Modification of Cement Composites with Hydrothermal Nano-${\mathrm{SiO}}_2$

The modification of cement concretes with various silica-containing additives is an urgent task that can significantly improve their physical and mechanical properties, as well as operational characteristics. In this paper, ${\mathrm{SiO}}_2$ sols and nanopowders based on a hydrothermal solution were obtained by a technological method using ultrafiltration membrane concentration. The physicochemical characteristics of silica nanoadditives were measured by dynamic light scattering, the BET (Brunauer–Emmett–Teller) method of low-temperature nitrogen adsorption, scanning and tunneling electron microscopy, and small-angle X-ray scattering and thermogravimetry. Sol and nanopowder with particles $6.6\mathrm{nm}$ in diameter, a specific surface area of ${410\mathrm{m}}^2/\mathrm{g}$, and a density of reactive silanol groups of Si-OH $4.9\mathrm{nm}$ were used as modifiers of the solutions. The level of increment in compressive strength $R_{\mathrm{com}}$ turned out to be lower for all dosages of ${\mathrm{SiO}}_2$ than after modification with the sol. At low ${\mathrm{SiO}}_2$ dosages (0.01%–0.25%), the increment in compressive $R_{\mathrm{com}}$, flexural $R_{\mathrm{flex}}$ strength, and density $\rho$ rose with an increase in the ${\mathrm{SiO}}_2$ dosage much more rapidly than at high dosages (0.5%–3.0%). The relations $\mathrm{\Delta }{R}_{\mathrm{com}}/{[\mathrm{SiO}}_2]$, $\mathrm{\Delta }{R}_{\mathrm{flex}}/{[\mathrm{SiO}}_2]$, and $\mathrm{\Delta }\rho /{[\mathrm{SiO}}_2]$ can be approximated by the same type of equations like $\mathrm{\Delta }R/{[\mathrm{SiO}}_2{]([\mathrm{SiO}}_2])=A/{[\mathrm{SiO}}_2{]}^z$, where coefficients $A$ and $z$ depends on the age of the solid samples. The level of ($R_{\mathrm{com}}^1/R_{\mathrm{com}}^{28}$) for the nanopowder was lower than for sol, especially at the lower dosages of ${\mathrm{SiO}}_2$ 0.01%–0.5% by weight. The ratio $R_{\mathrm{flex}}^{28}/R_{\mathrm{com}}^{28}$ changed in a relatively small range of values from 0.14 to 0.18 for sol and 0.16–0.23 for nanopowders. A modification with the hydrothermal ${\mathrm{SiO}}_2$ sol and nanopowder at dosages of 0.25%–0.50% and higher allows one to bring the grade of cement material to a level of low abrasion. The possibility of modifying concrete products with large dimensions by dosing liquid hydrothermal sol ${\mathrm{SiO}}_2$ and increasing the product’s durability by 3–6 times in a uniform manner is shown.