Bentonites are in worldwide use as candidate materials for the encapsulation of high-level radioactive waste (HLRW). To effectively seal waste canisters, bentonite is compacted to bentonite blocks which can be used to build a wall around the canisters. Compaction significantly improves the swelling pressure, which is currently considered as one of the most important parameters for assessing barrier performance. Most of the studies on compressibility of bentonites consider a few different materials only, which does not lead to a general understanding of bentonite performance. In order to identify the actual compressibility differences of different bentonites, a sizeable set of well characterized materials was investigated with respect to the dry densities after compaction.

Different results were obtained for bentonites that had been dried and bentonites that were equilibrated at 70% r.h. (relative humidity) prior to compaction. The dry density of dried bentonites depends on total porosity and particle density. However, the dry density of microporous bentonites depends on the microporosity rather than total porosity because microporosity is not reduced upon compaction. On the other hand, for the samples previously equilibrated at 70% r.h., the water content is most important. However, the water content, i.e. the water uptake capacity at 70% r.h., in turn largely depends on the CEC but also on microporosity. Therefore, under a given load, the 36 bentonites studied showed a significant range of resulting dry densities, depending on water content, CEC and porosity.

In conclusion, for a given bentonite, the dry density after compaction explains some geotechnical parameters such as the swelling pressure. However, for reasons explained in the present study, the dry density cannot be generally used to predict these parameters.

The elastic and structural behaviour of dehydrated analcime in compression in a non-penetrating medium up to 3 GPa was studied in a diamond anvil cell using in situ synchrotron powder diffraction. A first-order phase transition at 0.4–0.7 GPa is accompanied by a symmetry change from monoclinic (I2/a) to pseudo-rhombohedral (R3) due to trigonalization of the aluminosilicate framework. This is due to the migration of cations to new positions close to the 6-membered rings forming the channels. The reduction of the mean aperture of the structure-forming 6- and 8-membered rings, as a result of tetrahedral tilting, leads to a 7.5% reduction in volume at the phase transition. The bulk modulus values are 38(2) GPa for the low pressure (LP) phase [fitted with a Murnaghan equation of state, K' = 4 (fixed)] and 11(4) GPa for the high pressure (HP) phase [fitted with a third-order Birch–Murnaghan equation of state, K' = 9(1)]. The elastic behaviour of the LP phase is anisotropic, with compressibilities βa:βb:βc in the ratio 1:4:2; the most compressible direction b coinciding with the orientation of empty 8-membered rings. The compressibility of the HP phase is isotropic. Trigonalization appears to be the most effective (and probably unique) mechanism of radical volume contraction for the ANA structure type.

This is a review of the elastic behaviour and pressure (P)-induced structural evolution of zeolites and presents a comparative analysis of the deformation mechanisms of the Si/Al-framework and the rearrangement of the extra-framework species in response to applied pressure. The interaction between P-transmitting fluids and zeolites, which can lead to phenomena such as ‘P-induced over-hydration’, is described. The comparative elastic analysis and the high-P structural data of zeolites reported so far allow us to make some generalizations: (1) The range of compressibility among this class of openframework silicates is large, with bulk moduli ranging between 15 and 70 GPa; (2) Microporosity does not necessarily imply high compressibility, as several zeolites are less compressible than other nonzeolitic rock-forming minerals; (3) Compressibilities of zeolites do not seem to be directly related to microporosity, at least if we model microporosity with the ‘framework density’; (4) The flexibility observed in zeolites under hydrostatic compression is mainly governed by tilting of rigid tetrahedra around O atoms that behave as hinges within the framework. Pressure-induced tilting commonly leads to continuous rearrangement of the framework without any phase transition. More rarely, tilting induces displacive phase transitions and isothermal P-induced reconstructive phase transitions (i.e. with change in framework topology), have not been reported in this class of materials; (5) Deformation mechanisms in response to applied pressure are generally dictated by the topological configuration of the framework rather than the Si/Al-distribution or the extra-framework content. The channel content governs the compressibility of the cavities, leading to different unit-cell-volume compressibilities in isotypic structures.

Ambient temperature X-ray diffraction data were collected at different pressures from two crystals of β-As4S4, which were made by heating realgar under vacuum at 295ºC for 24 h. These data were used to calculate the unit-cell parameters at pressures up to 6.86 GPa. Above 2.86 GPa, it was only possible to make an approximate measurement of the unit-cell parameters. As expected for a crystal structure that contains molecular units held together by weak van der Waals interactions, β-As4S4 has an exceptionally high compressibility. The compressibility data were fitted to a third-order Birch–Murnaghan equation of state with a resulting volume V0 = 808.2(2) Å3, bulk modulus K0 = 10.9(2) GPa and K' = 8.9(3). These values are extremely close to those reported for the low-temperature polymorph of As4S4, realgar, which contains the same As4S4 cage-molecule. Structural analysis showed that the unit-cell contraction is due mainly to the reduction in intermolecular distances, which causes a substantial reduction in the unit-cell volume (∼21% at 6.86 GPa). The cage-like As4S4 molecules are only slightly affected. No phase transitions occur in the pressure range investigated.

Micro-Raman spectra, collected across the entire pressure range, show that the peaks associated with As–As stretching have the greatest pressure dependence; the S–As–S bending frequency and the As–S stretching have a much weaker dependence or no variation at all as the pressure increases; this is in excellent agreement with the structural data.

