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We have shown, assuming a well-defined thickness and conductivity, how the presence of the interfacial zone cement paste influences the overall conductivity of a mortar. Although the aggregate size distribution used was typical of a mortar, the same general behavior is expected of a concrete. Since the interfacial zone cement paste occupies a significant fraction of the total cement paste phase, up to one quarter or one third, and since it is percolated, the higher conductivity of this phase in a plain portland cement mortar or concrete will cause the overall conductivity of the mortar or concrete to be higher than that expected for just a simple two-phase (bulk cement paste plus aggregate) composite. Just how much higher is determined by the ratio of σs / σp, the ratio of the interfacial zone cement paste conductivity to that of the bulk cement paste. For small sand fractions, of the order of 5% or so, the overall conductivity can be predicted analytically, for any given sand size distribution, while for larger sand volume fractions, numerical computation is necessary. The random walk algorithm discussed in this paper can perform this task accurately.
We are grateful to Daniel Hong for useful conversations concerning the random walk algorithm in two-conducting-phase materials. Part of this work has been supported by the National Science Foundation, via the Science and Technology Center for Advanced Cement- Based Materials. LMS thanks NIST for partial support of his sabbatical leave from Schlumberger.