Modification of concrete by hydrothermal nanosilica
The paper shows the possibility to use hydrothermal nanosilica as a modifying additive to increase the strength of concrete based on portland cement. The technology of obtaining hydrothermal nanosilica in the form of sols and nanopowders by ultrafiltration membrane concentration and cryochemical vacuum sublimation is proposed. The results of increased strength of concrete under compression by the addition of hydrothermal nanosilica Sol are presented. The experiments were performed on highly mobile concretes (ST = 10-19 cm) on equal-moving mixtures with water-cement ratio W/C = 0.61-0.71 at a dose of SiO<sub>2</sub> 2 wt.% and the rate of superplasticizer polycarboxylate 0.95±0.05 mas.% by cement. At the initial stage of hardening when the age is 1 day, the increase in the compressive strength of concrete reached 90-128% compared to the control sample. At the age of 28 days the increase in strength was 40%. High chemisorption activity of hydrothermal nanopowder with respect to Ca(OH)<sub>2</sub> was determined in the experiment with lime medium. This indicates that the amorphous nanoadditive SiO<sub>2</sub>, which has a high specific surface area, causes the formation of high-strength hydrates of calcium silicates in the cement (lime) medium as a result of the pozzolan reaction, and this causes an increase in the strength of concrete.