Ikaite (CaCO3.6H2O), which forms in cold carbonate-rich marine environments, also crystallizes from calcite under aqueous conditions above ∼0.5 GPa at room temperature. Using synchrotron X-ray powder diffraction, measurements have been made of pressure-induced changes in unit-cell dimensions of ikaite contained in a diamond anvil cell. Ikaite shows anisotropic compressibility along the crystallographic axes in the order a > c > b up to 4 GPa. Comparison with other phases shows the relative volume compressibility of ikaite to be greater than that of gypsum (CaSO4.2H2O), calcite and aragonite. The volume of ikaite is less than that occupied by equivalent volumes of CaCO3 (calcite/aragonite) + 6H2O at all pressures below the freezing point (H2O(l) to ice VI). The bulk modulus of ikaite has been obtained from a fit to the Vinet equation of state, giving V0 = 760.3±0.1 Å3, K0 = 21.3±1.4 GPa, and K0' = 11.7±1.7. Dissolution of CaCO3 in H2O (l) at high pressure resulting in crystallization of CaCO3.6H2O suggests equilibrium behaviour between a carbonate-rich fluid species and the condensed hydrated calcium carbonate. The potential for ikaite as a candidate phase for water transport in cold subduction zones is considered.

Calcigel bentonite-polyacrylamide (PAA) composites were investigated for their formation characteristics and their macroscopic hydro-mechanical and microscopic structural properties. The composites prepared were different in terms of the polymer charge, i.e. cationic (PAA+), anionic (PAA−), non-ionic (PAA°) and in terms of the polymer-to-clay ratio. The latter varied according to the individual maximum sorption capacity established from adsorption isotherms. Oedometer and waterretention testswere conducted on composites prepared under initial slurry conditions. The microstructure of the composites was investigated using environmental scanning electron microscopy (ESEM). At low stress, distinctly increased void ratios were found for all types of the composites, whereas under highstress conditions only the PAA+-composite prepared at 100% of the maximum sorption capacity showed an increase in void ratio. Analyses by ESEM on this composite indicated structural changes related to the preferential face-to-face aggregation, which help to explain the macroscopic behaviour and also account for the increased hydraulic permeabilities observed.

The commonly used incompressible phase field models for non-reactive, binary fluids, in which the Cahn-Hilliard equation is used for the transport of phase variables (volume fractions), conserve the total volume of each phase as well as the material volume, but do not conserve the mass of the fluid mixture when densities of two components are different. In this paper, we formulate the phase field theory for mixtures of two incompressible fluids, consistent with the quasi-compressible theory [28], to ensure conservation of mass and momentum for the fluid mixture in addition to conservation of volume for each fluid phase. In this formulation, the mass-average velocity is no longer divergence-free (solenoidal) when densities of two components in the mixture are not equal, making it a compressible model subject to an internal con-straint. In one formulation of the compressible models with internal constraints (model 2), energy dissipation can be clearly established. An efficient numerical method is then devised to enforce this compressible internal constraint. Numerical simulations in confined geometries for both compressible and the incompressible models are carried out using spatially high order spectral methods to contrast the model predictions. Numerical comparisons show that (a) predictions by the two models agree qualitatively in the situation where the interfacial mixing layer is thin; and (b) predictions differ significantly in binary fluid mixtures undergoing mixing with a large mixing zone. The numerical study delineates the limitation of the commonly used incompressible phase field model using volume fractions and thereby cautions its predictive value in simulating well-mixed binary fluids.

A problem of thermal instability of a compressible micropolar fluid layer heated from below in the presence of suspended particles has been investigated. Dispersion relation is derived and Rayleigh number curve is then plotted against the wavenumber at different values of compressibility parameter for a model example. Compressibility is found to be responsible to destabilize the system in the presence and absence of suspended particles for both stationary and over stationary convection.

The effect of pressure on amide 15N chemical shifts was studied in uniformly 15N-labeled basic pancreatic trypsin inhibitor (BPTI) in 90%1H2O/10%2H2O, pH 4.6, by 1H-15N heteronuclear correlation spectroscopy between 1 and 2,000 bar. Most 15N signals were low field shifted linearly and reversibly with pressure (0.468 ± 0.285 ppm/2 kbar), indicating that the entire polypeptide backbone structure is sensitive to pressure. A significant variation of shifts among different amide groups (0–1.5 ppm/2 kbar) indicates a heterogeneous response throughout within the three-dimensional structure of the protein. A tendency toward low field shifts is correlated with a decrease in hydrogen bond distance on the order of 0.03 Å/2 kbar for the bond between the amide nitrogen atom and the oxygen atom of either carbonyl or water. The variation of 15N shifts is considered to reflect site-specific changes in φ, ψ angles. For β-sheet residues, a decrease in ψ angles by 1–2°/2 kbar is estimated. On average, shifts are larger for helical and loop regions (0.553 ± 0.343 and 0.519 ± 0.261 ppm/2 kbar, respectively) than for β-sheet (0.295 ± 0.195 ppm/2 kbar), suggesting that the pressure-induced structural changes (local compressibilities) are larger in helical and loop regions than in β-sheet. Because compressibility is correlated with volume fluctuation, the result is taken to indicate that the volume fluctuation is larger in helical and loop regions than in β-sheet. An important aspect of the volume fluctuation inferred from pressure shifts is that they include motions in slower time ranges (less than milliseconds) in which many biological processes may take place.