Influence of Gypsum on the Hydrothermal Reaction of Lime-Quartz System and on the Strength of Autoclaved Calcium Silicate Product

摘要

The influence of adding gypsum on the chemical reactions and strength of lime-quartz based autoclaved board samples were investigated. The samples were prepared by adding 0, 5, 10, 20 mass percent of gypsum (converted to CaSO_4) under saturated steam pressures at 180 deg C for various times from 0 approx 160 h and 140 deg C for 0 h. The starting materials of Ca (OH)_2 and quartz with Ca/Si = 0.8 were mixed with gypsum, cellulose fiber and water, followed by molding to a density of 1000 kg centre dot m~(-3). The demolded boards were then hardened by autoclaving. The specimens without gypsum reacted hydrothermally: Ca-rich C-S-H +quartz->C-S-H->tobermorite with 1.1 nm spacing. Adding gypsum to the starting mixtures, hemihydrate dehydrated form of the gypsum reacted with lime and quartz to form hexagonal prisms of hydroxyl ellestadite which grew radially, and inhibited the formation of C-S-H and tobermorite. This inhibition resulted in the delay of the elevation of the strength. As the hydrothermal reaction progressed, hydroxyl ellestadite reacted with quartz and decomposed into tobermorite crystals and anhydrite. A finer microstructure developed, resulting in a high strength. Further hydrothermal treatment increased the strength of specimens prepared with higher quantities of gypsum, indicating higher tobermorite crystallinity. Prolonged treatment of specimens prepared with 0, 5 or 10 mass percent of gypsum caused tobermorite to decompose into gyrolite, and led to a coarser microstructure and reduction in strength. However, gypsum that was added to constitute 20 percent give tobermorite to remain stable and inhibited the decomposition into gyrolite.