This paper deals with the effect of the ITZ on the leaching process of cement based materials.

In order to characterize this effect, we have performed experiments on mortar samples and pure cement paste samples. Two aggressive waters were used, the first one with pH=4.5 and the second one with pH=8.5. These two aggressive waters were modelled by a solution of ammonium nitrate. Compression tests were conducted on micro-cylinder samples (10, 12 and 14 mm of diameter) because of the slow kinetics of degradation due to the leaching.

In the case of a sample made with cement paste only, the leaching process leads to a uniform degradation of the samples. The thickness of this degraded zone is the same whatever the size of the sample. The decrease in compression strength is thus completely explained by this degraded zone. In the case of a sample made with mortar, the leaching process leads also to a degradation zone whose thickness is the same whatever the size of the sample. But, the decrease in mechanical properties and the increase in irreversibility couldn't be explained for the different sizes of samples by the same thickness of leaching zone. This is due to the fact that the chemical attack progresses through the interfacial zones because they have a higher porosity than the bulk paste. The relative volume of interfacial zones linked together with the aggressive environment is more important in the case of the smaller sample.