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By combined 4-point dc resistance and 2-point impedance spectroscopy measurements, the electrical properties of cement-based composites with mono-size steel or glass spheres were measured over a wide range of volume fractions (up to 42%). A highly resistive passive oxide film on the steel particles causes the composites to behave like the corresponding glass particle composites. With increasing frequency, however, displacement currents through the oxide films short them out, rendering the particles "conductive" relative to the matrix. This behavior is consistent with the previously proposed "frequency-switchable coating model."
The steel particle-cement matrix system is unique in that no percolation threshold is observed in dc or ac measurements. This is due to the combination of the passive oxide film on the steel particles (no dc percolation, even with immediate contact) and the unavoidable coating of large particles with cement particles during processing. This keeps adjacent particles apart during mixing. Later, hydration products replace the parent cement grains in the inter-particle spaces.
The conductivity vs. volume fraction behavior of spherical particle composites was investigated in both the insulating particle and conductive particle regimes, and compared with various mixing law and effective media theories. The behavior was best described in both regimes by the model of Meredith and Tobias. In the dilute limit, the "intrinsic conductivities" were 3 and −3/2 for conducting and insulating spheres, respectively.
High-current 4-point dc resistance measurements were found to yield different conductivities for steel particle composites, as compared to the impedance derived values. This is thought to be due to permanent changes induced in the matrix phase by the high current densities employed. Therefore, impedance spectroscopy is the only reliable means of measuring conductive particle composite behavior in cement-based matrices.
Next: Acknowledgments Up: Main Previous: Results and Discussion