Impedance spectra of hydrating cement pastes

Complex impedance spectra were obtained over the frequency range 5 Hz to 13 MHz on Portland cement pastes with water/cement ratios of 0.3, 0.35, and 0.4 at various hydration times from 6 h to 24 days. Features of the spectra which could be associated with the bulk material and which could be separated from the electrode arc, were studied. The overall bulk resistance of each paste was thereby determined as a function of hydration time. Bulk features evolved from a simple high-frequency intercept to an intercept with a single arc, then an intercept with two overlapping arcs, and back to an intercept with a single arc. A plausible equivalent circuit was developed involving an electrode R/C network and constant phase element in series with one or two bulk R/C networks and a bulk resistor. Possible physical interpretation is discussed but assignment of equivalent circuit elements to microstructural features and/or processes will require microstructural characterization and a knowledge of pore-phase chemistry and properties.