Pure water leads to a progressive decalcification of hardened cement pastes. The physical and chemical processes of this degradation are relatively well defined today, but the consequences of the leaching on the porosity increase and the loss of strength have been only partially studied. Our work consists of correlating the alteration of these properties with the rate of degradation of the material. The leaching is achieved by an accelerated process. Compressive strength and water porosity are measured after different processing times in an aggressive solution. A physical phenomenon analysis leads us to propose a stress distribution model at rupture, and a calculation model of the porosity increase, taking into account the nature of the degraded hydrates (CSH).