Crystalline admixtures are a class of materials used to reduce the water permeability of concrete when exposed to hydraulic pressure. According to the American ACI Standard, they are classified as PRAH (permeability reducing agents under hydrostatic pressure). Their principle of operation is based on the formation of salts that block the pores when the concrete comes into contact with water under pressure. This work examines the effect of crystalline admixtures on the durability of concrete mixtures against carbonation and against chloride penetration. For this purpose, six concrete mixtures were prepared, with and without the use of crystalline admixtures. The resistance indices against chloride penetration and against carbonation were measured in these mixtures. It was observed that the addition of crystalline admixtures to the concrete mixture improved the resistance indices in each case, thus contributing to the extension of the service life of the structures. The percentage increase depended on the water-to-cement ratio of the concrete as well as the type of corrosion mechanism.