Molecular Dynamics Simulation of Nanoscale Elastic Properties of Hydrated Na-, Cs-, and Ca-Montmorillonite

The knowledge of nanoscale mechanical properties of montmorillonite (MMT) with various compensation cations upon hydration is essential for many environmental engineering-related applications. This paper uses a Molecular Dynamics (MD) method to simulate nanoscale elastic properties of hydrated Na-, Cs-, and Ca-MMT with unconstrained system atoms. The variation of basal spacing of MMT shows step characteristics in the initial crystalline swelling stage followed by an approximately linear change in the subsequent osmotic swelling stage as the increasing of interlayer water content. The water content of MMT in the thermodynamic stable-state conditions during hydration is determined by comparing the immersion energy and hydration energy. Under this stable hydration state, the nanoscale elastic properties are further simulated by the constant strain method. Since the non-bonding strength between MMT lamellae is much lower than the boning strength within the mineral structure, the in-plane and out-of-plane strength of MMT has strong anisotropy. Simulated results including the stiffness tensor and linear elastic constants based on the assumption of orthotropic symmetry are all in good agreement with results from the literature. Furthermore, the out-of-plane stiffness tensor components of C33, C44, and C55 all fluctuate with the increase of interlayer water content, which is related to the formation of interlayer H-bonds and atom-free volume ratio. The in-plane stiffness tensor components C11, C22, and C12 decrease nonlinearly with the increase of water content, and these components are mainly controlled by the bonding strength of mineral atoms and the geometry of the hydrated MMT system. Young’s modulus in all three directions exhibits a nonlinear decrease with increasing water content.

Effect of Pore water expression on solid composition of cement paste

Pore solution expression is the most commonly used method to obtain aqueous phase in cementitious material. However, the high pressure applied on the sample may affect solid phase composition. This paper presents an experimental study on the chemical and mineral composition of cement paste before and after the expression. Results indicate that a small part of the alkali contained in samples can be excluded during the pore solution expression, mainly depending on alkali concentration in the pore solution. Due to the expulsion of interlayer water in C-S-H under high pressure, the pore solution expression reduces bound water content measured by TGA. The portlandite content determined by TGA is not affected by pore solution expression, but it leads to an overestimation of portlandite in QXRD, because of the enhanced preferred orientation of the (001) plane under the applied high pressure. In addition, the size of portlandite crystal decreases to some extent due to the creep caused by pressure. The content of hemicarbonate phase is found to decrease slightly after pore solution expression, which may be aroused by the increased solubility under high pressure.

In Situ Measurements of the Hydration Behavior of Compacted Milos (SD80) Bentonite by Wet-Cell X-ray Diffraction in an Opalinus Clay Pore Water and a Diluted Cap Rock Brine

Compacted bentonite is currently being considered as a suitable backfill material for sealing underground repositories for radioactive waste as part of a multi-barrier concept. Although showing favorable properties for this purpose (swelling capability, low permeability, and high adsorption capacity), the best choice of material remains unclear. The goal of this study was to examine and compare the hydration behavior of a Milos (Greek) Ca-bentonite sample (SD80) in two types of simulated ground water: i) Opalinus clay pore water, and ii) a diluted saline cap rock brine using a confined volume, flow-through reaction cell adapted for in situ monitoring by X-ray diffraction. Based on wet-cell X-ray diffractometry (XRD) and calculations with the software CALCMIX of the smectite d(001) reflection, it was possible to quantify the abundance of water layers (WL) in the interlayer spaces and the amount of non-interlayer water uptake during hydration using the two types of solutions. This was done by varying WL distributions to fit the CALCMIX-simulated XRD model to the observed data. Hydrating SD80 bentonite with Opalinus clay pore water resulted in the formation of a dominant mixture of 3- and 4-WLs. The preservation of ca. 10% 1-WLs and the apparent disappearance of 2-WLs in this hydrated sample are attributed to small quantities of interlayer K (ca. 8% of exchangeable cations). The SD80 bentonite of equivalent packing density that was hydrated in diluted cap rock brine also contained ca. 15% 1-WLs, associated with a slightly higher concentration of interlayer K. However, this sample showed notable suppression of WL thickness with 2- and 3-WLs dominating in the steady-state condition. This effect is to be expected for the higher salt content of the brine but the observed generation of CO2 gas in this experiment, derived from enhanced dissolution of calcite, may have contributed to the suppression of WL thickness. Based on a comparison with all published wet-cell bentonite hydration experiments, the ratio of packing density to the total layer charge of smectite is suggested as a useful proxy for predicting the relative amounts of interlayer and non-interlayer water incorporated during hydration. Such information is important for assessing the subsequent rates of chemical transport through the bentonite barrier.

