Dielectric spectra of bound water in hydrated Ca-smectite

Water contained in larger pores is considered to have the properties of free water, but water associated with colloid surfaces is often called bound water. The objective of this study is to utilize dielectric spectra to elucidate properties of various forms of bound water in Ca-smectite. The <2 μm fraction of Wyoming bentonite was Ca-saturated and prepared as oriented films. The loosely-stacked oriented films were equilibrated above saturated salt solutions to control relative humidity (RH), then compressed with a hydraulic press, and finally packed into coaxial cells. A vector network analyzer was used to indirectly determine the complex dielectric spectra. The gravimetric water contents ranged from 0.209 to 0.421 kg kg−1 for RH ranging from 54% to 98%, and static electrical conductivity (EC) ranged from 0.002 to 0.051 S m−1. Volumetric water contents ranged from 0.203 to 0.506 m3 m−3, and variable amounts of air were present in the different samples, ranging from 0.040 to 0.438 m3 m−3. Increased water content increased the EC because of greater continuity of water and lower tortuosity. Both the real and imaginary component of the permittivity were about an order of magnitude smaller at 54% RH than at 98% RH.