Formation of gyrolite during hydrothermal synthesis in the mixtures of CaO and amorphous SiO 2 or quartz

Abstract

Parameters of gyrolite hydrothermal synthesis were determined when primary mixtures consisting CaO and amorphous SiO 2· nH 2O or quartz and a sequence of intermediary compound formation were examined and explained. The molar ratio (C/S) of the primary mixtures was 0.66 (C—CaO; S—SiO 2) and the water/solid ratio (W/S) of the unstirred suspension was 10. Hydrothermal synthesis was carried out in saturated steam at 150, 175, and 200 °C temperatures. The duration of isothermal curing was 4, 16, 24, 32, 48, 72, and 168 h. Gyrolite does not form even after a week in the mixtures of CaO and amorphous SiO 2 at 150 °C temperature in saturated steam. Increase in the temperature positively affects the synthesis of this compound—pure gyrolite at 175 °C was obtained after 72 h at 200 °C—after 32 h. It should be noted that while synthesizing gyrolite, intermediary compounds C-S-H (I) and Z-phase are always formed. The mechanism of hydrothermal reactions and the sequence of compounds to be formed in the mixtures of CaO and quartz are totally different. Due to low quartz solubility rate at temperature range from 150 to 200 °C, neither Z-phase nor gyrolite forms even during 72 h of hydrothermal curing. In the beginning of the synthesis, α-C 2S hydrate prevails, which gradually recrystallizes into 1.13 nm tobermorite and xonotlite. Almost half of the quartz reacts during the first 4 h at 150 °C temperature, and the further decrease of its quantity depends much on the duration of hydrothermal curing. However, about 10% of the quartz does not react at all, when the C/S in the products approach approximately 0.8, stable calcium silicate hydrates—1.13 nm tobermorite and xonotlite—are formed. They are relatively stable. Experimentally obtained data and preconditions were approved by thermodynamic calculations.