Impact Resistance of the Cement–Mortar Composite Modified with ${\mathrm{SiO}}_2$ Nanoparticles and Microfiber

Portland cement composites modified with hydrothermal ${\mathrm{SiO}}_2$ nanoparticles (0.01–3.0 wt.%) and basalt microfiber (1.5 wt.%) with $\mathrm{W}/\mathrm{C}=0.4$ were developed. At different doses of ${\mathrm{SiO}}_2$ nanoparticles aged 1–28 days in combination with the microfiber, the compressive strength ${\mathrm{F}}_{\mathrm{com}}$ (up to 51.1 MPa, 1.5 times higher than the specimen without nanoparticles and microfiber) and flexural strength ${\mathrm{F}}_{\mathrm{flex}}$ (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 ${\mathrm{N}}_{\mathrm{ff}}$ and before ultimate failure ${\mathrm{N}}_{\mathrm{cd}}$, impact coefficient ${\mathrm{N}}_{\mathrm{iv}}{=\mathrm{N}}_{\mathrm{cd}}/{\mathrm{N}}_{\mathrm{ff}}$ (up to 30; 2.72 times), and specific energy of impact destruction (up to $199.4\mathrm{kJ}/{\mathrm{m}}^2$, 22.2 times) were calculated. Synergetic effects of the combined action of different scale modifiers on ${\mathrm{F}}_{\mathrm{flex}}^{28}$ (1.29 times) and on ${\mathrm{N}}_{\mathrm{cd}}$ and ${\mathrm{E}}_{\mathrm{im}/}{\mathrm{S}}_{\mathrm{c}}$ (1.95 times) were revealed. Statistical correlations with high ${\mathrm{R}}^2$ values were obtained between the characteristics, $({\mathrm{N}}_{\mathrm{cd}}{,\mathrm{N}}_{\mathrm{ff}}{)–(\mathrm{F}}_{\mathrm{com}}^{28}{,\mathrm{F}}_{\mathrm{flex}}^{28})$ and (${\mathrm{F}}_{\mathrm{flex}}^{1,7,28}$, ${\mathrm{F}}_{\mathrm{com}}^{1,7,28}$), (${\mathrm{N}}_{\mathrm{ff}}$, ${\mathrm{N}}_{\mathrm{cd}}$, ${\mathrm{N}}_{\mathrm{iv}}$) and dose of ${\mathrm{SiO}}_2$ 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.