Investigation into the microstructural and mineralogical changes induced by internal swelling reactions in concrete used for nuclear power plants – a mock-up study

Extending the lifetime of existing nuclear facilities is becoming in-creasingly relevant to ensure the continued generation of electricity with a low carbon footprint. For this purpose, a thorough assessment is required of the potential pathologies that may arise and affect the integrity of existing concrete support structures. Of particular interest are internal swelling reac-tions, such as delayed ettringite formation (DEF) and alkali-silica reaction (ASR), which can cause swelling and cracking of concrete. While these pro-cesses have been well-researched at a laboratory scale, controlled studies at a larger scale are more scarce. To simulate the effects of these processes on a larger scale, concrete mock-ups were constructed, subjected to elevated tem-perature and humidity, and monitored, after which the mock-ups were sam-pled for further characterization. The purpose of this study was to conduct a detailed investigation of this type of concrete to assess whether the macro-scopic effects of the swelling reactions had a significant impact on the mi-crostructure and mineralogy, i.e. to investigate the effect of the dissolution or precipitation of phases induced by internal swelling reactions. The findings indicated that the bulk mineralogy and pore size distribution of the concrete samples were not significantly altered by either DEF or ASR. However, elec-tron microscopy imaging revealed significant amounts of cracking across all samples, which was associated with infilling by either ettringite crystals or ASR gel, depending on the mock-up. The infilling of cracks explains the lim-ited effect of either process on the overall pore size distribution. Although the overall volumetric expansion in the mock-ups was significant, it was primarily associated with heterogeneous large-scale cracks, with only minor contributions from the microscopic fractures.