Study of the dielectric characteristics of cement paste

The characterization of the microstructure of cementitious materials by means of Impedance Spectroscopy (IS) is attracting an increasing interest. This is due not only to the importance of the cement cover on steel corrosion, but because of the possibility of characterise aspects like hydration degree of cement or water content. The present study analyses the impedance of cement paste specimens previously oven-dried or conditioned at 50% and 100% RH, in order to calculate their dielectric constant, epsilon. The results show that when oven-dried the cement paste has an almost perfect capacitive behaviour with reasonably low epsilon values. However, when the cement paste contains a certain amount of moisture, the shape of the IS diagram changes significantly and, according to the literature, abnormally high epsilon values are obtained. These changes are considered to be due mainly to some bulk and interfacial phenomenon still not well identified. Appropriate electrical insulation of the electrode/paste interface by means of mylar sheets allows to obtain reasonable epsilon values for moisten specimens and to advance in the identification of the phenomenon defining the shape of the obtained IS diagrams.