Peculiarities of xonotlite synthesis from the raw materials with different SiO₂ activities

In this work, the peculiarities of xonotlite hydrothermal synthesis at 200 °C from lime and SiO₂ materials with different pozzolanic activities (A_P) were investigated: calcinated opoka (A_p = 170.1 mgCaO kg⁻¹), granite sawing waste (A_p = 52.2 mgCaO g⁻¹) and reagent SiO₂·nH₂O (A_p = 336.8 mgCaO kg⁻¹). By XRD, DSC, TG, SEM, FT-IR methods have been shown that the formation of crystalline calcium silicate hydrates and the sequence of the intermediate phases existence are influenced not only by SiO₂ component activity, but by other factors too. The use of the most active raw meal with SiO₂·nH₂O results in a very rapid formation of z-phase, C–S–H(I) and gyrolite, which hardly recrystallize into thermodynamically stable mineral—xonotlite. The impurities in the starting materials may promote the formation of some other compounds and retard the synthesis of stoichiometric ones: high content of Al-containing minerals in granite sawing waste (15.41% of Al₂O₃) predetermines that 1.13 nm tobermorite even after 72 h of hydrothermal curing did not recrystallize into xonotlite. Regardless of its average activity, calcinated opoka is an excellent material for the synthesis of crystalline calcium silicate hydrates. Amorphous SiO₂ from opoka begins to react first, followed by tridymite and cristobalite. 1.13 nm tobermorite and xonotlite are formed at the beginning of the hydrothermal synthesis (4 h), and this greatly reduces the probability of the existence of amorphous phases.