Microstructure-Electrical Propertyrelationships in Cement-Based Materialss
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ABSTRACT
There has been much recent progress on the application of impedance spectroscopy (IS) to the study of microstructure and transport in cement-based materials. The IS spectrum allows for the precise determination of bulk resistance, which is a measure of the pore phase interconnectivity, and calculation of the relative dielectric constant, which is related to the capillary pore size and distribution. High values of the relative dielectric constant (-105) observed in cement paste at early hydration times are the direct result of the microstructure inducing dielectric amplification. Solvent exchange and freezing experiments, combined with digital-image-based computer modeling, have confirmed the role of large capillary pores in the dielectric amplification in young pastes. The conductivities (a) and relative dielectric constants (er) of ordinary portland cement (OPC) pastes were monitored during cooling and solvent exchange with isopropanol and methanol. Dramatic decreases in a and £r, in some cases over two orders of magnitude, occurred at the initial freezing point of the aqueous phase in the macropores and large capillary pores. The same dramatic decreases in a and e r were observed at the onset of solvent exchange. Both effects provide experimental support for the dielectric amplification mechanism within the microstructure on the gm-scale. A secondary dielectric amplification was observed in the frozen and solvent exchanged pastes, which produced dielectric constants on the order of 103. This effect is attributed to amplification on the nm-scale within the layered calcium silicate hydrate (C-S-H) gel microstructure. Additional insight into the variable nature of the C-S-H microstructure was obtained by comparing the dielectric behavior of methanol-exchanged OPC pastes to isopropanolexchanged OPC pastes.
INTRODUCTION
Impedance spectroscopy is a non-destructive electrical technique that has been used to characterize the microstructure of cement-based materials, providing useful information about the relationships between microstructure, electrical properties, and chemical processes during hydration [1-13]. IS consists of applying a small amplitude alternating current to a cement paste sample and measuring the impedance as a function of 255
Mat. Res. Soc. Symp. Proc. Vol. 370 01995 Materials Research Society
frequency. The impedance measurement can be made rapidly and easily while the sample is hydrating, without drying the sample or altering its microstructure. Recently, it was discovered that the relative dielectric constant of cement paste can attain high values [1-3, 9]. As shown in Figure 1, the value of Er for ordinary portland cement paste increases sharply during the first 10-15 hours of hydration to a maximum near 10f, then decreases gradually to a value around 103. This is surprising, since the component with the highest dielectric constant is the aqueous phase (Sr f 80). Even the C-S-H gel (Cr f 103) [9], cannot account for Er values of 105. Christensen et al. [9] and Coverdale et al. [11] p
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