| Abstract [eng] |
The parameters of α-C2S hydrate hydrothermal synthesis (4–72 h; 175–200 °C) from a non-stoichiometric composition (the molar ratio CaO/SiO2 = 1.5–1.75) of CaO/Ca(OH)2 and SiO2·nH2O mixture in unstirred suspension were determined and a sequence of intermediary compounds formation was examined and explained. It was determined that in unstirred Ca(OH)2–SiO2nH2O–H2O suspensions, calcium containing compound – portlandite – reacted slowly. It was estimated, within 4 hours of isothermal curing at 175 C -C2S hydrate, C-S-H(II) and a low base semi-crystalline C-S-H(I) type calcium silicate hydrate were formed. Within 48 h of hydrothermal synthesis, Ca(OH)2 was fully reacted and semi-crystalline C-S-H(II) recrystallizes into stable compounds – -C2S hydrate and C-S-H(I). It was estimated, within only after 8 h of isothermal curing at 200 C α-C2S hydrate start to dominate in the products together with C-S-H(I) and C-S-H(II). It should be underlined that after 24 h of isothermal curing C-S-H(II) becomes metastable and together with mentioned compounds xonotlite was formed. It was estimated that in the mixture (CaO and SiO2·nH2O) with C/S = 1.5–1,75, in the synthesis products α-C2S hydrate dominates at 175 °C after 8–72 h as at 200 °C – after 8–24 h. When duration of hydrothermal treatment was shorter – C-S-H(I) and C-S-H(II), as longer – kilchoanite and C8S5, prevail in the products. In this work, a study of water vapour sorption by α-C2S hydrate in a different humidity was done, and the stability of mentioned calcium silicate hydrate structure dependence on the partial pressure of water vapour was examined. Also, the theoretical calculations of hypothetical reactions parameters between water and α-C2S hydrate are presented. It was determined that at 25 oC temperature, the water vapour pressure (p/p0) over α-C2S hydrate influences the quantity of adsorbed water in the samples. Moreover, at lower p/p0 pressures (0.355 to 0.87), monolayer of adsorbate is formed, which can be characterized by the heat of physical adsorption. Meanwhile, by increasing p/p0, the layer of adsorbate grows, i.e. polymolecular layer is formed and the heat flow of the process is increased, in which value approximates towards the heat flow of water vapour condensation. The binder material (BM) was prepared by intensive milling and burning of synthetic α-C2S hydrate (C/S = 1.5; 16 h, 200 OC) and quartz mixture (1:1 by mass). It was estimated that the heat evolution and compressive strength properties are characteristic to created binder material. |
