AbstractPortland cement composites modified with hydrothermal SiO2 nanoparticles (0.01–3.0 wt.%) and basalt microfiber (1.5 wt.%) with W/C=0.4 were developed. At different doses of SiO2 nanoparticles aged 1–28 days in combination with the microfiber, the compressive strength Fcom (up to 51.1 MPa, 1.5 times higher than the specimen without nanoparticles and microfiber) and flexural strength Fflex (up to 13.2 MPa, 3.4 times) after 28 days of aging were determined. In addition, the impact viscosity, the number of blows before the first fracture Nff and before ultimate failure Ncd, impact coefficient Niv=Ncd/Nff (up to 30; 2.72 times), and specific energy of impact destruction (up to 199.4 kJ/m2, 22.2 times) were calculated. Synergetic effects of the combined action of different scale modifiers on Fflex28 (1.29 times) and on Ncd and Eim/Sc (1.95 times) were revealed. Statistical correlations with high R2 values were obtained between the characteristics, (Ncd,Nff)–(Fcom28,Fflex28) and (Fflex1,7,28, Fcom1,7,28), (Nff, Ncd, Niv) and dose of SiO2 nanoparticles, which can used for the design of concrete structures and reduction of cement consumption. The mechanism of the strong synergetic effect of this combination can be explained by the increased volume fraction of the high density (HD) phase of the calcium silicate hydrates (CSH) gel with more packed nanogranules in a basic volume of cement matrix and interfacial transition zone (ITZ) and the increase in the shear stress of the CSH gel relative to the lateral microfiber surfaces inside the HD phase volume.