title: Evolution of porosity, diffusivity and mineralogy of different cement-clay interfaces
Creators: name: Luraschi, Pietro
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 550 Earth sciences &amp; geology

description: Several countries are planning to store their radioactive waste in deep geological formations. In Switzerland, the repository will be made of tunnels and caverns and will be located several hundred meters underground within the Opalinus Clay (OPA). Concrete and cementitious materials will be used in large amounts inside the repository and will come in contact with OPA and bentonite. There will therefore be several regions where clays (OPA and bentonite) and cementitious materials will form a reactive interface. In fact, when cement and clay come in contact, the strong chemical differences between these two materials lead to mass fluxes and dissolution-precipitation reactions. The result is the formation of thin regions (mm to cm) with altered porosity, mineralogy and transport properties. The understanding of the behavior of such altered regions is relevant for the safety assessment of the nuclear waste repository. In this thesis, different cement paste-clay samples were produced and experimentally investigated in a laboratory by means of different analytical techniques: neutron imaging, X-ray tomography, SEM/EDX, TGA, XRD and through-diffusion experiments. The results are presented and discussed in four main chapters.
The first two chapters focused on high porosity Ordinary Portland Cement paste (OPC) in contact with Na-montmorillonite. The results evidenced a strong decrease of the sample’s diffusivity for both tritiated water (HTO) and chloride tracers already within the first 12 months of interaction. The chloride diffusivity was strongly reduced, suggesting that the newly formed phase precipitated in the free porosity (Chapter 3).
Mineralogical and porosity investigations on the same samples allowed detecting a low-porosity region on the montmorillonite side. This was mainly related to the precipitation of C-S-H phases within the first mm from the contact with the cement paste (Chapter 4). The cementitious side showed local porosity variation and various mineralogical alterations in the interface proximity (e.g., portlandite dissolution, Friedel’s salt precipitation).
Subsequently, interface samples composed of more realistic materials were produced and investigated by means of different analytical techniques during two years interaction at room temperature and in an oxygen-free atmosphere (Chapter 5). The cement side of the interfacing materials was composed of an OPC or low-pH ESDERD mortar, whereas bentonite or OPA composed the clay side. The experiments allowed to quantify the decrease of the diffusive properties of the different samples and to characterize the mineralogical alteration occurring at the interface. The mineralogical alteration was dependent on the interfacing materials.
In the last part (Chapter 6), new interface samples composed of OPC paste in contact with different clays (montmorillonite, bentonite, OPA) were produced and let react at 70°C for eight months, while the porewater chemistry and the pH were monitored. The experiments allowed gaining information about the chemical evolution of the different systems, and on the effect of the increased temperature on the interface evolution. It was demonstrated that high temperature experiments accelerate the samples reactivity, which is nevertheless comparable to the reactivity at room temperature in terms of mineralogical evolution.
The results of the different performed experiments significantly improved the knowledge regarding the modification occurring when cement and clay are put in contact. The gained information can be implemented in models and further experiments, and they also help understanding relevant processes occurring in a real repository.