Electrochimica Acta 51 (2006) 1831–1841 Impedance spectroscopy to characterize the pore structure during the hardening process of Portland cement paste M. Cabeza a , M. Keddam b , X.R. N ´ ovoa a,∗,1 , I. S´ anchez a , H. Takenouti b a Universidade de Vigo, E.T.S.E.I., Campus Universitario, 36310 Vigo, Spain b UPR15 CNRS, L.I.S.E., Universit´ e Pierre et Marie Curie, 75252 Paris Cedex 05, France Received 23 June 2004; received in revised form 16 November 2004; accepted 8 February 2005 Available online 25 August 2005 Abstract The relationship between the pore structure and the dielectric properties of cement paste is studied in this work. Dielectric properties clearly reflect the pore structure development and the degree of pore occupancy by electrolyte. Impedance spectroscopy (IS) measurements in the high frequency range (10 kHz–15MHz) have been performed in samples with varying pore occupancy. For cement pastes with a fully developed pore structure (curing time >28 days), when all pore families are filled with electrolyte, the main characteristic factor is the asymmetry observed in Cole–Cole plots, whereas as these samples are dried off or the pore structure is not fully developed (curing time <15 days), the asymmetrical dispersion decreases and the symmetrical dispersion becomes more important. The relationship between Cole–Cole plots dispersions and pore size distribution in samples is shown by performing bi-dimensional simulations. © 2005 Elsevier Ltd. All rights reserved. Keywords: Cement paste; Impedance spectroscopy; Electrical properties; Microstructure; Time constant dispersion 1. Introduction Impedance spectroscopy measurements performed on concrete are of great importance due to the possibility of correlating the dielectric properties and the microstructure of the system [1,2]. In a first stage of these studies, the presence of only one time constant in the high frequency loop was put forward, and the high values obtained for the dielectric constant were explained in terms of the dielectric amplification factor [3]. Some recent works on this field have demonstrated the existence of two time constants in the high frequency domain (10 kHz–15 MHz) [4–7]. The capacitance associated to one of these time constants correlates well with the fraction of solid phase in the cement paste [6–8], while the other one has been associated to some contribution of the electrolyte filling the pores [5,9]. ∗ Corresponding author. Tel.: +34 986 81 22 13; fax: +34 986 81 22 01. E-mail address: rnovoa@uvigo.es (X.R. N´ ovoa). 1 ISE member. The model proposed in a previous work [6] for the interpretation of the impedance spectra obtained in a “blocking” configuration corresponds to the Debye type depicted in Fig. 1. The capacitance C 1 accounts for the solid phase contribution while C 2 and the resistance R 2 are parameters associated to the liquid phase filling pores. While the relationship between C 1 and the fraction of solid phase is clear [6,8], the meaning of C 2 and R 2 , and the dis- persion in the corresponding time constant, is not so clear at present. The parameters R 2 and C 2 have both a strong dependence on the amount of water in the sample as well as on pore structure. This aspect is illustrated in Fig. 2 (taken from Ref. [6]) where it can be seen that the shape of the Cole–Cole plot changes from symmetrical dispersion (Cole–Cole type, Fig. 2A) to asymmetrical dispersion (Cole–Davidson type, Fig. 2B) as the pore structure develops. The present paper explores the effect of the pore occu- pancy and the pore size and dispersion in the impedance of the system. Those effects are evaluated in terms of 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.02.125