Concrete as carbon dioxide trap – right at the plant

6 to 8 percent of man-made CO₂ emissions worldwide are attributable to cement, the indispensable binder for the hard and versatile building material – but at the same time it is able to chemically bind the climate gas emitted during cement production again after manufacture; at least partially: between 11 and 30 percent, depending on the recipe and conditions. “Carbonation” is the name given to this process, in which limestone is formed from calcium hydroxide in concrete and CO₂ – a leisurely reaction that takes years and whose pace depends on numerous factors. For a long time, experts have been thinking about using it to improve the carbon footprint of concrete – and now, in the DemoUpCARMA project, Empa experts are working with partners under the leadership of ETH Zurich to test whether and how the process can be used and, above all, accelerated in a real concrete plant.

CO₂ savings of a good 15 percent

Specifically: in a specially installed plant of the company Kästli Bau AG in Rubigen in the canton of Bern – and with recycled material from deconstructed concrete structures. The carbon dioxide with which this material is “fed” comes from the nearby wastewater treatment plant and is delivered in a liquefied state. At the plant, it is then stored in a special silo from where the recycled granules are continuously “flooded” using a process developed by the Bern-based company Neustark AG – in other words, exposed to pure CO₂ gas and the uptake precisely measured. The team of experts researched the processes that take place in detail during the CO₂ uptake of the recycling granulate. They also investigated how so-called “recycling water” from water, cement and sand, which is produced for example during the cleaning of concrete vehicles and mixing plants, can be used to bind carbon dioxide.

Test results show that the process can make concrete more climate-friendly in two ways. Firstly, by absorbing CO₂ to “relieve” the atmosphere: Up to 10 percent of the emissions that were released into the atmosphere during the production of the cement for the original concrete can be absorbed by the novel building materials. Secondly, by reducing the cement content in recycled concrete – by 5 to 7 percent – thanks to its higher strength. The bottom line, according to the Empa experts, is that the potential for CO₂ savings are a good 15 percent.

Further information can be found on the Empa website.