Influence of SiO2 modification on hydrogarnets formation during hydrothermal synthesis

Formation and stability of hydrogarnet and Al-substituted tobermorite were examined at 175 °C temperature in saturated steam environment processing CaO-quartz and CaO-amorphous SiO₂ suspensions. A large quantity of Al₂O₃ was added to the starting mixtures molar ratio A/(S+A)=0.10, duration of hydrothermal synthesis—from 0 to 24 h]. It was determined that hydrogarnets always tend to form more rapidly than 1.13 nm tobermorite. However, later, with extension of synthesis duration, they start to fracture and their quantity reduces almost in half during 24 h. CaO is present in the further reaction with SiO₂ forming hydrated calcium silicates, and released Al³⁺ ions are inserted into Al-substituted tobermorite crystal lattice. Using amorphous SiO₂·nH₂O as SiO₂ component, starting raw materials react considerably quicker—the total Ca(OH)₂ is joined already while increasing the temperature up to 175 °C. Meanwhile, in the mixtures with quartz when their composition is described by the molar ratio C/(S+A)=1.0, traces of Ca(OH)₂ are found even after 24-h isothermal treatment at 175 °C temperature. Moreover, it depends on SiO₂ modification the hydrogarnets of what type are to be formed. Si-free hydrogrossular forms in the mixtures with quartz and katoite in the mixtures with SiO₂·nH₂O. Si⁴⁺ ions are inserted into the crystal lattice of the latter compound while the first one remains undisturbed. This is presumably related to the lower solubility of the quartz. It was also noticed that an isomorphic Si⁴⁺ ions substitution with Al³⁺ ions in the hydrated calcium silicate lattice is considerably quicker when an amorphous SiO₂ is used as SiO₂ component instead of quartz.