Dezember 2022 - by Johannes Tiefenthaler, Co-Founder and Co-CEO at Neustark
With over 1 billion tons per year, concrete from construction and demolition is the world's single largest waste stream. ETH spin-off Neustark developed a method to help turn this waste stream into a carbon sink, thus permanently removing CO2 from the atmosphere and generating negative emissions. To achieve that, a solution was conceived that mineralizes CO2 and stores it in recycled concrete aggregate. In 2017, the company started developing this process, and in 2022, they were able to deploy it at industrial scale within the framework of DemoUpCARMA.
The recycling streams: working with concrete producers
To investigate the domestic Carbon dioxide Capture, Utilization and Storage (CCUS) pathway, Neustark collaborates with the concrete producer Kästli. Kästli produces two recycled products that can fix CO2, which is of biogenic origin in the DemoUpCARMA project: concrete aggregate and concrete slurry.
Concrete aggregate is produced by crushing demolition concrete – originating from the demolition of concrete structures – for the subsequent re-use in road construction or for the production of new concrete. After crushing, the concrete aggregate is transported via conveyor belts to silos where it is temporarily stored. Concrete slurry is a side product from washing trucks and concrete batching systems. This slurry is typically water with a solid content (mainly cement and sand) of less than 10 % in mass. It is typically collected in a basin and reused as a pure-water replacement for ready mix concrete.
The physics: transforming CO2 into rock
Both concrete aggregate and concrete slurry contain hydrated cement phases. These hydrated cement phases are in contact with water – and thus in a solid-liquid equilibrium. Part of the hydrated cement is dissolved in the water and is present as ionic. As CO2 is also dissolved in this water, a new mineral that exhibits a lower solubility than the hydrated cement phases precipitates, – and voilà, calcium carbonate (CaCO3) is formed. Thus, the CO2 and the hydrated cement undergo a chemical transformation to form rock. This so-called carbonation reaction of 1 kg CO2 releases heat such that the temperature of 1,000 kg of concrete increases by 4 to 5°C.
CaCO3 is considered to be amongst the most permanent ways to sequester carbon. Only temperatures above 600°C or very strong acids could trigger the release of CO2.This ensures that the CO2 remains stored in the concrete, even if it is demolished again after being reused.