Effect of hydrothermal nanosilica on the performances of cement concrete

Abstract

A new technological method for producing a nano-modifier for cement concretes from renewable source of hydrothermal solution using the processes of polycondensation of orthosilicic acid and ultrafiltration membrane concentration is proposed. According to the data of dynamic light scattering, tunneling electron microscopy, and low-temperature nitrogen adsorption, it is shown that in sol samples the average diameters of amorphous SiO2 nanoparticles can be varied in the range 10–100 nm, the specific surface area of ​​SBET is 50–500 m2 / g, the density of chemically active surface silanol Si-OH groups up to σ = 4.9 nm−2. Increasing the compressive strength of concrete specimens modified with SiO2 sol (0.5–3.0 wt%) and polycarboxylate superplasticizer (0.2–3.98 wt%), liquid-binder ratio L/B = 0.5–0.7, with normal hardening and with accelerated hardening with heat-moisture treatment, it reached 128% at the age of 1 day, and up to 40% at the age of 28 days. According to the kinetic curves of water absorption, a significant decrease in the volume, average pore diameter and an increase in the uniformity of the distribution of pores along the diameter of the modified concrete were established, which led to an increase in its water impermeability. The kinetics of the pozzolanic reaction with the formation of crystalline calcium hydrosilicates has been studied by the methods of X-ray diffraction analysis, energy-dispersive spectroscopy, scanning electron microscopy, and thermogravimetry. The chemo-sorption capacity of hydrothermal SiO2 nanoparticles for binding Ca(OH)2 in CaO solution and cement matrice was determined, which, due to the high SBET and σ values ​​for the first 24 h of the reaction, is many times higher than that of silica fume. In modified Portland cement matrice at the age of 1 day, the δCaO value was 750 [mg CaO / g SiO2], which corresponded to a response of 60–70% of the introduced nano-SiO2 and an increase in the rate of alite hydration by 20–30%, and rised at the age 100 days to 1200 [mg CaO / g SiO2]. Hydrothermal sols and SiO2 nanopowders have prospects for the creation of cement composites.