Dear Sir, please see:

Interfacial transition zone in concrete

By: J.P.Ollivier, J.C.Maso and B.Bourdette

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. 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.