The physicochemical properties of amorphous silica precipitated from a hydrothermal solution were studied. Low-temperature nitrogen adsorption in conjunction with the BET method was used to determine the specific surface area of this silica. Based on thermogravimetry data, the total content of water was estimated. A comparison of the thermogravimetry data with the Zhuravlev physicochemical constants made it possible to determine the temperature dependences of the concentration of surface and internal silanols over a temperature range of from 200 to 1200°C. A new type of amorphous silica with enhanced internal water content was revealed. The distinctions between the mechanisms of the removal of surface and internal water were established.

The technological mode of obtaining amorphous SiO₂ nanopowders based on hydrothermal solutions is proposed in this study. Polycondensation of orthosilicic acid as well as ultrafiltration membrane separation, and cryochemical vacuum sublimation were used. The characteristics of nanopowders were determined by tunneling electron microscopy, low-temperature nitrogen adsorption, X-ray diffraction, and small-angle X-ray scattering. The scheme allows to adjust density, particle diameters of nanopowders, specific surface area, as well as diameters, area and volume of the pore. Thus, the structure of nanopowders is regulated—the volume fraction of the packing of spherical particles in aggregates and agglomerates, the size of agglomerates, and the number of particles in agglomerates. The pour densities of the nanopowders depend on the SiO₂ content in sols, which were 0.02 to 0.3 g/cm³. Nanoparticles specific surface area was brought to 500 m²/g by low temperature polycondensation. Nanoparticle aggregates specific pore volume (0.2–0.3 g/cm³) weakly depend on powders density. The volume fraction of the packing of SiO₂ nanoparticles in aggregates was 0.6–0.7. Solid samples of compacted nanopowders had a compressive strength of up to 337 MPa. Possible applications of hydrothermal SiO₂ nanopowders are considered.

The approach of numerical simulation of orthosilicic acid OSA polymerization and SiO₂ nanoparticle formation in hydrothermal solution have been developed based on the model of the homogeneous stage of nucleation and the subsequent growth of particles. The influence of surface tension on the interface of SiO₂–water, the rate of molecular deposition, and Zeldovich factor Z were evaluated. Temperature dependence on time, pH, initial OSA concentration, and ionic strength are the main parameters that determine the kinetics of colloid phase formation, the final average size of SiO₂ nanoparticles, and the particle size distribution and its polydispersity index. The results of the numerical simulation were verified with experimental data on OSA polymerization and measurement of nanoparticles sizes using the method of dynamic light scattering in a wide range of temperatures of 20–180 °C, pH = 3–9, SiO₂ content C_t of 300–1400 mg/kg, and ionic strength I_s of 0.0001–0.42 mol/kg. The results obtained can be used in the technology of hydrothermal synthesis of sols, gels, and nanopowders to regulate the kinetics of OSA polymerization and SiO₂ nanoparticle growth, particle size distribution, morphology, and structure of products.

We used SiO₂ nanoparticles, introducing them as stable aqueous sols from hydrothermal solutions, to enhance the mechanical properties of concrete. SiO₂ nanoparticles 10–100 nm in size with a specific surface of 60–500 m²/g were brought in the cement-sand-water system ranging from 0.01 to 0.3 wt % (with respect to the amount of cement). The nanoparticles were homogeneously distributed, applying a Relamix superplastificator ranging from 0.8 to 1.0 wt % (with respect to the amount of cement). The concrete was prepared by accelerated high-temperature hardening. The compression and bending durability were measured as the functions of the weight fraction of nanoparticles; the compression durability of solid samples was found to significantly depend on this parameter. We also studied the influence of the water/cement ratio on the properties of concrete.