Induced polarization of cementitious materials. Part I. Modeling their complex conductivity

Summary Geophysical electrical methods are increasingly being used in civil engineering to characterize and monitor cementitious materials. However, there is currently a lack of understanding of the role of their electrical surface conductivity and there is no quantitative model explaining their complex conductivity (induced polarization) spectra. Therefore, our goal is to propose and to validate a mechanistic model. We prepared 20 cement paste samples of well-established cement compositions (named CEMI and CEMV in the cement nomenclature) and 16 corresponding mortar samples (labeled MORI and MORV), all cured for 60 days, with water-to-cement (w/c) ratios ranging from 0.35 to 0.60. Complex conductivity spectra were measured at 21°C in the frequency range 10 mHz-45 kHz. For the cement pastes, both the in-phase conductivity and the magnitude of the quadrature conductivity increase systematically with the increase of the w/c ratio. The electrical properties of the mortars scale proportionally with those of the corresponding cement pastes, and the proportionality coefficient can be predicted from the volume fraction of cement and the model. We observe that the normalized chargeability is proportional to the quadrature conductivity, consistent with theoretical expectations. The relationship between the normalized chargeability and the surface conductivity and between the normalized chargeability and the Cation Exchange Capacity (CEC) are explained using a dynamic Stern layer model associated with the polarization of the inner component of the double layer coating the surface of the minerals. In other words, the dynamic Stern layer initially developed for colloidal solutions and geomaterials can be applied to cementitious materials opening new doors in their non-intrusive monitoring. To our knowledge, this is the first study to provide a physically-based interpretation of the complex conductivity spectra of cement pastes and mortars. These results demonstrate that induced polarization displays strong potentials for imaging water content and the Cation Exchange Capacity (CEC) (alternatively the specific surface area) of cementitious materials at various scales. This opens new perspectives regarding the quantitative non-invasive geophysical monitoring of cement and concrete for both civil and nuclear engineering applications.