Disclosed is an aqueous additive or sealing composition for cementitious compositions comprises a water-soluble salt of carbonate or hydrogen carbonate, a silica-based material, a specifically selected dispersant, at least one specifically selected thickening agent, and water. Furthermore, a method of manufacturing the aqueous additive or sealing composition, a cementitious composition comprising a hydraulic cementitious binder, a mineral aggregate, the aqueous additive composition, and optionally water and its preparation method is described. Moreover, a method of treating a surface of a cementitious structure by applying the aqueous additive or sealing composition on a surface of a cementitious structure as surface treatment agent or coating material is disclosed. The disclosed additive and sealing composition can be used as liquid crystalline waterproofing admixture composition in several applications.