Contact capacitance effect in measurement of a.c. impedance spectra for hydrating cement systems

An attempt is made to clarify an argument related to the utilization of a.c. impedance spectroscopy for hydrating cement systems. The question relates to which electrode configuration, 2-point, 3-point or 4-point measurement, is pertinent for impedance spectrum measurement. Theoretical analysis and experiment indicate that these electrode configurations should in principle give the same results. The impedance spectra from 3- or 4-point measurement are, however, strongly influenced by the contact areas between the specimen and potential sensors. This influence is attributed to the potential sensor-specimen contact capacitance effect. The experiment indicates that when contact capacitance, or area, between the specimen and potential sensor is small enough the impedance spectrum in the Nyquist plot is characterized by an almost perfect semi-circle and negligible high frequency offset resistance. These are the typical characteristics of the spectra obtained from 3- and 4-point measurements with point contact between the potential sensors and specimen. The 2-point and 4-point measurements give the same spectra when contact capacitance approaches a sufficiently large value. It is apparent that the impedance spectra from 3- and 4-point measurements with point contact cannot reflect true information about hydrating cement systems. They are experimental artifacts. The 2-point measurements, however, can give more reliable results.