Conceptual models of smectite hydration include planar (flat) clay layers that undergo stepwise expansion as successive monolayers of water molecules fill the interlayer regions. However, X-ray diffraction (XRD) studies indicate the presence of interstratified hydration states, suggesting non-uniform interlayer hydration in smectites. Additionally, recent theoretical studies have shown that clay layers can adopt bent configurations over nanometer-scale lateral dimensions with minimal effect on mechanical properties. Therefore, in this study we used molecular simulations to evaluate structural properties and water adsorption isotherms for montmorillonite models composed of bent clay layers in mixed hydration states. Results are compared with models consisting of planar clay layers with interstratified hydration states (e.g. 1W–2W). The small degree of bending in these models (up to 1.5 Å of vertical displacement over a 1.3 nm lateral dimension) had little or no effect on bond lengths and angle distributions within the clay layers. Except for models that included dry states, porosities and simulated water adsorption isotherms were nearly identical for bent or flat clay layers with the same averaged layer spacing. Similar agreement was seen with Na- and Ca-exchanged clays. While the small bent models did not retain their configurations during unconstrained molecular dynamics simulation with flexible clay layers, we show that bent structures are stable at much larger length scales by simulating a 41.6×7.1 nm2 system that included dehydrated and hydrated regions in the same interlayer.

Cementitious materials and their alkaline pore fluids can change the structure of bentonite used as a raw material for road embankments or concrete storage of garbage cans. This study investigated the alteration of montmorillonite-rich bentonite from northeast Morocco (Trebia deposit, Nador) in alkaline media rich in Ca2+, Mg2+, Na+, or K+. Specimens based on raw bentonite mixed with variable proportions of oxides (CaO, MgO) or hydroxides (NaOH, KOH) and water were prepared and aged for 28 days. Mineralogical composition by X-ray diffraction (XRD) was determined on raw bentonite and specimens to follow phase changes. Chemical composition and thermal characteristics were determined for raw bentonite and specimens by Fourier-transform infrared spectroscopy (FT-IR) and thermogravimetric/differential thermal analysis (TGA/DTA). Microstructural evolution and alteration of the external surface of bentonite were evaluated using scanning electron microscopy coupled with energy dispersive X-ray (SEM/EDX) analysis. XRD results of bentonite-CaO mixture demonstrated the formation of gels (e.g. C-S-H) and calcite. When the amount of CaO added increased, excess portlandite and the precipitation of calcite in the outer surface of bentonite occurred, stopping pozzolanic reaction and consequently decreasing the compressive strength of specimens. On the other hand, the addition of MgO allowed the formation of brucite. Sodalite and cancrinite were neoformed with the addition of 32 wt.% NaOH after 28 days of hydration. The addition of hydroxides (NaOH or KOH) to bentonite did not reveal any setting due to the absence of the formation of cementitious phases.

Adsorption isotherms for water vapor, c-spacing and heat of immersion in water of mixed Na/Ca-montmorillonite were measured at 25°C at various RH. There was good agreement between the calorimetric data, the heat calculated from the isotherms by use of BET equation, and the calculations from the ion-dipole model. It was concluded that the electrostatic forces between the adsorbed cations and the water molecules are the dominant forces in the hydration of the clay. Thus, at low moisture content, only the adsorbed Ca-ions are hydrated. The heat released when Na-platelets condense to form Ca-packets was measured, and it was suggested that this energy term is the driving force for the demixing phenomena.

Li-bearing smectite minerals occurring as hydrothermal alteration products of magnesium silicate minerals in skarns associated with the Moldova Nouǎ, Romania, porphyry copper deposit were examined by X-ray powder diffraction, infrared spectroscopy, and thermal and chemical analyses. Li-bearing smectite containing 0.45–0.50 Li/unit cell is common, whereas smectite containing 0.21–0.33 Li/unit cell is less common. Both materials coexist with talc and kerolite. The Li-bearing smectite minerals (b = 9.111 Å) contains semi-ordered or ordered stacking and is highly crystalline, similar to saponite. After 3-yr storage under laboratory conditions in an air-dried state (RH = 50%) or after heating for 2 hr at 100°, 200°, 300°, or 400°C, the Li-bearing smectite minerals showed charactenstics of a regular 1:1 interstratification of anhydrous and dihydrate layers. Some segregation of the anhydrous, monohydrate, and dihydrate layers was noted.

The amount of Li-for-Mg substitution was found to be close to that in hectorite, and the number of octahedral vacancies was similar to that in stevensite. This Li-bearing smectite apparently formed directly from colloidal suspensions at atmospheric temperature and pressure.

