Experimental and Computer Simulation Results for the Electrical Conductivity of Portland Cement Paste
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EXPERIMENTAL AND COMPUTER SIMULATION RESULTS FOR THE ELECTRICAL CONDUCTIVITY OF PORTLAND CEMENT PASTE B.J. Christensen, T.O. Mason, and H.M. Jennings Northwestern University, Department of Materials Science, Evanston, IL D.P. Bentz and E.J. Garboczi National Institute of Standards and Technology, Building Materials Division, Gaithersburg, MD. ABSTRACT The electrical conductivity of portland cement paste is an important transport property, especially since, when properly normalized by the pore fluid conductivity, it is equivalent to the normalized ionic diffusivity of the material via the Nernst-Einstein relation. This paper presents experimental and computer simulation results for /alo, where a is the conductivity of the bulk paste as determined from impedance spectroscopy, and ao is the conductivity of the pore solution. Comparison between simulation and experiment is carried out for an 0.5 water:cement ratio white cement paste as a function of capillary porosity. The quantitative agreement between theory and experiment is reasonable. INTRODUCTION The dc electrical conductivity of cement paste, a, is an important transport parameter, and can be used as a probe of microstructural development [1]. Also, of particular practical significance, ionic diffusivities can be obtained from the Nernst-Einstein relation [2] by using the relative conductivity oral, where orois the pore fluid conductivity. Scientifically sound quantitative relationships between microstructure and a/a., have never been made. This is because of both the lack of an adequate theory, and the lack of a program of careful, systematic measurements. This paper presents a preliminary comparison between simultaneous experimental measurements of a and a. using impedance spectroscopy, and computer simulation predictions of a/a . . ELECTRICAL CONDUCTIVITY AND MICROSTRUCTURE The dc electrical conductivity of a complicated composite material, like cement paste, depends on microstructure in several ways. In general, for a material containing pores that are filled with a conductor, the bulk conductivity a, averaged over the sample dimensions, can be written [3] as a = f 4,a., where 4) is the porosity, a. is the conductivity of the material filling the pores, and 1 is a dimensionless factor summarizing parameters like pore connectivity and tortuosity. The factor 4) corrects for the cross-sectional area available for flow. In porous sandstone rocks, for example, where much work has been carried out in relating microstructure and electrical conductivity, the dimensionless quantity F = aja, called the formation factor, has been defined [4]. For the relatively low porosities encountered in sandstones, a power law in * is often found to be a reasonable description of experimental values of F [4]. This general relationship is usually called Archie's law [5]. The dc electrical properties of sandstone rocks can be described electrically by a two-phase composite model, where the sand grains are insulating and the pore fluid, which is usually a brine, is a conductor [6]. Th
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