This paper deals with the effect of the leaching process of cement based materials on their mechanical properties. This process induces mainly a total leaching of Ca(OH)2 and a progressive decalcification of C-S-H which leads to a gradient of ratio in the leaching zone. In a previous work, we venture the hypothesis that the dissolution of calcium hydroxide was the essential parameter governing both decrease in strength and increase in porosity in the case of a paste sample made with an OPC cement which leads to a 20% content of calcium. In order to quantify the effect of the decrease of ratio in C-S-H, we have performed experiments on paste sample with the admixture of silica fume to reduce the content of calcium hydroxide and thus emphasize the effect of ratio decrease of the C-S-H. The leaching process was achieved by the use of a 50% concentrate solution of ammonium nitrate. Compression tests were conducted on micro-cylinder samples (10, 12, 14, 20 and 30 mm of diameter) because of the slow kinetics of degradation due to the leaching. The deterioration of the cement paste and the mortar exposed to the action of the ammonium nitrate was indicated by a peripheral zone of less resistance. The experimental results allow the modeling of the mechanical behavior of cement pastes in relation to the ratio of degraded area over total area of the sample . The model thus defined allows to separate the effect of calcium hydroxide leaching and C-S-H leaching, and shows the importance of the first one.