Abstract
Waste silica with 5.6μm in the mean particle diameter was mixed with lime, shaped to 7×11×70mm and autoclaved at the range of temperature between 130 and 240°C for several hours under saturated vapor pressure. When the waste silica was mixed with lime in the molar ratio CaO/SiO2=0.4, calcium silicate hydrate gave the greatest bending strength.
The observation with a scanning electron microscope (SEM), TG, DTA and X-ray analysis were performed to determine the microstructure, component and crystallinity of the reaction products. The reaction products autoclaved were mainly tobermorite groups and the bending strength reached maximum when the specimens were autoclaved at 180°C for the time more than 48hr. The increase in the ratio of lime tended to raise water absorption and porosity, but to lower bulk density.
The thermal expansion of the products was measured by thermomechanical analysis (TMA) in the temperature range of 50 to 700°C. Calcium silicate hydrate expanded slightly from room temperature to 250°C, and contracted at temperatures higher than 250°C, because of its dehydration. The residual silica expanded abruptly at 573°C, because of the transition of α to β type quartz. Calcium silicate hydrate autoclaved at 200°C in the molar ratio of CaO/SiO2=0.4 gave the least thermal expansion and contraction.
The addition of sodium hydroxide accelerated the hydrothermal reaction and decreased the water absorption and porosity of the resulting calcium silicate hydrate. The effect of sodium hydroxide on the relationship between the maximum bending strength and autoclave temperature is ambiguous.
