Recyclability of low-clinker cements using "carbon capture and utilisation"- Roads to a sustainable concrete production

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The high consumption of natural resources and the high CO2 emissions caused by the construction sector are the biggest challenges to be overcome in order to achieve climate neutrality. The integration of concrete waste into recycling processes is an essential step to achieve the defined goals. “Carbon capture and utilization” (CCU) offers a promising approach. By carbonating the fines of concrete waste (hardened cement paste) and then using it as a reactive cement substitute, CCU can be part of the solution to both challenges. The use of modern clinker-reduced composite cements is expected to result in significant differences in the chemical and mineralogical composition of the fine fractions of concrete waste that will be produced in the future. However, systematic investigations are still lacking to which extend clinker-reduced cements are suitable for carbonation and subsequent use as a reactive supplementary cementitious materials (SCM). This is the onset of the present research project starts. In the first part, the relationships between chemistry and mineralogy of the cements used and the properties as well as the reactivity of the resulting recycled cement paste (recycSCM) are investigated. For this purpose, modern cements (CEM II/C-M and CEM VI) are artificially produced, hydrated and carbonated. In the following step, the performance of the recycSCM is systematically investigated by applying a hydration model to the first 48 hours and using the activity index according to DIN EN 197-1 after 28 days. The uptake of CO2 during the carbonation of the hardened cement paste and the subsequent use of the recycSCM as a reactive SCM cause a considerable reduction of the clinker content and thus offer a significant CO2 saving potential. This savings potential will be compared to the amount of CO2 emitted during the production of the different types of cement. Thus, the CO2 balance of high and low clinker cements with the addition of the different recycSCMs can be presented in detail and, further on, the ecological footprint for concretes produced using these cements can be predicted exemplarily. In summary, the key parameters reactivity, hydration behavior and performance as well as the ecological efficiency of recycSCMs will be investigated. With the results of the project, recycled fines of future concrete waste in the compositions commonly expected to be used worldwide can be assessed in advance with regard to their contribution as a reactive binder component.