Influence of SiO2 modification on hydrogarnets formation during hydrothermal synthesis

Formation and stability of hydrogamet and Al-substituted tobermorite were examined at 175 degreesC temperature in saturated steam environment processing CaO-quartz and CaO-arnorphous SiO2 suspensions. A large quantity of Al2O3 was added to the starting mixtures [molar ratio A/(S + A) = 0.10, duration of hydrothennal 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 SiO(2)(.)nHO as SiO2 component, starting raw materials react considerably quicker-the total Ca(OH)(2) is joined already while increasing the temperature up to 175 degreesC. 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 degreesC temperature. Moreover, it depends on SiO2 modification the hydrogamets of what type are to be formed. Si-free hydrogrossular forms in the mixtures with quartz and katoite in the mixtures with SiO(2)(.)nH(2)O. 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. (C) 2003 Elsevier Ltd. All rights reserved.