Hydrophobic substances contain in its structure non-polar groups by which have the capability to "repel" water molecules. Because of them the material become hydrophobic, i.e. it is not water-wettable. Hydrophobic additives are used in various areas, including buildings, bridges, ceramic materials, underground constructions and marine constructions. The use of hydrophobic admixtures is one of the ways to protect building elements from adverse water effects, thereby increasing their durability by reducing the water absorption. In our study we investigated the influence of siloxanes on compressive strength and absorbability of cement mortar. The modification of cement mortar microstructure due to addition of organosilicon compounds was analyzed by mercury intrusion porosimetry. Finally we assessed the effectiveness of poly(dimethylsiloxane) and water solution of methyl silicon resin as hydrophobic admixture by measuring the water absorption coefficient.