Electrical conduction and dielectric behavior of cement-based materials
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This thesis addresses the interplay between electrical conduction behavior and the dielectric behavior of cement-based materials, with emphasis on the electrical resistivity, inherent voltage as observables that depend on the electric current during and before the electrical measurement, the age, the composition, the moisture content, the electrical contact ionicity and the electrical contact scheme. The inherent voltage observed in this work is associated with electret formation, with its polarity opposite to the voltage due to polarization and its magnitude increasing with prior current (e.g., 0.1 mA) application and decreasing with increasing water content. These observables are scientifically and technologically important. In spite of the intricate multivariable dependencies that have been systematically addressed, correlations have been found that shed new light on the control of the conduction and dielectric behavior. The combination of conduction and dielectric behavior is modeled as a voltage source (the inherent voltage) and a resistance (the true resistance) in series, with the polarity of the voltage the same as that of the resistance when a current is applied, indicating that the voltage is due to electret formation. The true resistance is given by the slope of the voltage-current curve, though the measured resistance obtained at a fixed current may differ substantially from this value. The lower is the current, the more is the difference. A current of 0.1 mA is sufficiently high for the difference to be small. The model enables calculation of the inherent voltage from the resistivity measured at a fixed low current (e.g., 0.01 mA). The inherent voltage is given by the voltage intercept of this curve, though the actual value during a resistance measurement at a fixed current (preferably small, such as 5 nA) may differ from this value. Measurement of either resistance or inherent voltage conducted by using two electrical contacts gives results that are not reliable. Four contacts should be used. A high resistivity correlates with a high inherent voltage. The level of prior current affects the inherent voltage. Age tends to increase both the resistivity and the inherent voltage. Moisture decreases both the resistivity and the inherent voltage, whether the moisture comes from moisture exposure or a high water/cement ratio. This is because a reduction in resistivity results in less ability to sustain a substantial inherent voltage. In contrast, moisture is known to increase the polarization, which has opposite polarity from the electret voltage.