This diploma thesis deals with crystallization additives used to modify cement composites. Describes their principle of operation, application methods and also mentions the different manufacturers with their products. In this work were made test samples with Xypex Admix crystallization additive which were tested for mechanical and physical properties depending on the amount of crystallization additives, aggressive environment and the duration of exposure in this environment. Selected samples were analyzed with RTG, DTA, SEM and porosimetry for detailed examination of the microstructure.