Waste silica with 5.6μm mean particle diameter was mixed with lime and autoclaved in alkaline solutions at 180°C for several hours under saturated vapor pressure. The observation under a scanning electron microscope (SEM), DTA and X-ray analysis were performed to determine the microstructure, phase component and crystallinity of the reaction products.
The additions of sodium and potassium hydroxides accelerated the hydrothermal reaction. On the contrary, lithium hydroxide served as an inhibitor, because of the formation of Li2SiO3 on the surface of silica. Glauconite (mica group), an impurity in waste silica, retarded the crystal growth of calcium silicate hydrate, because of the formation of analcite (zeolite group) by sodium hydroxide.
The use of low concentration of NaOH or KOH resulted not only in a considerable reduction in autoclaving time, but also in increasing the bending strength of calcium silicate hydrate briquettes. The addition of 1 wt% potassium titanate whisker (K2O·6TiO2) into the waste silica and lime caused the bending strength of briquettes to increase by about 40%.
