Towards a scientific-based assessment of long-term durability and performance of cementitious materials for radioactive waste conditioning and disposal

Because of its solid structure, low permeability, slow diffusion and good chemical retention for many radionuclides, cementitious materials are often used for conditioning radioactive waste and are omnipresent in many radioactive waste disposal facility concepts as engineered barriers. Cementitious materials are not in geochemical equilibrium with the surrounding disposal environment and will undergo slow but significant geochemical, physical and mechanical changes. The evolution and the consequences on their performance have to be assessed over extreme long time scales ranging from a few hundreds of years to several tens of thousands of years. For a deep understanding of the long-term geochemical and microstructural alteration processes, a synergy between (i) experimental studies, (ii) multi-scale modelling and (iii) coupled reactive transport models is necessary and a prerequisite to assess the long-term performance. This paper presents an overview of some recent developments in these three approaches. From an experimental point of view, well-designed accelerated degradation tests are required as a basis for mineralogical and microstructural characterization and for measurement of macroscale properties as diffusion coefficient, permeability and water sorptivity. From multiscale modelling perspective, deployment of thermodynamic model, continuum scale, mesoscale and pore scale models are key to advancing the understanding of material evolution. In particular, an overview of pore-scale models is presented that explicitly accounts for the heterogeneity of cement paste for calculating flow, transport and ageing processes and effects on microstructure and macroscopic properties. Some of these developments are discussed in the context of long-term carbonation and leaching of cementitious materials. Finally, the paper discusses opportunities in the context of immobilization of waste, including some recent modelling work.