Impedance characterization of ordinary Portland cement-pulverized fly ash binders: Fid-Bau Portal

Impedance characterization of ordinary Portland cement-pulverized fly ash binders

McCarter, W.J. / Starrs, G.

This work focuses on a.c. impedance response of a binary mixture of ordinary Portland cement (OPC) and pulverized fuel ash (PFA) (as a partial replacement for OPC) showing the potential application of this technique in studying the pozzolanic reaction and as a means of detecting PF within concrete. Considering the latter, this could be developed as a means of quality control in the production of PFA concretes. The impedance response can be devided into two distinct regions: a low-frequency arc associated with electrode polarization effects, and a flat, high-frequency portion associated with the bulk material. The frequency of applied field which delineates these two regions is 1.10 kHz at 1 h, reducing to 0,67 kHz at 10 h. Examination of the high-frequency portion at 1 h reveals three identifiable arcs all with their centres depressed below the real axis. It is postulated, that the part of the high-frequency arc dominating at frequencies above 4,43 MHz, is associated with the inclusion of the resistive aggregate phase within the cementious binder thereby creating a more tortuous, continuous pathway in comparison to neat cement pastes. The existence of two, partially merged arcs within the frequency range 1.10 kHz to 4.43 MHz is a new feature and must be as a direct result of the presence of PFA particles, because they are not present in the plain OPC mortar. The reason why PFA particles produce a distinctive region could be due to their distinctive spherical shape (cenospheres) although the overall particle size is similar to OPC. The spheres tend to have a smooth, glassy surface and are hollow (resulting in a lower specific gravity than OPC). Charges electrostatically held on to the particle surface could move over the particle surface in presence of an applied electrical field, thereby including large dipole moments (double-layer polarization). The pozzolanic reaction is very slow and the PFA particles remain within the pores until activated by the calcium hydroxide released during the hydration of OPC.