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Every year, around six million tons of bottom ash are produced in Germany from the incineration of municipal waste. These are treated in specialized processing plants in order to recover metals and process them into a purified finished ash through targeted screening and sorting steps. “The mineral components it contains are able to absorb CO2 and bind it permanently – known as carbonation. In our project, we want to develop a practical carbonation process and test whether our end product is suitable for use in road construction or concrete production,” explains Prof. Dr. Björn Siebert from the Institute of Construction Materials, Geotechnics, Transport and Water at TH Köln.

Different approaches in the test facility

To this end, a technical test facility is to be built by the two scientific project partners TH Köln and RWTH Aachen at the Leppe waste disposal center. This will be operated by the consortium leader, the Bergischer Abfallwirtschaftsverband (BAV), near Lindlar. As an industrial partner, refer GmbH provides the starting material, processed incinerator bottom ash from its grate ash processing plant.

The key technical question is which method can be used to bind the CO2 in the ash most efficiently. “We are pursuing two approaches, each of which has specific advantages and disadvantages: Wet carbonation under water allows more carbon dioxide to be absorbed, but requires energy for subsequent drying. In wet carbonation with little moisture, a relatively dense, carbonized layer forms on the surface of the ash, so that less CO2 is stored as it cannot penetrate into the interior,” says Prof. Dr. Axel Wellendorf from the Institute of General Mechanical Engineering at TH Köln.

Use in road construction or concrete production

Initially, the partners will test different carbonation processes and set up a flexible, practical test facility for this purpose, with which the processes can be set up and compared under the most realistic conditions possible. The focus then shifts to the application of the material obtained: “Our aim is to substitute raw materials that would otherwise be obtained using large amounts of energy. In unbound form, our carbonated ash could replace gravel or sand in road construction or earthworks, for example,” says Siebert.

At the same time, the researchers are examining the potential for CO2-intensive concrete production. Here, the ash could act as a binding agent. The prerequisite for this application is a defined, consistent material quality. For this reason, the project is also carrying out tests on further processing or pure crushing. “If we succeed in making carbonated ash available that meets the relevant standards and environmental requirements, this would be an important step for the circular economy and climate protection,” emphasizes Wellendorf.