In this research a hydrothermal method was applied to synthesis of wollastonite. Silica, nano silica and calcium carbonate were used as raw materials. Different slurries were prepared with SiO2/CaO molar ratios of 0.54, 0.8, 1 and a solid content of 50 wt%. Then the slurries were hydrothermally treated in an autoclave for 2 h at 200 degrees C at pressures of 3, 5 and 7 atm. In the next step, the samples were dried at 150 degrees C for 8 h and then were calcined at 1000 degrees C for 5 h. The microstructure and phase analyses were investigated using SEM and XRD. The results showed that by using both silica and nano silica, with increasing pressure, the amount of wollastonite increased. At all pressures using silica, slurry with 50 wt% of solid content, SiO2/CaO molar ratio of 0.8 gave the optimum results while by using nano silica the optimum SiO2/CaO molar ratio was 1.

Keywords: silica; nano silica; wollastonite; hydrothermal