CO2 removed from the atmosphere has been successfully stored in concrete for the first time, according to the US team behind the trial.

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The project saw California-based Heirloom – a specialist in direct air capture (DAC) carbon removal – team up with CarbonCure, whose reclaimed water technology injects CO2 into concrete wastewater, where it mineralises. This CO2/concrete slurry is a permanent store for the carbon and can be used in new concrete mixes. The trial took place at a plant in San Jose, California belonging to Central Concrete, which used the resulting concrete in a range of construction projects across the Bay Area.

“This demonstration project is a global milestone for carbon removal technology that confirms concrete’s enormous potential as a climate solution that can permanently store carbon in our most essential infrastructure - from roads and runways to hospitals and housing,” said Robert Niven, chair and CEO of CarbonCure Technologies. “We’re thrilled to be collaborating with Heirloom and Central Concrete on this groundbreaking world first.”

CarbonCure’s model of transforming CO2 from a waste stream into a resource for the construction industry has won several awards, including the Carbon X-Prize. It takes wastewater collected from washing out the inside of mixing trucks, then combines this with CO2 to create an upcycled construction resource. The company claims its technology can reduce the use of fresh water by 17-20 per cent, cut virgin cement use by 8-10 per cent, and save 15kg of CO2 for every cubic metre of concrete.

Heirloom’s DAC process uses limestone and electric kilns to remove CO2 from the atmosphere in a cycle that mimics nature, but which takes days rather than years. Crushed limestone is heated in the kilns, producing CO2 – which is captured - and calcium oxide, which is mixed with water to form calcium hydroxide. This is then spread on to large trays where it reacts with CO2 in the air to form limestone over the course of around three days. That limestone then gets sent to the kiln and the cycle begins again.

“The science is clear: In order to reach climate goals we must remove billions of tons of already emitted CO2 from the atmosphere each year,” said Shashank Samala, CEO of Heirloom. “This is an important step toward that future and shows the promise of DAC technologies combined with smart, permanent methods of sequestration.”

Even the most aggressive emissions reduction projections from the UN Intergovernmental Panel on Climate Change require the removal of 6-10 billion tons of CO2 per year by 2050 to stick to the 1.5˚C of warming set out in the Paris Agreement. DAC technologies are likely to play a key role in CO2 removal, and have recently received large investments from the US government through the Department of Energy’s $3.5 billion DAC hub program and the Inflation Reduction Act.