V. V. Potapov

Influence of silica nanoparticles on the strength characteristics of cement samples

The influence of silica nanoparticles on the properties of systems of the cement-sand-water type was investigated experimentally. Nanoparticles were separated from natural hydrothermal solutions according to the two-stage technological scheme including membrane concentration and cryochemical vacuum sublimation drying of silica sols with the preparation of nanopowders. Silica nanopowders with a particle size of 10–100 nm, a specific surface area of 100–400 m2/g, and an average pore diameter of 3–10 nm were introduced into the cement-sand-water system in amounts of 0.001–0.200 wt % with respect to concrete. The nanopowders were mixed with water intended for tempering cement to obtain a stable suspension with a uniform distribution of particles over the volume, which was introduced into the mixture of cement and sand. The characteristics of the solid samples were measured at different ages from 3 to 28 days. It was established that the nanoadditive affects the density, the rate of strength development, and the final compressive strength of cement samples.

Removal of Silica from Geothermal Brine

The problem of removing silica from geothermal brine is considered. The colloidal and chemical properties of the brine are studied. Silica precipitation by electrocoagulation is described, and the process parameters are optimized. Factors in the profitability of the multipurpose utilization of the hydrothermal heat carrier are discussed.

Studying the Interaction between Silica Nanoparticles and Metals by Spectrophotometry

The optical absorption spectra of water silica sols containing nanoparticles (NPs) of metals (Ag, Pd, Fe, and Pt) are investigated. Silica sols are obtained from natural hydrothermal solutions via membrane concentration (ultrafiltration). Water sols of silica with specific sizes, pH values, ζ potentials of SiO2 NP surfaces, and low concentrations of SiO2 NPs are used. Plasmon resonance in optical absorption spectra is used to study the interaction between silica and metal NPs. Parameters of plasmon resonance (position, height, and half-width of optical absorption bands), from which the degree of interaction is assessed, are determined. Relationships between the optical properties of the surfaces of nanoparticle-size silica particles, the method of their production, and the effect of adsorbed metal particles on these properties are established.

Enhancement of concrete durability by introducing SiO2 nanoparticles

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.

Polycondensation Kinetics of Orthosilicic Acid in a Hydrothermal Solution

The polycondensation kinetics of orthosilicic acid in hydrothermal solutions was studied experimentally at different temperatures, pH, and ionic strengths. As a result of the mathematical processing of experimental data, equations approximating the time dependence of the polycondensation process and the dependence of the polycondensation rate constant on the solution ionic strength were obtained. Experimental data on the kinetics of the reverse process of dissolving colloidal silica were obtained at different temperatures and pH.

Polycondensation of orthosilicic acid in hydrothermal solutions at different temperatures, pH values, and ionic strengths

The kinetics of polycondensation of orthosilicic acid in hydrothermal solutions is investigated at different temperatures, pH values, and ionic strengths. The equations approximating the time dependence of the polycondensation and the dependence of the polycondensation rate constant on the ionic strength of the solution are determined by the mathematical processing of the experimental data. The experimental data on the kinetics of the inverse process, i.e., the dissolution of colloidal silica, are obtained at different temperatures and pH values.

Physicochemical Processes during Precipitation of Silica from Geothermal Brine

Physicochemical properties of the silica–geothermal brine colloidal system are investigated. The reaction order and the rate constants for silica polymerization are determined, the radius of colloidal particles is measured, and their charge is estimated. The mechanism of coagulation and precipitation of colloidal silica by metal cations is experimentally studied. On the basis of the results obtained, a basic process to recover silica from the geothermal heat-transfer medium is designed.

Characteristics of NanoSilica Produced from Hydrothermal Solutions

SiO2 sols and nanopowders are produced from hydrothermal solutions using the following processes: polycondensation of orthosilicic acid, ultrafiltration membrane concentration, and cryochemical vacuum sublimation. The physical–chemical characteristics of SiO2 sols and nanopowders are determined: particle size distribution, specific surface area, viscosity, and impurity ions content. It is demonstrated that the size of SiO2 nanoparticles in sols and nanopowders could attain 5–100 nm. The possibility of applying the produced silica to strengthen the concrete is justified.

Obtaining of Silica Nanopowders from Natural Hydrothermal Solutions

Obtaining of nanodispersed silica powders from natural hydrothermal solutions is described. Hydrothermal solutions contain colloid silica forming as a result of the polycondensation of the molecules of orthosilicic acid. Via ultrafiltration with a membrane concentration of hydrothermal solutions, silica sols with SiO2 contents up to 600 g/dm3 (43.0 wt %) and particle radii of 29–135 nm are obtained. The silica powders with the specific surface area of 110–400 m2/g, average pore diameter of 3–10 nm, and pore volume of 0.2–0.3 cm3/g are obtained via the cryochemical vacuum-sublimation drying of sols with the use of liquid nitrogen. The particle size in the powders is in the range from 10 to 100 nm.

The optical properties of nanodisperse silica in hydrothermal solutions

The optical properties of aqueous solutions of colloidal silica were studied by photon correlation spectroscopy and spectrophotometry. Photon correlation spectroscopy measurements were taken at a 633 nm wavelength of monochromatic laser radiation. Autocorrelation function and scattered light amplitude plots were constructed, and the mean radii and diffusion coefficients of particles were determined. Spectrophotometric measurements were taken over the electromagnetic radiation wavelength range 200–1000 nm. The positions of optical density maxima were determined. The influence of various factors, including concentration, the size of nanoparticles, temperature, and pH, on the optical properties of aqueous solutions of silica was estimated.