Effect of microstructure on the electrical conductivity of clay-rich systems (original) (raw)
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Journal of Geophysical Research, 2002
1] The influence of clay and water content in the electrical conductivity of rocks and soils has been experimentally established and is expressed by simple empirical laws: the Archie's law and the addition law between volume water conductivity and surface shale conductivity. Two independent numerical modeling techniques, the moment method and the finite difference method, are presented here and are used, first, to verify the agreement between Maxwell's equation based theoretical approaches and the empirical laws and, second, to begin to investigate for a possible effect of the microscopic geometry over macroscopic conductivity. A good agreement between simulation results and Archie's law is obtained when both randomly distributed isotropic and elongated microvolumes of conducting water are considered and a slight difference appears between these two microstructures. For low clay contents in clay-dispersed media, the clay-associated conductivity is shown to be proportional to a specific clay area, which is in good agreement with the addition empirical law.
Electrical conductivity of 1 : 1 and 2 : 1 clay minerals
Surface Engineering and Applied Electrochemistry, 2014
The A.C. impedance plots were used as tools to analyze the electrical response of two varieties of Tunisian halloysite 1:1 and illitic samples 2:1 as a function of frequency at different temperatures (80-800°C). The real and imaginary parts of the complex impedance trace semicircles in the complex plane. Except for the illite, It-1, the second sample analyzed in this study, these plots give evidence for the presence of both bulk and grain boundary effect, above 600°C onwards. The bulk resistance of the materials decreases with the rise in temperature. Impedance Spectroscopy data reveal a non-Debye type of dielectric relaxation. The Nyquist plots show the negative temperature coefficient of resistance of both pure Tunisian illite and halloysite samples. The results of bulk electrical conductivity and its activation energy are presented for the two mineral clay samples. For illite It-1, the activation energy values estimated from the AC conductivity pattern and modulus pattern are very similar and suggest a possibility of a long-range mobility of charge carriers (ions) via hopping mechanism of electrical transport processes at higher temperature. On the other hand, for the halloysite sample provided from kasserine, (Ha-Kass), the modulus analysis admit that the electrical transport processes of the material are very likely of electronic nature. Relaxation frequencies follow an Arrhenius behavior with the activation energy values not comparable to those found for the electrical conductivity.
Low-frequency complex conductivity of sandy and clayey materials
Journal of Colloid and Interface Science, 2013
Low-frequency polarization of sands and sandstones seems to be dominated by the polarization of the Stern layer, the inner part of the electrical double layer coating the surface of the silica grains and clay particles. We investigate a simple model of Stern layer polarization combined with a simple complexation model of the surface of the grains immersed in a 1:1 electrolyte like NaCl. In isothermal conditions, the resulting model can be used to predict the complex conductivity of clayey materials as a function of the porosity, the cation exchange capacity of the clay fraction (alternatively the specific surface area of the material), and the salinity of the pore water. A new set of experimental data is presented. This dataset comprises low-frequency (1 mHz-45 kHz) complex conductivity measurements of saprolites and sandstones that are well characterized in terms of their petrophysical properties (porosity, permeability, specific surface area or CEC, and pore size). This dataset, together with incorporating additional data from the literature, is used to test the Stern layer polarization model. We find an excellent agreement between the predictions of this model and this experimental dataset indicating that the new model can be used to predict the complex conductivity of natural clayey materials and clay-free silica sands.
Computation of hydraulic conductivity of montmorillonitic clays by diffuse double layer theory
Knowledge of hydraulic conductivity of clays is very important in many cases and in waste containment system in particular. Several investigations have been carried out on hydraulic conductivity in the past and it has been brought out that the hydraulic conductivity is strongly influenced by the pore fluid chemistry apart from the soil type and void ratio. Based on the theoretical considerations, the effect of dielectric constant of the pore fluid, valency and concentration of cations in the pore fluid, it is surmised that the hydraulic conductivity is significantly influenced by diffuse double layer thickness, which reduces the pore volume through which flow takes place. A study of Gouy-Chapman diffuse double layer theory show that the diffuse double layer thickness is negligible for non-polar fluid with very low dielectric constant (e.g. CCl 4 ). Considering the thickness of diffuse double layer contributed by CCl 4 as zero, the equivalent thickness of diffuse double layer as affected by dielectric constant, valency, concentration, hydrated radius of cation and specific surface has been estimated and presented for montmorillonitic clays. The effective void ratio has been evaluated and a unique relationship between effective void ratio and hydraulic conductivity has been brought out for montmorillonitic clays with different pore medium chemistry. The present study throws more light on the fundamental mechanisms controlling the hydraulic conductivity in clays.
A triple-layer model of the surface electrochemical properties of clay minerals
Journal of Colloid and Interface Science, 2004
We propose an electrical triple-layer model (TLM) to describe the electrochemical properties of clay minerals. This model includes a speciation model of the active crystallographic surface sites plus a classical description of the Stern and diffuse layers. In addition to the surface charges associated with the surface groups (aluminols, silanols, and >Al-O-Si< sites), the model takes into account the degree of isomorphic substitution rate inside the crystalline network. The model computes both the ζ potential and the low-frequency (few kHz) surface conductivity as these two properties are equally important to interpret electrokinetic properties of colloids and clay-rich porous materials. For surface conductivity, the model comprises two contributions; one is associated with the Stern layer (dynamic Stern layer) and the second is associated with the excess of counterions located in the diffuse Gouy-Chapman layer. The parameters of the TLM model are optimized using the Simplex algorithm. Comparison between the model and the experimental data shows good agreement for a reasonable choice of parameters close to the a priori values determined from quartz and gibbsite. Surface conductivity of smectite appears rather independent of salinity, while for kaolinite, surface conductivity increases with salinity. In both cases, the Stern layer contribution to surface conductivity dominates but the contribution associated with the diffuse layer cannot be neglected.
