On the Hydrothermally Cured Bricks Synthesized from the Mixture of Calcium Hydroxide and Silica Gel

Abstract

The compressed mixture of calcium hydroxide and silica gel powder was treated hydrothermally at various temperatures between 140°C and 300°C, in an autoclave filled up 12% water. The influences of CaO/SiO^2- ratio, moulding pressure, reaction period, reaction temperature, the amount of unreacted lime, the morphology and the kinds of reaction products on the compressive strength of cured bricks were inverstigated.
The compressive strength of the cured bricks varied with the CaO/SiO₂ ratio, and at the temperature below 180°C the maximum values were obtained in the bricks with CaO/SiO^2- ratio of 2/3 and 1/3. The maximum value was found in the lower CaO/SiO^2- ratio (1/3) bricks cured at an elevated temperature. For example, the maximum strength was 280 kg/cm² for 2/3 ratio cured at 140°C, and 560 kg/cm' for 1/3 ratio at 300°C. The bulk specific gravity and combined water content in bricks decreased with an increase of the curing temperature and the treating period. The unreacted lime in the reaction product decreased with the increase of the curing period, but it was not the case in product with CaO/SiO^2- ratio under 2/3. From the result of X-ray diffraction analysis, a compound like CSH (1) was found in the products cured below 180°C, and xonotlite or gyrolite were found in the product cured above 250°C. The highest compressive strength was attained in the brick containing gyrolite.
Polarizing microscopic observation made it clear that at lower temperature the reaction products were scattered around the original silica gel particles, and at higher temperature the boundary of silica gel disappeared and the products dispersed in the matrix. The bricks cured at high temperature gave good heat resistivity. The bricks with CaO/SiO^2- ratio 1/3 cured at 300°C for 6 hrs revealed only 3% contraction after heating at 800°C.