Impact of the cation exchange capacity on dielectric relaxation spectra of water saturated clays

Clay is a key mineral phase in soil science and geotechnical engineering due to its ubiquitous appearance in soils and sedimentary rocks. Under moist conditions exchangeable cations at clay mineral interfaces dissociate and form an electrostatic double layer. Hence, clay dielectric relaxation spectra contain valuable information of the material due to strong contributions by interactions between aqueous pore solution and mineral phases. Against this background, dielectric relaxation spectra of kaoline, illite and two bentonites were measured at water saturation with network analyzer technique in the frequency range from 1 MHz to 5 GHz. To relate physicochemical material parameters such as cation exchange capacity or specific surface area to the dielectric relaxation behavior, measured dielectric relaxation spectra were parameterized by a generalized fractional dielectric relaxation model (GDR), a mixture model in combination with a Cole-Cole relaxation function (augmented broadband complex dielectric mixture model - ABC-M) and a theoretical mixture equation (complex refractive index model CRIM). The spectra show clear signatures of the dominated clay mineral. There is evidence that relaxation parameters of the low frequency dispersion in terms of relaxation strength and apparent direct current conductivity are closely related to the cation exchange capacity.