Interpretation of the Impedance Spectroscopy of Cement Paste Via Computer Modelling.
Interpretation of the Impedance Spectroscopy of Cement
Paste Via Computer Modelling.
Olson, R. A.; Christensen, B. J.; Coverdale, R. T.;
Ford, S. J.; Moss, G. M.; Jennings, H. M.; Mason, T. O.;
Garboczi, E. J.
Journal of Materials Science, Vol. 30, 5078-5086, 1995.
Sponsor:National Science Foundation, Washington, DC
building technology; cement paste; electrical
properties; freezing; impedance spectroscopy;
The d.c. conductivity, sigma, and low-frequency relative
dielectric, kappa, constant of Portland cement paset
were monitored, using impedance spectroscopy, during
cooling from room temperature down to -50 DGC. Dramatic
decreases in the values of sigma and kappa, as great as
two order of magnitude, occurred at the initial frezzing
point of the aqueous phase in the macropores and larger
capillary pores. This result provides strong
experimental support for the dielectric amplification
mechanism, proposed in Part II of this series, to
explain the high measured low-frequency relative
dielectric constant of hydrating Portland cement paste.
Only gradual changes in the electrical properties were
observed below this sudden drop, as the temperature
continued to decrease. The values of sigma and kappa of
rrozen cement paste, at a constant temperature of -40
DGC, were dominated by properties of
calcium-silicate-hydrate (C-S-H) and so increased with
the degree of hydration of the paste, indicating a C-S-H
gel percolation threshold at a volume fraction of
approximately 15%-20%, in good agreement with previous
predictions. Good agreement was found between
experimental results and digital-image-based model
computations of sigma at -40 DGC. Freeze-thaw cycling
caused a drop in the dielectirc constant of paste in the
unfrozen state, indicating that measurements of kappa
could be useful for monitoring microstructural changes
during freeze-thaw cycling and other processes that
gradually damage parts of the cement paste