Abstract
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% of gypsum (converted to CaSO4) under saturated steam pressures at 180°C for various times from 0-160 h and 140°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· m-3. The demolded boards were then hardened by autoclaving. The specimens without gypsum reacted hydrothermally : Carich 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% 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% give tobermorite to remain stable and inhibited the decomposition into gyrolite.
