon comes from cement, which typically makes up less than 15 per cent of the mix by weight. So decarbonising cement itself remains the biggest and most impactful lever.
“The first option you have is decarbonising cement manufacturing,” he said.
“If you have less emissions from cement, you can go a long way.”
Nezhad said that over the past five-six years, Boral has “accelerated its alternative fuel program,” shifting away from coal toward waste-derived options.
“We are now using 114 kilo tonnes of waste material instead of coal,” he said.
“That’s a massive source of reduction.”
Boral is also investing in alternative raw materials.
“We’re replacing limestone with pre-carbonated materials. Where it doesn’t need to be decomposed into carbon dioxide,” Nezhad said.
While full decarbonisation of cement manufacturing remains Boral’s goal, Nezhad said that the company is also pursuing a second strategy: use less cement in concrete.
“A lot of our research and development goes into finding new ways to use less cement,” Nezhad said.
Nezhad said that supplementary cementitious materials (SCMs) such as fly ash and slag have long played that role, but supply constraints and performance limitations mean the industry needs to be innovative.
“One of Boral’s key focus areas is achieving high cement-replacement levels without compromising the properties of concrete,’ he said.
Nezhad said that the company is now working to push replacement rates to 70 per cent or higher, while also identifying new SCMs.
“One of the challenges we face is having access to enough materials to replace cement,” he said.
“We’ve made progress recently with calcined clay, but when cement manufacturing decarbonises fully, you’ll achieve your objective.” •
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