Surface Chemistry of Halloysite Nanotubes. As investigated by adsorption reactions and imaging analysis

Interest in halloysite clay minerals has increased steadily over the last 20+ years, in part, due to their nanotubular shape and size, high aspect ratio and potential technological applications (Churchman et al., 2016). Throughout this thesis the surface chemistry of halloysite nanotubes is investigated by employing several analytical and advanced microscopy techniques to obtain a greater understanding of their internal and external surface features and surface adsorption reactions. This multifaceted analysis approach investigates a range of tubular halloysites of two morphologies, cylindrical and polygonal prismatic, where comparison of the two forms a framework for the presentation and discussion of the results. The results presented in Chapter 3 demonstrate the phosphate adsorption capacity of halloysite nanotubes is influenced by pH and maximum adsorptions of 1.3 mg/g and 0.5 mg/g were obtained for the cylindrical and polygonal prismatic morphologies respectively. Use of advanced microscopy techniques in Chapter 4 showed the external surface of the polygonal prismatic nanotubes have multiple steps and edges, which may act as additional adsorption sites, as has been shown for other clay minerals (Siretanu et al., 2016). In addition, use of cross-section transmission electron microscopy provided evidence of the link between the two morphologies, where the cylindrical nanotubes appear to be the template from which the larger polygonal prismatic nanotubes grow, as previously postulated by Hillier et al. (2016). In further novel work (Chapter 5), gold nanotags have been functionalised with a variety of anionic and neutral terminated linkers and used as nanoscale probes to study adsorption to the surface of halloysite nanotubes Here, transmission electron microscopy proved that the adsorption occurred primarily on the nanotube inner lumen and edges. The final technical chapter, Chapter 6, focuses on the structure and dynamics of interlayer water in halloysite through the use of neutron scattering techniques, where the initial results demonstrated that the interlayer water appears to move via both rotations and translations within the interlayer. The research presented ascertains that the surfaces of the halloysite nanotubes are more complex than often depicted for technological applications and that the specific tubular morphology is important in the functionality and behaviour of the nanotubes. This fundamental work contributes towards optimisation of halloysite nanotubes for technological applications.

The hidden impact of structural water – how interlayer water largely controls the Raman spectroscopic response of birnessite-type manganese oxide

Birnessite-type manganese oxides, consisting of stacked MnOx sheets, separated by charge-balancing metal ions and structural water are potential candidates for electrochemical applications. Due to their structural complexity, Raman spectroscopy is...

Effects of Temperature on Structural Properties of Hydrated Montmorillonite: Experimental Study and Molecular Dynamics Simulation

Montmorillonite (MMT) is highly sensitive to environmental changes and therefore plays a key role in the structural evolution of rocks and soils and even damage and disasters. The effects of important environmental factors (the temperature and water content) on MMT structural properties require in-depth study. The structure and morphology of sodium montmorillonite (Na-MMT) and its thermal products (micro-nanoparticles) were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). A molecular dynamics (MD) simulation was performed to investigate how temperature (below the failure temperature of the Na-MMT crystal layer) affects the structural properties of hydrated MMT. (1) The laboratory results showed that increasing the temperature significantly affected water molecules, and the particle aggregates exhibited inhomogeneous thermal expansion. The interlayer structure collapsed at 500–700°C. (2) In the simulation, the pull-off force inhibited interactions among oxides, crystal layers on both sides of the sample, and the bonding structure of water molecules, thus exposing the stress on the bonding body for analysis. The MMT ultimate stresses in the X, Y, and Z directions all trended downward with increasing water content and temperature. (3) Environmentally induced damage was most likely to occur in the Z direction. Increasing the number of interlayer water molecules increased the layer spacing and considerably weakened van der Waals forces, such that the roles of the electrostatic force and the interlayer hydrogen bond network gradually became significant. The most significant impact of increasing the temperature was reflected in the hydrogen bonding network, resulting in the destruction of the interlayer water bridge, the gradual failure of the layered bonding structure, and the formation or development of cracks. This improved understanding of the structural properties of MMT aggregates under environmental change advances research on the evolutionary behaviour of nano-, micro-, and macrostructures of rocks and soils.