Roland Pusch

Microstructural evolution of buffers

A buffer consisting of blocks of compacted Na bentonite powder absorbs water by which the grains expand and exfoliate, yielding clay gels that occupy the voids between them. The expandability of the grains and thereby the bulk density and hydraulic conductivity depend on the density of the gel fillings and hence on the grain density. Since the porewater electrolytes affect the pore size distribution of the gels they have an impact on the hydraulic conductivity as well. The density distribution in matured MX-80 clay determines both the hydraulic conductivity and the ion diffusion transport as well as the swelling pressure. It can be evaluated from micrographs covering a sufficiently large fraction of a cross-section of the clay.

Electron microscopic examination of hydrothermally treated bentonite clay

Abstract Water-saturated Na bentonite clay with a bulk density of 2 g/cm3 was autoclaved at 150 and 200°C for up to 0.5 years and then left to rest under confined conditions at room temperature for several weeks. Transmission electron microscopy with elemental micro-analysis was applied to study hydrothermally induced changes, which were found to have the form of particle reorganization and, at 200°C, release and precipitation of silica. The precipitations are suspected to be hydrated amorphous silica gels. The microstructural alteration, which was more obvious at 200°C than at 150°C, involved regrouping of previously dispersed and expanded stacks of montmorillonite flakes to yield dense branches of interwoven, contracted stacks separated by large voids.

Physico/chemical stability of smectite clays

Abstract There is convincing evidence from field data that smectite clay undergoes conversion primarily to illite and chlorite if it is fully water-saturated and heated. The conversion may take place through mixed-layer formation with increasing illite/smectite ratio at higher temperatures and pressures. This process requires dehydration of the interlamellar space, for which either an external pressure or drying are needed. An alternative mechanism that takes place without dehydration, is dissolution of smectite and neoformation of illite. Both processes imply reorganization of the smectite crystal lattice for which the activation energy is fairly high, meaning that the conversion is negligible at temperatures lower than about 60°C. At elevated temperatures the conversion rate is controlled by the access to potassium for either mechanism. An ongoing detailed investigation of this subject has led to a tentative model for the smectite-to-illite conversion in natural sediments and in canister-embedding clay in high-level radioactive waste (HLW) repositories.

ASPECTS ON THE ILLITIZATION OF THE KINNEKULLE BENTONITES

Earlier interpretations of the conversion of the virgin smectite of the Ordovician Kinnekulle K-bentonites into the present mixed-layer illite/smectite imply that it took place through charge increase of the smectite with subsequent uptake and fixation of potassium. Recent analyses show that the layer charge of the smectite component of the I/S is in fact low and they suggest that neoformation of a separate illite phase took place. Pytte’s kinetic model gives good agreement with the actual conversion rate for an activation energy of about 25–27 kcal/mole, depending on the adopted rate parameters, temperature history and assumed potassium source. In the Kinnekulle case the rate-controlling factor appears to have been the supply of potassium, which is concluded to have required large-scale, heat-induced groundwater convection.

Quick-clay microstructure

Abstract The article deals with a quick clay which was originally deposited in sea water. By leaching in situ the salt content has been reduced to a very small value. By using a special technique for step-wise replacement of pore water with acrylate plastic, 500A˚thick sections were cut with a precision microtome. The sections were photographed in an electron microscope and the micrographs obtained could be used for a study of the clay microstructure. The microstructure is characterized by a linkage of groups or chains of small particles in and between denser flocs or aggregates or between bigger particles. There is no preferential orientation either of small or of bigger particles. A preliminary study of the quick clay and of unleached parts of the same clay stratum has not revealed any microstructural differences. The extreme thinness of the clay sections means that the micrographs reveal pores larger than about 500A˚. Thus the micrographs give a fairly complete picture of the size and shape variation of the micropores in the clay. By measuring the maximum dimension of all pores which could be identified and by using suitable methods for statistical condensation, representative values of mean pore size and two-dimensional “porosity” were obtained. These characteristics are discussed in relation to the permeability and strength properties of the quick clay and of some fresh- and brackish-water deposited clays which have been investigated previously. Finally, on the basis of the micrographs a hypothesis is made concerning the rate of settlement.

Basic model of water- and gas-flow through smectite clay buffers

Abstract Canister embedment of highly compacted Na bentonite forms a low-permeable medium of significant homogeneity both macro- and microscopically. However, despite the fact that a large part of the porewater is in interlamellar positions and not mobile by ordinary hydraulic gradients, there are still a number of pore passages that let water and gas through even at very high bulk densities. A preliminary model for flow and diffusion has been outlined on the basis of generalized, quantitative microstructural data and basic physical relationships. It accounts for actually recorded hydraulic and gas conductivities as well as for commonly measured swelling pressures. Qualitatively, it is in agreement also with published ion diffusion data.

THE SENSITIVITY OF ARTIFICIALLY SEDIMENTED ORGANIC-FREE ILLITIC CLAY

Abstract A series of experiments with artificially sedimented illitic clay was carried out with the purpose to investigate if quickness could be produced by leaching of a clay deposited in salt water. In order to reduce the influence of such factors as organic constituents, a high silt content or a particular mineral composition, a fine-grained organic-free illite clay and an artificially produced salt water were used. The elimination of organic substances was obtained by a prolonged treatment with hydrogen peroxide of the dry clay material. It was found that the organic matter was very firmly bound to the mineral phase. No marked sensitivity change was caused by the leaching. It was concluded that leaching may no be a sufficient condition for the formation of quick-clay. This is in direct contradiction with the results from similar tests by Bjerrum and Rosenqvist (1957) . The discrepancy is discussed with special reference to the fact that Bjerrum and Rosenqvist used a clay material which was known to be a quick- clay in its natural state. Such a material may have an organic content which governs the remoulded shear strenght in the leached state.

Microstructural stability controls the hydraulic conductivity of smectitic buffer clay

The hydraulic conductivity of smectitic buffer clays is determined by the volume fraction and continuity of permeable parts of the microstructure, i.e., soft and medium-dense clay gels. The bulk conductivity calculated by use of microstructural parameters agrees well with experimental data except for soft Ca bentonite, which is significantly less permeable than predicted. The microstructural stability of its softest parts is poor, which causes erosion, transport and accumulation of particles yielding clogging of voids and reduction in conductivity. The presence of soft parts explains why water under relatively high pressure can penetrate to a few centimeters depth in partly water-saturated clay and why gas makes its way through channel-like paths in saturated clay

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.

Experience from preparation and investigation of clay microstructure

Clays without expanding minerals can be prepared so that their microstructural constitution is preserved. If expansive clays are treated in the same fashion there is an obvious risk of very significant distortion of the particle arrangement because of drying, expansion and disintegration. It is also a risk that impregnated clay becomes too soft to be cut for microscopy, but experience shows that embedding in a suitable plastic monomer with subsequent polymerization gives sufficiently hard specimens to make ultramicrotomy applicable. Impregnation with this substance requires that the pore water is first removed, which can be done in several ways as described and discussed in this paper. The thickness of the thin section should be as small as possible for transmission electron microscopy, i.e. a few hundred angstroms, but thicker sections can be used for certain microstructural analyses.