Experiments were carried out to obtain sols and nanopowders of SiO2 on the basis of hydrothermal solutions. The processes of orthosilicic acid polycondensation, ultrafiltration membrane concentration and cryochemical vacuum sublimation were carried out to obtain the result. The physical and chemical properties of sols and nanopowders of SiO2 have been determined through a number of methods. It has been demonstrated that diameters of SiO2 nanoparticles in sols and nanopowders were within the range of 5-100 nm. The possibility of the use of the obtained silica as a modifying additive to increase the strength of concrete has been substantiated. Presented are the results of the test aimed to heighten concrete compressive strength by the use of nanosilica additive extracted from hydrothermal solution.