Electron spin resonance (ESR) spectra of Cu2+-hexadecylpyridinium (HDP) montmorillonites were investigated as a function of HDP+ content and the hydration state of Cu2+ at relative humidities of p/p0 = 0.52-8 × 10-7 at 298°K. The symmetry of the Cu2+ ESR spectra and the intensity of the ESR signal increased upon dehydration of the complex. The HDP+ cation caused an increase in the hydration state of Cu2+ at a given p/p0 and an increase in the covalency of the Cu-O bond.

The 001 spacing of Na-smectite was found to vary from 9.6 Å at 0% relative humidity (RH) to 12.4 Å at 60-65% RH. The 9.6-Å spacing corresponds to dehydrated Na-smectite, and the 12.4-Å corresponds to Na-smectite with one water layer. A regular series of intermediate values resulted from ordered interstratification of the 9.6- and 12.4-Å units. Ordered interstratification was confirmed by the presence of a 001 spacing of 9.6 + 12.4 Å = 22 Å. This peak appeared under experimental conditions at about 35% RH. It appeared for calculated simulations of ordered stacking of 50/50 mixtures (±10%) of 9.6- and 12.4-Å units. The 004 peak of this 22-Å spacing interacted with the 002 of the 9.6-Å spacing of ordered mixtures of more than 50% 9.6-Å units and with the 002 of the 12.4-Å spacing of ordered mixtures of more than 50% 12.4-Å units. The result of this interaction was a complex peak, the position of which was a function of the ratio of 9.6- and 12.4-Å units. This complex peak was noted for experimental and for calculated conditions. Calculated tracings assuming ordered stacking matched the experimental tracings closely, whereas those assuming random stacking did not.

Ordering was apparently due to the interaction of the positive charge of the interlayer cation repelling the positive charge of the hydrogens of the hydroxyl ions, one above and one below, closest to the interlayer space. The collapse of a single interlayer space (dehydration) brought the interlayer cation closer to the hydrogens of the hydroxyls causing the hydroxyls to rotate such that the hydrogens shifted toward the adjacent interlayer spaces. Collapse of these two interlayer spaces was therefore more difficult. This same mechanism helps explain ordering in illite/smectite. The difference is that hydration/dehydration is quick and reversible, whereas the change from smectite to illite is slow and irreversible.

The enthalpy of freezing of methyl salicylate sorbed by four commercial montmorillonite clays was used to estimate the absorption/adsorption capacity of the clay. Differential thermal analysis was therefore employed to give a quantitative estimate of the sorption capacity of the clay for methyl salicylate. The absorption capacity of the clay depended on the state of hydration and ranged from 30.4% for an 8% water content to 9.7% for a montmorillonite containing 24% water.

Hydroxychromium montmorillonites were prepared from solutions containing chromium nitrate and varying amounts of NaOH. The reactant Cr ions were hydrolyzed and, in some experiments, were dimerized to a significant extent. The extent of polymerization in the product formed from a solution containing a maximum amount of [Cr2(H2O)8(OH)2]4+ was compared with that in products obtained from solutions containing monomer, [Cr(H2O)5OH]2+, and lesser amounts of the dimer. Despite the uncertainty about the nature and amounts of product interlamellar species, the catalytic effect of the montmorillonite interlayer on the hydrolysis and polymerization of Cr(III) appears to be independent of the nature of the reactant species. The validity of this conclusion depends to a large extent on the reliability of water determination in the clay mineral species, which is estimated to be no better than 5%. The assumption that H2O+ is 'combined’ and H2O— is 'free’ water is shown to be partially true, thereby causing some uncertainty in the interpretations of interlayer compositions.

Secondary minerals formed during simulated weathering of nuclear waste glasses have been identified by analytical electron microscopy. A complete description of the reacted glass, from the outermost surface in direct contact with the leachant solution to the reacting front that migrates into the bulk glass, was obtained. Manganese and iron oxyhydroxide phases and saponite were found to have precipitated onto the residual glass surface from the leachant solution. Iron-bearing smectite, serpentine, and manganese and uranium-titanium oxyhydroxides formed in situ in the glass in several distinct bands at different depths beneath the original surface. This sequential development of secondary phases displays a clear trend toward more order and crystallinity in the phases farthest from the reaction front and indicates that complete restructuring of the glass into crystalline phases did not occur at the interface with fresh glass. Additionally, the formation of a discrete uranium-bearing phase, as opposed to uranium uptake by precipitated phases, suggests that stable actinide phase formation rather than ion exchange may be a possible mechanism for retarding radionuclide release to the environment.