Correlation of hydraulic conductivity of clay–sand compacted specimens with clay properties
Applied Clay Science, 2004
Hydraulic conductivity of compacted specimens consisting of 1 of 16 clay samples and siliceous sand/natural soils was measured using permeameters with flexible wall cells. Clay samples studied were eight natural bentonites, one purified montmorillonite, five standard clay samples, one synthetic hectorite, and two cation-exchanged bentonites. The clay content to whole solid was fixed at 11.5% w/w, and the mixtures were rammed in a mold to form disk compactions. A higher hydraulic conductivity was observed with increase in the Ca/Na ratio of clay. Hydraulic conductivity was also affected by bentonite content of the clays and the mineral content. A relation between the hydraulic conductivity and the characteristic index properties of the clays such as chemical concentration, methylene blue adsorption capacity (MB), and so on were elucidated, and then a polynomial expression was derived to reproduce measured hydraulic conductivity data. This relation was then utilized to estimate hydraulic conductivity of blended bentonites from characteristic properties of the component bentonites. D
Influence of texture on DC conductivity and dimensional changes of kaolin and illitic clay
Ceramics International, 2019
The influence of the technological texture on the DC conductivity of an illitic clay and kaolin was studied. Textured samples were prepared by layering a wet plastic mass made from distilled water and clay. Anisotropy of the samples was assessed by studying the DC conductivity and dimensional changes as functions of temperature in parallel and perpendicular directions to the basal planes of illite and kaolinite crystals. It was found that the shrinkage of the samples after firing is more significant in the direction perpendicular to the basal planes. The DC conductivity both in the illitic clay and kaolin is higher parallel to the basal planes of illite or kaolinite crystals than perpendicular to them. In both materials, the dominant charge carriers are K + and Na + ions, complemented by H + and OHions during the removal of the physically bound water and dehydroxylation. The influence of the texture remains observable in the kaolin samples even after a high-temperature treatment (1200 °C), contrary to the illitic clay where this influence is suppressed by the high amount of the glassy phase (~80 mass %) 2 after the thermal treatment at temperatures above 1000 °C. Compared to the illitic clay, the influence of the technological texture on the DC conductivity of kaolin is more significant.
Effect of Clay Type on the Diffusional Properties of a Clay-Modified Electrode
Clays and Clay Minerals, 1996
The response of two swelling clays (SWy-1 and SAz-1) and of one non-swelling clay (KGa-1) and of a series of mixtures of these clays to different electrolyte concentrations was examined using clay-modified electrode techniques. A non-interacting probe ion, Fe(CN)6 3-, was monitored via reduction for its arrival at a Pt electrode coated with thin films of the day mixtures. The three clays had both different temporal responses and different equilibrium currents. For SWy-1 the currents were developed over time and were dependent upon the electrolyte of the bathing solution, which was consistent with X-ray diffraction data literature for the interlayer dimension. Similar behavior was found for SAz-1, but for KGa-1, currents were instantaneous and were independent of the bathing electrolyte. This suggests that the pores controlling the probe ion transport were between particle or pinhole in nature. When mixtures of the clays were examined, it was found that KGa-1 caused defects within the structures of the mixed clay films. The SWy-1 mixture was not as affected by these disruptions as was the SAz-1 mixture. Key Words-Clay charge, Clay-modified electrodes, Clay swelling.
Advances in Materials Science and Engineering
The aim of this paper is to investigate the effect of pore-water salinity on the electrical resistivity (ER) of different compacted clay liners (CCLs) in terms of its mineralogical composition. For this purpose, an experimental programme was conducted where ERs of different kaolin-dominant CCL specimens, reconstituted using water having different concentrations of NaCl (0 M, 0.5 M, and 1.0 M), were measured. The kaolin-dominant CCL specimens tested in this study include pure kaolin, three different kaolin-bentonite mixtures, and three different kaolin-sand mixtures. The experimental results show that the ERs of CCL specimens decrease as the salt concentrations in pore water, moisture content, and dry density increase. At constant density and moisture content, the test results also indicate that increasing the sand content in kaolin-dominant CCL specimens increases its ER regardless of the water salinity level. This behaviour could be attributed to the lower surface conduction of san...
Electrochemical measurements on clay minerals for possible sensor applications
Ionics, 1999
Natural clays were characterized by XRD, TGA and SEM/EDX, then pressed to pellets and analyzed by impedance measurements under various defined gas compositions and temperatures. From the Nyquist plot, conductivity values were calculated for the different clay samples. A strong dependence of the conductivity on the water vapour concentration at low temperatures was found for all investigated clay samples. Partial substitution of water by methanol led to a decrease in conductivity at low temperatures, confirming the essential role water plays in the conductivity mechanism at these temperatures. Increasing the temperature resulted in a decrease in conductivity, until a turn-over point was reached. Beyond this temperature an Arrhenius behaviour was observed, suggesting a change in the conductivity mechanism.