The rationale for opting for cement-based materials as an engineered barrier in a geological disposal facility for radioactive waste

This paper provides experimental justification for selecting cement-based materials for engineered safety barriers (ESB) in geological disposal facilities (GDF) for radioactive waste. The long-term performance of three materials — Ordinary Portland Cement Concrete (OPCC), Calcium Aluminate Cement Concrete (CACC), and Nirex Reference Vault Backfill (NRVB) —was comparatively analyzed under conditions simulating groundwater impact in a crystalline rock massif. A one-year filtration experiment was conducted using synthetic groundwater at elevated pressure. The evaluation included comprehensive assessment of phase composition evolution, mechanical strength, hydraulic permeability, and chemical/electrochemical parameters of the interacting liquid phase. Results demonstrated that OPCC maintained increased content of strengthening phases (C-S-H, ettringite), enhanced mechanical strength exceeding the required minimum of 7.5 MPa, and consistently low hydraulic permeability (~10 -12 m/s), meeting ESB criteria for contact with bentonite buffer. The pH of its filtrate remained below 11.5, minimizing alkaline degradation risks for bentonite. In contrast, NRVB exhibited complete dissolution of C-S-H and portlandite, resulting in significant strength reduction and high hydraulic permeability, making it unsuitable for bentonite buffer contact. For aluminate concrete, cross-reaction of metastable phase CAH 10 into less dense C 3 AH 6 caused substantial strength decrease and increased hydraulic permeability exceeding maximum permissible values (10 -10 m/s), limiting its GDF application. Based on these findings, Ordinary Portland Cement Concrete emerges as the preferred material for ESB construction in GDF. These results establish foundation for subsequent geochemical modeling of long-term cement barrier evolution.