Next: Formula for interfacial
It is now well-established experimentally that interfacial transition zones (ITZ's) exist around aggregate (rock, sand) particles in concrete. This is mainly because the cement paste matrix is itself particulate. When the cement grains encounter the "wall" of the aggregate, a region of higher porosity near the aggregate surface will appear, due to the "packing" constraints imposed by the aggregate surface [1,2]. Because the average aggregate diameter is much larger than the average cement grain diameter, the aggregates on average will appear locally flat to the cement grains, so the ITZ thickness will depend on the median size of the cement grains, and not on the aggregate size . The median diameter of most cements in common use is around 10-30 micrometers, so this is typically the kind of width one finds associated with ITZ's. In the case where the cement grains are of the same order size as the aggregate particles, the whole idea of ITZ's, at least in the sense considered in this paper, loses its meaning. This case is not considered in this paper.
The restrained placement of cement around aggregates results in a gradient of porosity, and therefore a gradient of properties, around each aggregate. The high volume fraction of aggregates in a typical concrete (60-75%) means that the spacing between adjacent aggregates is only a few times the typical ITZ thickness. This fact implies that the cement paste in the ITZ's can have a significant volume fraction and can be percolated  and, therefore, can have a significant effect on properties. Of particular interest are elastic moduli, compressive strength, chloride and sulfate diffusivity, electrical and thermal conductivity, shrinkage, and creep. This paper is restricted to the (linear) properties of ionic diffusivity (which mathematically is the same as thermal or electrical conductivity [5,6]), elastic moduli, and thermal expansion.