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

为了实现气候中和，自然资源的高消耗和建筑行业造成的高二氧化碳排放是需要克服的最大挑战。将混凝土废料纳入回收过程是实现既定目标的必要步骤。“碳捕获与利用”（CCU）提供了一种很有前途的方法。通过碳化混凝土废料的细粒（硬化水泥浆），然后将其用作活性水泥替代品，CCU可以解决这两个挑战的一部分。使用现代熟料还原复合水泥，预计将在将来产生的混凝土废料的细组分的化学和矿物学组成方面产生重大差异。然而，目前还缺乏系统的研究表明，熟料还原水泥是否适合作为一种反应性补充胶凝材料（SCM）进行碳酸化和后续使用。这是目前研究项目开始的开端。在第一部分中，研究了所使用的水泥的化学和矿物学与所产生的再生水泥浆体（recycSCM）的性能和反应性之间的关系。为此，现代水泥（CEM II/C-M和CEM VI）是人工生产、水化和碳化的。在接下来的步骤中，通过在前48小时应用水化模型，并在28天后根据DIN EN 197-1使用活性指数，系统地研究了recycSCM的性能。在硬化水泥浆的碳化过程中，二氧化碳的吸收，以及随后将回收SCM作为反应性SCM的使用，导致熟料含量的显著降低，从而提供了显著的二氧化碳节约潜力。这一节约潜力将与生产不同类型水泥期间排放的二氧化碳量进行比较。因此，高熟料水泥和低熟料水泥的二氧化碳平衡，加上不同的回收scm，可以详细展示，进一步，使用这些水泥生产的混凝土的生态足迹可以举例预测。综上所述，本文将对再生scm的关键参数反应性、水化行为和性能以及生态效率进行研究。根据该项目的结果，可以提前评估世界范围内普遍使用的组合物中未来混凝土废料的回收细粒作为活性粘结剂成分的贡献。

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.