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The ion diffusion process in mortar is different from the one which occurs in cement paste. This difference is due to the presence of transition zones, which take place around the grains in mortar and which are very porous regions. Based on mercury intrusion porosimetry experimental data and on the analysis of percolation through a 3D mortar model, a computation of the transition zone porosity and of the bulk paste porosity has been carried out. The porosity of the transition zone has been analyzed as a function of the mortar composition and of the degree of hydration.

It seems probable that the higher porosity of the Interfacial Transition Zone (ITZ) affects concrete durability. An experimental program has thus been developed to provide more information in this area and to evaluate the influence of the transition zone on the transfer properties. A model based on the mercury intrusion porosity results shows that the ITZ is about 1.5 to 2.5 times more porous than OPC cement paste and in this zone the tritium diffusion coefficient is a factor of three higher.The tritium diffusion coefficient has been measured for mortars containing different sand concentrations. We have shown that the binder phase diffusivity decreases with the sand concentration down to a minimum, then increases when the 1TZ are interconnected. Under these conditions, the sand volume concentration is above 50% and for higher concentrations, the diffusivities of the binder phase and pure cement paste are similar. The effects of ITZ on the porosity compensate for the increasing tortuosity due to the interposition of particle aggregates.Measurement of the calcium leaching rates from mortar with high sand concentrations and pure cement paste with the same W/C ratio confirms these results: the leaching kinetics of the binder phase in mortar are very close to those of the pure cement paste.