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The nanoparticles of SiO2 were used in cement systems to modify the rheological behavior,to enhance the reactivity of supplementary cementitious materials,and also to improve the strength and durability.In this research,low-cost nano-SiO2 particles from natural hydrothermal solutions obtained by membrane ultrafiltration and,optionally,by cryochemical vacuum sublimation drying,were evaluated in portland cement based systems.The SiO2-rich solutions were obtained from the wells of Mutnovsky geothermal power station (Far East of Russia).The constant nano-SiO2 dosage of 0.25％ (as a solid material by weight of cementitious materials) was used to compare the cement systems with different nanoparticles against a reference mortar and a commercially available nano-SiO2.Nanoparticles were characterized by X-Ray Diffraction (XRD),BET Surface Area,Scanning Electron Microscope (SEM) and Fourier Transform Infrared (FTIR) spectroscopy techniques.It was demonstrated that the addition ofpolycarboxylate ether superplasticizer and the dispersion treatment using an ultrasound processor can be used to facilitate the distribution ofnano-SiO2 particles in the mixing water.The effect ofnano-SiO2 particles in portland cement mortars was investigated by evaluating the flow,heat of hydration and compressive strength development.It was demonstrated that the use ofnanoSiO2 particles can reduce the segregation and improve strength properties.