PHYSICO-CHEMICAL PROCESSES OF ALKALINE ACTIVATION OF SILICA DURING HEAT TREATMENT IN THE SIO2-NaOH-H2O SYSTEM

The physicochemical processes during alkaline activation of the SiO2-NaOH-H2O system with crystalline silica have been investigated. The studies were carried out using thermogravimetric, X-ray phase analysis methods and IR spectroscopy data. Compositions with different silicate modulus (SiO2/Na2O 4-7), the amount of the introduced silica fume (10-30%) in substitution of marshalite and the concentration of sodium hydroxide solution (30-60 wt.%) are considered. According to the results of the study, the reactions of interaction of the components and a model of phase transformations in the composition when it is heated to 850 °C are proposed. At the stage of interaction of the components (30-130 °C), hydration processes occur with the formation of sodium hydrosilicate Na2SiO3(OH), with on the surface of which a layer of crystallohydrate water and a water layer to be formed without formation of free water. When the composition is heated to temperatures of 130-300 °С the water layer and then the crystallohydrate water are removed. The removed water interacts with unreacted silica and forms hydrated forms of silica. Upon further heating to temperatures of 310-800 °C, OH groups are removed from sodium hydrosilicates and hydrated forms of silica and it turns into anhydrous silicates. Heating the composition to 850 °C leads to the formation of a pyroplastic mass from a eutectic melt (Na2O·2SiO2–SiO2) and residual silica. The two-stage mechanism of formation of a porous frame is established at the stage of decomposition of sodium hydrosilicate crystallohydrates (80-200 °C) and at the stage of melt foaming at high temperatures (790-850 °C). Foaming occurs due to the removal of water vapor (80-200 °C) and the expansion of the volume of gases (790-850 °C) in the porous structure formed at the first stage of foaming. The developed compound of the high-modulus composition (SiO2/Na2O 5,7) is a basis to obtain a porous glass composite using a two-stage alkaline technology with the introduction of additional oxides that increase chemical resistance. The composition includes the following components, wt.%: marshalite – 50, silica fume – 23, sodium hydroxide – 16, water – 11.