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In many designs for radioactive waste repositories, cement and clay will come into direct contact. The geochemical contrast between cement and clay will lead to mass fluxes across the interface, which consequently results in alteration of structural and transport properties of both materials that may affect the performance of the multi-barrier system. We present an experimental approach to study cement-clay interactions and the consequent mineralogical and structural evolution (dissolution/precipitation of minerals, porosity changes) as well as its feedback on transport properties (diffusion coefficients, accessible porosities) of both materials. A diffusion cell has been developed to accommodate interface samples consisting of a small (5 mm diameter, 5 mm length) clay plug (montmorillonite) and a small hardened cement plug (OPC) of the same size. It allowed performing both in situ measurement of water content across the sample using neutron radiography and measurement of transport parameters using through-diffusion tracer experiments and dynamic neutron imaging. The aim of the high-resolution neutron radiography experiments at the ICON beam line at SINQ (PSI) was to monitor changes in water content (porosity) and their spatial extent. Neutron radiographs of several evolving cement-clay interfaces over a period of three years delivered quantitative data which allow resolving local water contents within the sample domain. An increase in the water content of the sample at the cement side of the interface accompanied by a reduction of water content at the clay side was observed. In addition, Through-diffusion experiments showed that by increasing the reaction time the diffusive flux tended to decrease. No complete clogging was observed during the measurements (about one year), and the HTO tracer could always diffuse across the sample domain. To verify “the flux reduction across the whole sample domain” observed in the through-diffusion experiments and “the structural change” observed in time-resolved neutron imaging measurements, and also to quantify their effect on transport properties of the sample at the interface, a new diffusion experiment has been performed. Heavy water (D2O) was introduced to diffuse into the normal pore water (H2O) present in the cement and clay pore space. Advancement of a D2O front then, resolved in time and space, across several fresh and aged interface samples was measured using cold neutrons at ICON facility in SINQ, PSI. The difference in neutron scattering properties of these two types of water makes it possible to observe the spatially resolved dynamics of the D2O diffusion. Diffusion through the fresh interface samples occurred as expected from the original diffusion parameters of these materials. In contrast, the D2O front nearly stopped at the interface of the aged sample. This may be the result of mineralogical changes at the interface, for instance, a porosity reduction by mineral precipitation known as clogging. The methodology developed in the course of this thesis allows probing different systems for better constraining the numerical models. For example, the precipitation of BaSO4/SrSO4 in clay systems and its feedback on transport parameters was explored in collaboration with other groups. The relevant processes of these structural and transport parameters alterations have to be implemented into reactive transport simulations in order to improve the safety assessment for waste repositories and generally help enhancing its design. |
