Computational simulation of alternating current (AC) impedance of hardened cement mortar

The present study proposes a novel methodology of simulating the AC impedance of hardened cement mortar, through the technology of micro computed tomography based finite element analysis (μCT-based FEA). A three-dimensional (3D) physical meso-scale structure of hardened cement mortar is first segmented into multicomponent phases of air void, cement paste, sand aggregate, and interfacial transition zone (ITZ) considered with different thicknesses, and then subjected to current harmonic perturbations to acquire the AC impedance spectra. Compared to traditional impedance measurement, μCT-based FEA avoids the contributions of external electrode and hence improves the characterization of cementitious electrochemical system in its true state. The simulated complex impedance presents a depressed high-frequency loop and varies with the increasing ITZ thickness in the range of 26.6 to 106.4 μm. The numerical impedance plots are best fitted to a theoretical equivalent circuit model of R_s(R_ctQ) with the parameters of bulk ionic resistance (R_s), charge transfer resistance (R_ct) at the solid/liquid interface, and the non-Debye capacitance (Q). The changes in mesostructure of cement mortar with the thickened ITZs can be characterized by a linear reduction in R_s, a non-linear decrease in Q, and a limited increase in R_ct. Based on the electrochemical impedance modelling, the study establishes the mathematical relationship between the defect's volume (air voids plus ITZ) and the model parameters (R_s or R_ct) for prediction of material deterioration.