Geometry Effects for Specific Electrical Conductance in Clays and Soils
A systematic study of the influence of salinity and clay content on the electrical conductivity of sodium-illite clay gels shows that the geometry or “formation resistivity factors” of such systems can adequately be described by the model of oblate ellipsoids, used to simulate the shape of the clay particles. This conclusion is in agreement with the results previously obtained on kaolinite and montmorillonite clays. An axial ratio of 16 was found for the illite clay particles.
On the basis of Burger’s and Maxwell’s equations for electric flow through porous media, formulae are derived for calculating the electrical conductivity of mixed systems, i.e. “clay 1 + clay 2 + electrolyte solution” and “clay + spherical particles + electrolyte solution”. The electrical conductivity of these systems is expressed in terms of the shape parameters, surface conductances, specific surfaces and volume concentrations of the constituents. The type of equation can eventually lead to an explanation (in terms of a non-uniform particle shape distribution) of the anomalous geometry effects in some clays.
Experimental results conform fairly well to the equations. Glass powder at volume fraction between 0.18 and 0.60 added to a 17.4% montmorillonite gel acts as an inert diluent on the specific electrical conductivity of the mixture. For a mixture of 1 part montmorillonite to 9 parts kaolinite, the measured specific electrical conductivity agrees within about 10% with the predicted over the range studied (volume fraction of montmorillonite: 0.035 to 0.039; mixture porosity: 0.61 to 0.660; salt concentration range: 0.5 n to 2 n NaCl).
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