[en] Non-cementitious grouts have been tested in ONKALO for the sealing of fractures with the small hydraulic aperture. A non-cementitious inorganic grout material for sealing the fractures of the apertures less than 0.05 mm is colloidal silica called silica sol. In this work, the release of silica colloids from the silica sol gel, the stability of silica sol colloids and sorption of Europium-152 on silica sol colloids were determined in saline OLSO and low salinity Allard groundwater simulates, de-ionized water and NaCl and CaCl₂ solutions. The salinity of groundwater had a significant influence on the release and stability of silica colloids. In OLSO, particle size distribution was at first wide from nanometres to thousands of nanometres. According to the disappearance of large particles, aggregation and sedimentation had been occurred after one year. Zeta potential values around zero indicated the instability of colloids. In Allard, particle size distribution was narrow and the particle diameter remained less than 60 nm. High negative zeta potential values were in accordance with the existence of stable silica colloids. Europium sorption on silica sol colloids was significantly dependent on solution pH. The K_d-values were 2-10 m³ kg^-1 for OLSO pH 7-8 and Allard pH 8-9 and 100-140 m³ kg^-1 for OLSO pH 10-11. In OLSO pH 10-11 samples Eu-152 was rapidly sorbed onto silica sol colloids and measured radioactivity in a liquid phase was at the background level resulting in inaccurate K_d determination. The concentrations of the released silica colloids were some higher than concentrations of natural colloids determined in granitic groundwater. The main uncertainties remain in the quantification of colloid generation under realistic repository conditions and how mobile colloids are. Irreversible sorption on silica colloids may increase radionuclide transport if colloids are sufficiently stable. Under the prevailing medium saline to saline groundwater conditions in Olkiluoto, no significant release of colloids from silica sol is expected but the possible influence of glacial melt waters and the synergy of bentonite and silica colloids has to be considered. (orig.)