AbstractThe 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, SiO2 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  nm in diameter, a specific surface area of 410  m2/g, and a density of reactive silanol groups of Si-OH 4.9  nm were used as modifiers of the solutions. The level of increment in compressive strength Rcom turned out to be lower for all dosages of SiO2 than after modification with the sol. At low SiO2 dosages (0.01%–0.25%), the increment in compressive Rcom, flexural Rflex strength, and density ρ rose with an increase in the SiO2 dosage much more rapidly than at high dosages (0.5%–3.0%). The relations ΔRcom/[SiO2], ΔRflex/[SiO2], and Δρ/[SiO2] can be approximated by the same type of equations like ΔR/[SiO2]([SiO2])=A/[SiO2]z, where coefficients A and z depends on the age of the solid samples. The level of (Rcom1/Rcom28) for the nanopowder was lower than for sol, especially at the lower dosages of SiO2 0.01%–0.5% by weight. The ratio Rflex28/Rcom28 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 SiO2 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 SiO2 and increasing the product’s durability by 3–6 times in a uniform manner is shown.

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