Identification of Na-smectite hydration by use of “humid cell” high voltage microscopy

Abstract

Swelling of smectite clays is a property of fundamental importance for the use of such barrier materials in repositories for high level radioactive wastes and is readily observed macroscopically. The hydration process on a molecular scale, involving separation of adjacent smectite lamellae by penetrating water molecules, remains hypothetical but its detailed microscopic features have now been documented. A 3 MV electron microscope was used for the study, which comprised filling of Na-smectite in powder form in a closed cell that was penetrated by electron radiation in the microscope after exposing the clay to water and thereafter drying it. The study showed two practically important facts. Firstly, the wetting phase initially resulted in a thin hydrated “shell” of the dry aggregates. Micrographs taken after various periods of time demonstrated that the “shell” softened and expanded, thereby connecting clay aggregates that were initially separated. The inverse process took place in the drying phase, i.e. the very soft, relatively homogeneous clay gel was transformed to separate dense aggregates. Secondly, the expansion of the fully hydrated aggregates did not yield a homogeneous gel with uniform interparticle distance. Instead, the expansion of the aggregates was limited so that a stable condition was reached in which a small amount of the porewater was contained within the aggregates, while a major portion occupied larger inter-aggregate voids. This confirms the conclusion from various electron optical studies of smectites prepared by use of resin-embedding techniques, that it is necessary to distinguish between “internal” (i.e. interlamellar, also termed “interlayer”) and “external” water. Their ratio is a determinant of the percolation and diffusion properties in bulk.