Microstructural and micro-mechanical property changes of cement pastes for ILW immobilization due to irradiation

While degradation of concrete due to irradiation is a widely studied subject, the deterioration and changes in microstructure of cement pastes immobilizing evaporator concentrates due to gamma radiation is a very specific subject because of the nature of the solidified radioactive waste. The cement paste proposed as possibly suitable binder for cementitious composites immobilizing evaporator concentrates (marked as NP in further text), a paste with the same composition but mixed with simulated evaporator concentrates (NP_C) and a reference paste (PC) were subjected to gamma irradiation from Co-60 source at doses of 2 MGy and studied in terms of microstructure changes using Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDX), X-ray diffraction (XRD) and nanoindentation. SEM-EDX revealed rise of new phases on the NP_C samples rich in Na⁺ compounds due to the irradiation. Results from nanoindentation indicated a formation of carbonates in the irradiated samples, but XRD analysis did not find a significant rise in CaCO₃ minerals. The possible explanation is a rise of alkali carbonates in their amorphous form. (author)