Electrode configurations and impedance spectra of cement pastes

Electrode effects on impedance spectra of cement pastes were investigated by two-, three-, and four-point measurements without a potentiostat over the frequency range 0.01 Hz-10 MHz. Electrode immittance effects arising from highly resistive/capacitive contacts cannot be fully corrected by nulling procedures. Two-point measurements are much more susceptible to such effects than three- or four-point measurements. The three- and four-point results on pastes suggest that there is negligible high-frequency "offset" resistance, and that bulk paste arcs are not significantly depressed below the real axis in Nyquist plots. The important impedance-derived equivalent circuit parameters are bulk resistance and capacitance; offset resistance and arc depression angle may not be physically meaningful parameters. Whereas all electrode configurations give reliable values of bulk paste resistance, only the three-point configuration provides the total paste/electrode dual arc spectrum involving a single electrode. Multielectrode (three- or four-point) measurements may be necessary to establish the true bulk paste dielectric constant.