Radiation-Induced Micromechanical Degradation of Cement Paste Under Gamma and Neutron Exposure Relevant to Long-Term Concrete Performance in Nuclear Power Plants

Abstract This study examines the effects of gamma and combined neutron–gamma irradiation on cement paste and mortar, focusing on the micromechanical response of the main hydrated phases and associated microstructural changes. Samples were irradiated under both neutron and gamma conditions. Nanoindentation was performed to assess the mechanical behavior of individual phases. Results show that irradiation induces stiffening of the primary hydrates and causes compaction and densification of C–S–H gels, including partial breakdown of fibrillar structures and reorganization of colloids into clustered, rosette-like morphologies. Irradiation also led to the formation of microcracks within the cement paste. While these microcracks did not affect the micromechanical response of individual phases, they are likely responsible for the reductions in compressive strength observed in cement composites under irradiation. The findings demonstrate that irradiation produces irreversible nanoscale and microscale modifications in cementitious materials, and that local phase stiffening combined with microcrack formation provides a mechanistic explanation for the degradation of bulk mechanical properties.