The microstructure of bentonite clay

Abstract

Bentonite is a common clay having many applications. One of them will be in the spent nuclear fuel repository, as a buffer material between host rock and the canisters containing the radioactive waste. This application has high requirements for the material used, ranging from, among others, appropriate mechanical properties, chemical stability, fine porous structure to sufficient hydraulic conductivity. The material should maintain its properties over time as the life span of the repository is hundreds of thousands years. The aim of this thesis was to investigate the structure of bentonite clay in a condition similar to expected in the repository. The research focused mainly on the pore structure, and if the existence of narrow, slit-like pores between clay layers influenced the pore water properties. Several experimental methods had been employed: small-angle X-ray scattering, transmission electron microscopy, atomic force microscopy, nuclear magnetic resonance and anion exclusion measurement. MX-80 bentonite was the main material studied, but sodium and calcium montmorillonites obtained by purification of MX-80 were used as a simplified study case. Several sample preparation routines have been tested. Additionally, MX-80 bentonite from long-term experiment was analysed and the results were compared to those obtained in short-time experiments. It was shown that the purification process significantly alters the clay structure, making the purified clay unsuitable as simplified analogue of natural bentonite in structural studies. The influence of different preparation procedures on clay microstructure was decreasing with increasing compaction of the samples. Porosity had been characterised using interlamellar and non-interlamellar porosity. With the increase of density, as total porosity of the sample decreases, the proportion of interlamellar porosity increases.