Production and characteristics of hydrothermal nanosilica

V. Potapov, D. Gorev
Research Geotechnological Center, Far Eastern Branch of Russian Academy of Sciences,
Russian Federation

Keywords: hydrothermal solution, orthosilicic acid, SiO2 nanoparticles, nanopowders, agglomerat

Summary:

Experiments were performed to obtain SiO2 sols, gels, and mesoporous nanopowders based on hydrothermal solution. To achieve this, polycondensation of orthosilicic acid (OSA) at certain pH and temperature, as well as ultrafiltration membrane concentration, sol-gel transition, and cryochemical vacuum sublimation were done. The dynamic light scattering, scanning and tunneling electron microscopy, low-temperature nitrogen adsorption, and others methods determined the physical and chemical characteristics of nanosized SiO2 samples. By choosing the pore size of the ultrafiltration membranes at the sol concentration stage, one can control the ratio of the SiO2 content to the total salt content and provide a zeta potential of nanoparticles sufficient for the stability of the sols. It was shown that by varying the temperature at the polycondensation stage from 20 to 90°С at pH = 8.5-9.3, it is possible to control the final average diameter of SiO2 particles in the range from 5 to 100 nm, respectively. The technological scheme allows you to adjust density, particle diameters of nanopowders, specific surface area, as well as diameters, area and pore’s volume. Thus, the structure of nanopowders is regulated - the volume fraction of the packing of spherical particles in aggregates and agglomerates, and the size of agglomerates. The pour density of the nanopowders depends on the SiO2 content in sols and were 0.02–0.3 g / cm3. 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. Fractal dimension of nanoparticles agglomerates was 2.04-2.21. Solid samples of compacted nanopowders had a compressive strength of up to 337 MPa. Possible applications of hydrothermal nano-SiO2 are considered. One way of using of SiO2 sols and nanopowders having a high and chemically active surface is modification of portland cement concrete to rise it’s compressive strength by acceleration kinetics of cement hydration and changing the structure of CSH gel.