Vadim Potapov, Professor, Doctor of Technical Sciences, Head Scientist, Research Geotechnological Center of the Russian Academy of Sciences, Russian Federation

Amorphous nanodispersed silicon dioxide SiO2 is currently one of the most common nanomaterials. The relatively low cost of production and its physico-chemical properties make it possible to obtain and use nanosilica in large quantities. In various forms of nanopowders, gels, xerogels, aerogels, sols and sol-gel synthesis products, nanosilica is used as an independent product or one of the components of the final product. A new renewable source for the production of nanosilica is a hydrothermal solution exists in the depths of deposits as a result of the interaction of water with rock minerals at elevated temperatures and pressures. Silica come in the hydrothermal solution as orthosilicic acid molecules.

A reagent-free technology for the production of a new type of amorphous nanosilica in the form of stable aqueous sols, gels and nanopowders has been developed (Fig. 1) [1–3]. Its physical and chemical characteristics are studied and directions of application in industry are proposed: first of all, in the production of Portland cement concrete, growth stimulators of agricultural plants and medical preparations.

The production technological scheme includes the processes of polycondensation of silicic acid molecules in a hydrothermal solution and the assembly of SiO2 nanoparticles with an adjustable size [1–3]. By the selection of the pore diameter at the stage of ultrafiltration membrane concentration [3], SiO2 nanoparticles and ions of dissolved salts are separated in such a way that the degree of concentration of SiO2 nanoparticles becomes many times higher than that of ions. As a result, the thickness of the double electric layer and the zeta potential of the surface of the nanoparticles ensure the stability of concentrated aqueous sol with a SiO2 content of up to 40 wt. % and higher. By the process of sol-gel transition with a decrease in pH values in the range of 5-6, SiO2 gels are obtained. Using the processes of coagulation and vacuum sublimation, SiO2 nanopowders are extracted from sols. The technology allows you to adjust the structural and morphological characteristics of nanopowders and gels. Three forms of hydrothermal nanosilica – sols, gels, nanopowders – have physical and chemical properties that allow us to find a number of applications. Hydrothermal nanosilica has a high specific surface area of up to 500 m2/g (Fig. 2), high density of surface silanol groups up to 4.9 nm-2, low degree of crystallinity, small size of fractal agglomerates, high sorption ability and chemical activity of the surface, non-toxicity [2]. The exclusion from the technological scheme of traditional chemical reagents such as sodium silicate, acids, alkalis, ion exchange resins and low specific consumption of electrical energy lead to a low production cost of hydrothermal nanosilica.

In Portland cement concretes, hydrothermal SiO2 nanoparticles, due to their high and chemically active surface, accelerate the kinetics of the formation of calcium hydrosilicates gel, reduce capillary porosity, regularize the gel structure, increasing the volume packing density of the gel nanogranules and increasing its modulus of elasticity and modulus of shear [4, 5]. Nanomodification of concrete with hydrothermal SiO2 leads to an acceleration of compressive strength gain: by 2 times at the age of 1–3 days and by 40–60 % at the age of 28 days. An increase in bending strength and axial tension strength is achieved. The mechanism of material destruction changes and the fracture toughness and fracture energy coefficients increase. The most powerful effect of the influence of nanoparticles on the structure of concrete is expressed in an increase in the ability to dissipate the impact energy of destruction and an increase in the impact viscosity. Modification with nanosilica increases the durability of concrete: water resistance, frost resistance and abrasive resistance. The modification of concrete at different scale levels has a significant prospect (Fig. 3). When modifying concrete with a combination of hydrothermal nanoparticles SiO2 – microfiber, a synergistic effect is manifested: the bending strength increases by 3.4 times, the specific energy of impact destruction – by 22.2 times, the impact viscosity coefficient – by 2.7 times. Nanomodification with hydrothermal SiO2 nanoparticles is used in heavy weight concrete, and also has broad prospects for use in concrete with cement-substituting materials – silica, fly ash, slag, aluminosilicate additives, in monolithic and block construction, construction of transport bridges, 3D-printing.

When introducing non-toxic hydrothermal SiO2 nanoparticles with a high and biochemically active surface into the cells of agricultural plants, the effects of stimulating the growth and increasing the crop of a wide range of agricultural plant were obtained: potatoes, cabbage, zucchini, lettuce, radish, wheat, etc. The effects are achieved as a result of the interaction of SiO2 nanoparticles with intracellular structures and due to the intensification of photosynthesis by luminescent reemission of solar radiation energy in the wavelength range corresponding to the maximum values of the absorption coefficients of photosynthetic pigments – chlorophylls a and b. With an increase in yield, there is an increase in the biochemical parameters of plants: the contents of starch, protein, fat, oil, vitamins, etc. [2]. When germinating wheat seed varieties, the effects of enhancing the action of hydrothermal SiO2 nanoparticles in combination with light-diode radiation of a certain wavelength were established. Methods of using hydrothermal nanosilica as a feed additive that increases the productivity of agricultural animals: cows, pigs, chickens, bees have been developed.

Approaches for the use of hydrothermal nanosilica as anti-inflammatory agents, enterosorbents and drugs that affect the bone structure are proposed.

References

1. Potapov V.V., Fediuk R.S., Gorev D.S. Obtaining sols, gels and mesoporous nanopowders of hydrothermal nanosilica. Journal of Sol-Gel Science and Technology. 2020. V. 94. pp. 681–694. https://doi.org/10.1007/s10971-020-05216-z.
2. Potapov V.V., Fediuk R.S., Gorev D.S. Hydrothermal nanopowders: obtaining them and their characteristics // Nanomaterials. 2020. V. 10(4), 624. pp. 1–28. doi:10.3390/nano10040624.
3. Potapov V.V., Fediuk R.S., Gorev D.S. Membrane concentration of hydrothermal SiO2 nanoparticles // Separation and Purification Technology. 2020. 251. 117290. pp. 1–15. https://doi.org/10.1016/j.seppur.2020.117290.
4. Potapov V.V., Efimenko Yu. V., Fediuk R.S., Gorev D.S. Effect of hydrothermal nanosilica on the performances of cement concrete // Construction and Building Materials. 2021. V. 269. 121307. pp. 1–15. https://doi.org/10.1016/j.conbuildmat.2020.121307.
5. Potapov V., Yuriy Efimenko Yu., Fediuk R., Gorev D., Kozin A., Liseitsev Yu. Modification of Cement Composites with Hydrothermal Nano-SiO2 // Journal of Materials in Civil Engineering. 2021. doi 10.1061/(ASCE)MT.1943-5533.0003964.

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