Evaluation of Phase Diagrams with Regard to Alkali Resistance of Calcium Aluminates at High Temperatures
1 TU Bergakademie Freiberg, Institut für Keramik, Glas- und Baustofftechnik, 09596 Freiberg/Germany
2 Calsitherm Silikatbaustoffe GmbH, 33175 Bad Lippspringe/Germany
Revision 02.11.2012, 07.11.2012
Volume 5, Issue 1, Pages 113 - 117
Abstract
The paper is a critical view on calcium aluminates concerning their thermal stability and possibility of reaction with alkalis in regard to the evaluation of phase diagrams in subsolidus field but at temperatures of 1100 °C to 1200 °C. In this solid state the volume changes followed by density changes from original to the formatting compounds due to the alkali corrosion. The main topic is the calculation of type and amount of the formatting alkali compounds and the volume change of the solid phase mixture in each of the five calcium aluminates herein. Due to the CA6 reaction with alkalis to β-alumina the linear expansion is about 14 % and the solid phase is destroyed as well. The other four calcium aluminates have linear expansion of maximal 2,6 % or shrinkage of maximal 3,7 % respectively. Therefore one can expect an alkali stability of these four calcium aluminates somewhat. In the CaO-Al2O3 system the lowest eutectic melts at 1435 °C between C3A and C12A7. So the densification by sintering can start nearly 200 to 300 K below the temperature of the first melt. Therefore porous calcium aluminate thermal insulating materials have a possible service temperature of 1100 to 1200 °C.
Keywords
calcium aluminates, alkali, corrosion,calculation of volume changes, high temperature thermal insulation
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