INFLUENCE OF PROCESS CONDITIONS ON THE CRYSTALLIZATION OF CALCIUM SILICATES IN THE STIRRING AUTOCLAVE AND THEIR IMPACT ON THERMAL STABILITY

The thermal stability of calcium silicates is on the one hand limited by the phase transformation of its main phases xonotlite and tobermorite and other, more crystal water containing calcium silicate hydrates (C-S-H) into wollastonite at higher temperatures. 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. Next to reasons related to raw materials, the process conditions play an important role for the crystallization of the calcium silicates. In the present work, several trials were carried out in a laboratory scale stirring autoclave to show the effects of changing process parameters like ramp and dwell time, temperature / pressure, water-solid ratio and stirring speed on the formation of calcium silicates. The synthesised powder was analysed with use of quantitative and qualitative XRD, specific surface measurements and SEM. Furthermore, in a second step, the filter pressed products were investigated to find out the influence on the thermal stability behaviour.