A high-pressure, high-temperature cell was used to monitor the basal X-ray powder diffraction spacing of Na-saturated Cheto montmorillonite in contact with NaCl solutions at temperatures as high as 200°C and hydraulic pressures as high as 6700 psi (456 bar). The 003 and 005 reflections were used to determine d(001) of the smectite. The montmorillonite, in 1 molal NaCl, exhibited a d(001) of 15.4 Å at room temperature and pressure and a d(001) of 15.6–15.7 Å under 500–2200 psi hydraulic pressure. The basal spacing of the clay in 5 molal NaCl was 15.2 Å and 15.33–15.45 Å at 1 bar and 750–6700 psi (53–456 bar), respectively. Because no changes in the basal spacing with increasing temperature to 200°C were detected in any of the experiments, this Na-smectite probably exists as a two-water-layer complex under diagenetic conditions.

Smectites synthesized from experiments at 5.5 GPa and 1500°C are of high quality, crystals are large at >10 μm, and the 2:1 layers may have a homogeneous charge distribution. Smectite was exchanged with various cations (Na+, Li+, K+, Ca2+, and Mg2+) and the hydration behavior of each sample was observed by an in situ powder X-ray diffraction method under precisely controlled relative humidity (RH). The smectite showed distinct stepwise (discontinuous) hydration versus RH. During the transition between two hydration states, the coexistence of the two states was observed. Randomly interstratified structures with one and two planes of H2O are time-dependent phenomena and relate to hydration and dehydration processes.

A new method for the prediction of Gibbs free energies of formation for hydrated clay minerals is proposed based on the parameter ΔGO= Mz+(clay) characterizing the oxygen affinity of the cation Mz+. The Gibbs free energy of formation from constituent oxides is considered as the sum of the products of the molar fraction of an oxygen atom bound to any two cations multiplied by the electronegativity difference defined by the ΔGO= Mz+(clay) between any two consecutive cations. The ΔGO= Mz+(clay) value, using a weighting scheme involving the electronegativity of a cation in a specific site (interlayer, octahedral, or tetrahedral) is assumed to be constant and can be calculated by minimization of the difference between experimental Gibbs free energies (determined from solubility measurements) and calculated Gibbs free energies of formation from constituent oxides. Results indicate that this prediction method compared to other determinations, gives values within 0.5% of the experimentally estimated values. The relationships between ΔGO= Mz+(clay) corresponding to the electronegativity of a cation in either interlayer or octahedral sites and known ΔGO= Mz+(aq) were determined, thereby allowing the prediction of the electronegativity of transition metal ions and trivalent ions in hydrated interlayer sites and octahedral sites. Prediction of Gibbs free energies of formation of any clay mineral with various ions located in the interlayer and with different cations in octahedral sites is possible. Examples are given for Al-rich montmorillonite from Aberdeen, transition element-exchanged montmorillonite, and Ni-rich stevensite, and the results appear excellent when compared to experimental values.

The objective of this study was to investigate the influence of layer charge on the hydration of Mg-saturated expandable 2:1 phyllosilicates. Water retained by 12 Mg-saturated clays at 54% relative humidity was quantified gravimetrically. X-ray diffraction and total chemical analysis were used to determine the hydratable surface area (447–759 m2 g−1) and layer charge [0.327–0.754 electrons per formula unit (e f.u.−1)] of each sample. Water retained by the clays increased with both hydratable surface area and layer charge of the clays. However, the increase in H2O content with layer charge occurred only on external surfaces of the clays. This result suggests that the H2O on external surfaces is localized around the cation/charge sites rather than forming multi-layers as was suggested previously. A model is proposed for the hydration of expandable 2:1 phyllosilicates. The model assumes that interlayer volume controls interlayer hydration and that the number of cation/charge sites on external surfaces controls hydration of external surfaces.

Nonexchangeable polymers in interlayers of expansible phyllosilicates influence thermal dehydration in ways not completely understood. Thermal dehydration of hydroxy-interlayered vermiculite (HIV) from Florida soils, for example, results in irreversible d001 shifts. This study was conducted to characterize HIV dehydration as a function of time (t) and temperature (T), and to determine how reversibility of dehydration is affected by elevated T. Clay-sized HIV from 3 soils was heated incrementally and d-spacing shifts (Δd) were monitored by X-ray diffraction (XRD) at low relative humidity (RH). Samples were then mounted on a metal heating strip in the XRD focal plane and scanned repeatedly at constant T levels to monitor Δd with t. Finally, Δd in response to RH shifts from <5% to >85% was determined at 25°C and at elevated temperatures. Incremental heating revealed a Δd plateau roughly corresponding to the z dimension of hexameric octahedrally coordinated Al. The initial slope of Δd-vs-t curves increased with T. The same maximum Δd reached at 200°C was reached at 160°C, but more slowly. All samples exhibited reversible and irreversible dehydration, the former being attributable to sites in equilibrium with external vapor and the latter to sites requiring heat for desorption. Reversible sites were not perturbed by moderate heating, but were apparently eliminated by polymer dehydroxytation. The dehydration behavior of HIV could be explained by steric resistance of water vapor diffusion within a tortuous interlayer polymeric network. Alternatively, new polymer/oxygen-surface bonds exceeding the hydration energy of interlayer components could form via heat-induced re-articulation of polymer/oxygen-surface bonds at smaller basal spacings.

