Project summary

The K4 Project (Carbon Dioxide Reduction through Low-Lime Clinker and Carbonation Curing in Concrete Production) combines the direct avoidance of process related CO₂ emissions (Carbon Direct Avoidance) in clinker production with the permanent binding of carbon dioxide in concrete through carbonation curing.

In this project, the team at TUM has investigated the permanent binding of CO₂ in concrete and its effects on durability. In addition, TUM carried out an ecological assessment (LCA).

The carbonation degree of two binders, a CEM I 52.5 N and a belite rich K4 cement, were investigated. Carbonation curing is affected by the binder composition, relative humidity, CO₂ concentration, and carbonation duration. The results showed that under optimal conditions (80 % relative humidity, 50 % CO₂), the carbonation degree increases, while expanded pre-curing at low relative humidity reduces the carbonation degree. 

In the next step, the effect of carbonation curing on microstructure, phase development, and durability was examined. Carbonation curing densified the microstructure, which had a positive impact on durability. However, due to its mineralogical composition, the K4 cement shows a low hydrate phase formation, resulting in reduced strength. Overall, we demonstrated that carbonation cured concretes meet durability standards and thus maintains the service life of the concrete samples.

At TUM, we conducted the life cycle assessment (LCA) at both material and product levels. K4 cement, produced with recycled material and at lower calcination temperatures, reduced the global warming potential (GWP) compared to conventional CEM I. Concrete samples with K4 cement showed higher CO₂ uptake than CEM I at the industrial experiments. The use of green electricity in CO₂ capture significantly lowered the GWP.

The K4 project was funded by the Federal Ministry of Research, Technology and Space (BMFTR) within the KlimPro-Industrie program and carried out with Heidelberg Materials AG, Lithonplus, IBU-tec, Kraft Curing Systems GmbH, and Friedrich-Alexander-Universität Erlangen-Nürnberg from 2021 to 2025.