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.

Figure 1 – Microscopic view of concrete aggregate before (left) and after (right) injecting CO2:  the calcium carbonates are visible as cube shaped crystals.

The technology: making ‘sinking CO₂ into concrete’ an integral part of recycling

Neustark’s ambition is to broadly deploy this process and the underlying technology. One of the key success factors to industrial deployment is a seamless integration of the CO₂ storage technology within existing concrete producers’ facilities.

This integration process is investigated as part of DemoUpCARMA project: The existing aggregate storage silos at Kästli’s concrete plant have been retrofitted with a CO₂ silo in order to integrate Neustark’s procedure directly at the plant. Additionally, a monitoring system has been installed to observe and control the CO₂ uptake. Moreover, the concrete slurry carbonation plant was designed as a palette-sized add-on to the slurry collection basin, which can carbonate the slurry on a continuous basis. It is connected to the same CO₂ silo and evaporator, which supplies the CO₂ also for the concrete aggregate carbonation plant.

The installation and implementation of the prototype at the Kästli plant indicates that Neustark’s technology is replicable and can be integrated into existing infrastructures without interfering with current operations. As part of DemoUpCARMA, the new plant fixing CO₂ in recycled concrete aggregate and concrete slurry is currently being operated at Kästli for the course of one year. The project goal is to store over 500 tons of CO₂ within that timeframe.

Figure 2 - The silo and outside view of the installation at Kästli.

The value chain: sourcing biogenic CO₂ from nearby facilities

Neustark sources its CO₂ for this project from the nearby wastewater plant Ara Region Bern. The biogenic CO₂ produced by the plant – which Ara Region Bern was releasing into the atmosphere before this collaboration – is captured from a point source and liquefied for transportation. Afterwards, a biogas-fueled truck brings the CO₂ to the storage plant using dedicated vessels of 20 tons of liquid CO₂.