Measuring the effect of the ITZ on the transport related properties of mortar using electrochemical impedance

In this paper, electrochemical Impedance spectroscopy (EIS) is used to investigate the effect of the interfacial transition zone (ITZ) on the transport related properties of Portland cement mortars. The ITZ between the cement paste and the aggregate provides a significant pathway for chloride transport which causes corrosion of reinforcing steel. The EIS experiments were used to determine how transport properties are influenced by the microstructure. The compressive strength, capillary porosity, chloride migration and impedance were measured on several mortar mixes which had different aggregate sizes at a constant aggregate fraction. An equivalent circuit model was fitted to experimental impedance spectra in order to obtain the electrical properties from the EIS results. The parameters of the equivalent circuit (resistances and capacitances) were correlated to physical conditions of the material microstructure. The results showed a clear relationship between the physical transport properties and the modelled equivalent circuit elements. This provides an explanation for the electrical impedance of mortar along conductive and non-conductive paths.