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Interpretation of Impedance Spectroscopy of Cement Paste via Computer Modeling
Part I: Bulk Conductivity and Offset Resistance

R.T. Coverdale, B.J. Christensen, H.M. Jennings
T.O. Mason
Department of Materials Science and Engineering
Northwestern University
Evanston, IL 60208

D.P. Bentz, and E.J. Garboczi
National Institute of Standards and Technology
Building Materials Division
Bldg. 226, Room 348 B
Gaithersburg, MD 20899

Abstract

Computer simulation of the impedance spectroscopy (IS) of hydrating cement paste, using a three dimensional, four phase model, is described. Two puzzling features of experimental IS results, the possible offset resistance in the Nyquist plot and the sharp decrease in normalized conductivity within the first 50 hours of reaction, are studied with the computer simulation model. Insight is provided into these features using the ability of the model to quantitatively compare microstructure and properties. It is concluded that the offset resistance is an experimental artifact, and does not directly relate to microstructure. The drop in conductivity during the first fifty hours is shown to be a consequence of a gradual shift from parallel-dominated to series-dominated behavior of the electrical conductivity, as microstructural modifications take place during hydration, causing the capillary pore structure to become more tortuous. This tortuousity can also explain the high frequency impedance behavior in terms of a two-arc response.

• Introduction
• Puzzling Experimental Features
• Description of the Computer Model
• Results
• Conclusions
• Acknowledgments
• References