Synthesis of insulating calcium silicates hydrates in a laboratory scale and determination of technological properties

Abstract

The production of fiber-cement plates began in the late 19th century. Calcium silicate technology is an important component of many construction materials. Portland cement contains in its matrix calcium silicate hydrates (CSH). Calcium silicate insulating materials are produced in a wide range of bulk density from 50 to 2100 Kg/m3. They are made of lime, silica sources, cellulose or other fibers (glass or carbon), additives and water. Several methods are used for the production of light calcium silicates. The first step, dosing and mixing the raw materials, is similar in all processes, followed by a variety of possible process steps like hydrothermal synthesis in a stirring autoclave, instant casting or pre-reaction of raw materials. This thesis is based on the determination and study of the technological properties about the calcium silicates hydrates. In this work, tests carried out use the pre-reaction method to receive light-weight materials with about 300 Kg/m3. If the raw materials are pre-reacted, they are activated prior to processing and brought to pre-reaction under intensive stirring at temperatures around 100°C. A water-rich CSH (II) gel is the result. Afterwards the product is hydrothermally treated in an autoclave using different pressure, temperature and time conditions. For the industrial application of calcium silicates the following technological properties are the most important: · Low thermal conductivity · High modulus of rupture · Low shrinkage at higher temperatures The thermal stability of calcium silicates is on the one hand limited by the phase transformation of its main phases: xonotlite, tobermorite and other more crystal water containing calcium silicate hydrates (C-S-H) into wollastonite at higher temperatures. On the other hand, impurities leading to a lower melting point. To improve the thermal stability, it is necessary to decrease the amount of amorphous or poorly crystalline phases which are mainly responsible for high shrinkage values. As for xonotlite, another advantage is the small difference in the crystal structure compared to wollastonite, which results in less shrinkage associated with the phase dominant at high temperatures. Next to reasons related to raw materials, the process conditions play an important role for the crystallization of calcium silicate hydrates. To influence the crystallization, the autoclaving conditions as well as the raw materials are the main parameters to control or to vary, respectively. Due to the fact that changing one of the various boundary conditions can affect the process in an unpredictable way, systematic research regarding the specific parameters is necessary.