The application of the Frenkel-Halsey-Hill (FHH) formalism to the water desorption isotherms obtained for the whole range of the activity of water with the pressure membrane device (0.98 < aw < 1) and with the desiccator (0 < aw < 0.98) gives information concerning the nature and the relative importance of the 2 mechanisms involved in the dehydration—hydration processes: adsorption and capillary condensation. The state and location of water are described in each domain. An equation that gives the thickness t of the film of water adsorbed on the walls of pores versus the activity of water is developed. This t-curve is used to get, from the desorption isotherm, the pore size distribution curve of the studied hydrated materials. Then concepts of surface and fabric of clay pastes are discussed as a function of hydration and a mechanism is proposed to explain swelling and shrinkage of finely divided materials. Three kinds of surfaces, related to the aggregate fabric, are defined as a function of their capacity to adsorb water. Each kind of surface is determined by a specific technique: the total surface area (St) by ethylene glycol adsorption, the external surface area of particles (Ss) by nitrogen adsorption and the external surface area of aggregates (Se) by hydraulic conductivity measurements. As a consequence it is only with completely dispersed clays that swelling is a function of St. With unwell-dispersed clays, water adsorption, which induces swelling, successively occurs on St, Ss and Se surfaces.

The in-depth perturbation of vicinal water by the surfaces of montmorillonite layers was investigated by relating the swelling pressure, Π, of the montmorillonite layers to the H-O-H bending frequency, ν2, of the interlayer water. For this purpose, an oriented montmorillonite gel was deposited on a porous filter in an environmental chamber. On its underside the filter was in contact with a solution maintained at atmospheric pressure. By admitting nitrogen gas at a known pressure to the environmental chamber, water was squeezed from the gel into the solution until equilibrium was reached and Π equalled the applied pressure. Then the gel was divided into 2 parts. One part was used for the gravimetric determination of the water content, mw/mc. It was possible, therefore, to determine mw/mc as a function of Π. The other part of the sample was transferred to an FTIR spectrometer where the ν2 of the water within it was measured by attenuated total reflectance. Thus, the same samples were used to determine the dependence of both Π and ν2 on mw/mc. It was found that Π and ν2 were both exponential functions of mc/mw and so a linear relation was found between ln(Π + 1) and ln(ν2/ν2°), where ν2° is the H-O-H bending frequency of bulk water. These results strongly support the conclusion that the in-depth perturbation of the water by the surfaces of the montmorillonite layers is primarily responsible for both the development of Π and the departure of ν2 from ν2°.

An environmental infrared microbalance (EIRM) cell was used to study H2O sorption on two montmorillonite samples as a function of water content and type of exchangeable cation. The vibrational spectra showed that H2O sorbed to the clay at low-water content was strongly influenced by the exchangeable cation and by the close proximity to the clay surface. At water contents <6 H20 molecules per exchangeable cation, the H-O-H bending mode of H2O (v2 mode) shifts to a lower frequency and is characterized by an increase in molar absorptivity. In contrast, the positions of the asymmetric and symmetric OH-stretching modes of sorbed water (v1 and v3 modes) shift to higher energies. These observations indicate that H2O molecules sorbed to the clay surface at low-water content are less hydrogen bonded than in bulk H2O. In addition, the vibrational-stretching and bending bands of the structural OH groups of the 2:1 layer are also strongly influenced by H2O content and type of exchangeable cation. By using the EIRM cell, the molar absorptivities of the structural OH-bending vibrations were measured as a function of H2O content. The position and molar absorptivity of the structural OH-bending bands at 920, 883, and 840 cm-1 are strongly influenced by H2O content and type of exchangeable cation. The molar absorptivity of the 920-cm-1 band, which is assigned to the AlAlOH group, decreased strongly at low-H2O content. This reduction in intensity is assigned to a dehydration-induced change in orientation of the structural OH groups resulting from the penetration of H2O molecules into siloxane ditrigonal cavities that are not associated with a net negative charge from isomorphous substitutions.

Hydration behavior of Na-smectite crystals synthesized at a pressure of 5.5 GPa and temperatures of 1400°–1500°C was examined by X-ray powder diffraction at various relative humidities (RH) in the range of 0–100%. The basal spacing of the Na-smectite crystal increased stepwise with increase in RH. The reflections observed were only normal reflections of a single or dual hydration states of smectite. No irrational, intermediate, or asymmetrical reflections were observed. The simple hydration behavior, not known for natural smectite with fine particle sizes and low crystallinity, indicates that the Na-smectite crystals are as perfect as common inorganic crystals with an ordered structure.