Exposure of concrete to the atmosphere changes the microstructure of the cement paste. However, knowledge about the alteration of the porosity due to drying is typically not considered in deterioration processes of the hardened cement paste. The purpose of this article is to emphasize the importance of the porosity changes due to drying. 1 mm thick discs specimens made with four different pastes including three blended cements were exposed to CO2-free atmosphere at 70% relative humidity. The consequences of drying during one year of exposure were studied using XRD, and SEM for the phase assemblage, while porosity alteration was examined by MIP and high-resolution microscope. This study showed that the significant coarsening of the pores occurs during exposure at 70% RH which should be taken account of in durability studies.