Non-steady state migration (NSSM) test is commonly employed to evaluate chloride ion ingress into concretes. This paper quantifies the microstructural changes in concretes subjected to NSSM tests, induced by the chemical constitution of the cement and replacement materials. Electrical impedance spectroscopy (EIS) and equivalent electrical circuit modeling are used to obtain characteristic features of the concrete pore structure that aid in this quantification. A methodology is provided in this paper to convert the bulk resistances of the specimens under the action of any applied voltages to the ones corresponding to a 30V applied potential. The relationships between: (i) bulk resistance of the specimens before the NSSM test and the NSSM coefficient D(nssm), (ii) resistance of the connected pores and the microstructural parameter phi beta (product of porosity and pore connectivity), and (iii) bulk resistance after the NSSM test corresponding to a 30V potential and the chloride penetration depths, demonstrate the effect of chloride binding. The capacitance related to the pore-solid interface obtained from the equivalent circuit is used to indicate that the chloride binding products are formed at the interfaces and influence the tortuosity of the pore system. The experiments and the model suggest an average reduction of about 10% in phi beta values after the NSSM test. (C) 2011 Elsevier B.V. All rights reserved.