- Author
-
Olson, R. A.
|
Christensen, B. J.
|
Coverdale, R. T.
|
Ford, S. J.
|
Moss, G. M.
|
Jennings, H. M.
|
Mason, T. O.
|
Garboczi, E. J.
- Title
- Interpretation of the Impedance Spectroscopy of Cement Paste Via Computer Modelling.
- Coporate
- Northwestern Univ., Evanston, IL
National Institute of Standards and Technology, Gaithersburg, MD
- Journal
-
Journal of Materials Science,
Vol. 30,
5078-5086,
1995
- Sponsor
- National Science Foundation, Washington, DC
- Keywords
-
building technology
|
cement paste
|
electrical properties
|
freezing
|
impedance spectroscopy
|
percolation
- Identifiers
- freezing of water in cement paste proes; dielectric amplification mechanism; electrical properties of frozen cement paste at - 40 ºC; freeze-thaw cycling experiment
- Abstract
- 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 ºC. 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 ºC, 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 ºC. 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 microstructure.