Impedance Spectroscopy of Hydrating Cement‐Based Materials: Measurement, Interpretation, and Application

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Abstract
This work concerns the state of the art for use of impedance spectroscopy for studying the evolving microstructure of cement‐based materials during hydration. Features of the spectra are discussed and related to components of the microstructure with the assistance of pixel‐based computer modeling techniques. It is proposed that the enormously high relative dielectric constants (∼105) observed just after set are the result of dielectric amplification and are related to the distribution of pore sizes and the thickness of product C─S─H layers separating the pores. The conductivity is related to the volume fraction of porosity, the conductivity of the pore solution, and the interconnectivity of the porosity. The conductivity, when normalized by that of the pore solution, i.e., inverse formation factor, is a measure of this interconnectivity and can be used to predict such engineering properties as ionic diffusivity and water permeability. Composite mixing laws are employed to aid in explaining the behavior of the conductivity and to obtain a qualitative measure of the pore shape with hydration. Procedures for predicting the conductivity of the pore solution and for subtracting out electrode lead effects at high frequency are discussed.