INTERPRETATION OF THE IMPEDANCE SPECTROSCOPY OF CEMENT PASTE VIA COMPUTER MODELING .3. MICROSTRUCTURAL ANALYSIS OF FROZEN CEMENT PASTE

The d.c. conductivity, sigma, and low-frequency relative dielectric co nstant, k, of Portland cement paste were monitored, using impedance sp ectroscopy, during cooling from room temperature down to -50 degrees C . Dramatic decreases in the values of sigma and k, as great as two ord ers of magnitude, occurred at the initial freezing point of the aqueou s phase in the macropores and larger capillary pores. This result prov ides strong experimental support for the dielectric amplification mech anism, proposed in Part II of this series, to explain the high measure d low-frequency relative dielectric constant of hydrating Portland cem ent paste. Only gradual changes in the electrical properties were obse rved below this sudden drop, as the temperature continued to decrease. The values of a and k of frozen cement paste, at a constant temperatu re of -40 degrees C, were dominated by properties of calcium-silicate- hydrate (C-S-H) and so increased with the degree of hydration of the p aste, indicating a C-S-H gel percolation threshold at a volume fractio n of approximately 15%-20%, in good agreement with previous prediction s. Good agreement was found between experimental results and digital-i mage-based model computations of sigma at -40 degrees C. Freeze-thaw c ycling caused a drop in the dielectric constant of paste in the unfroz en state, indicating that measurements of k could be useful for monito ring microstructural changes during freeze-thaw cycling and other proc esses that gradually damage parts of the cement paste microstructure.