Authors:
Dr. Ralf Simmat | Forschungsgemeinschaft Feuerfest e. V. at the European Centre for Refractories | Germany
Dr. Christian Dannert | Forschungsgemeinschaft Feuerfest e. V. at the European Centre for Refractories | Germany
Dr. Kerstin Hauke | Forschungsgemeinschaft Feuerfest e. V. at the European Centre for Refractories | Germany
Juliane Paul | Hochschule Koblenz | Germany
Kariman Abdelgawad | Hochschule Koblenz | Germany
Prof. Dr. rer. nat. Olaf Krause | Hochschule Koblenz | Germany
Natural andalusite is an important refractory raw material that is widespread in use for manufacturing industrial refractory unshaped products and fired brick materials. However, andalusite cannot be synthesized for refractory purposes and natural ressources are limited. FGF, Hochschule Koblenz and InisMa together investigate in depth the thermal phase evolution and, in parallel, the thermomechanic properties to understand the outstanding properties of andalusite bearing refractory castables. It is well known that andalusite decomposes at temperatures above ~1300 °C, resulting in dense, felted aggregates of mullite and a highly viscous silica phase. Theses aggregates still provide high refractoriness and mechanical strength. The volume increase of about 4.5 % during this transformation works against high temperature shrinkage.
The phase changes occuring with increasing temperatures, involving the andalusite transformation that can be influenced by other components of the castables, as well as reactions of transformed andalusite with the surrounding matix, influence the thermomechanical properties of the castables. With the aim to develop alternatives to andalusite bearing castables, these processes must be entirely understood.
The method of monotonic heating reveals important mineralogical phase changes in two commercial castables (ULCC and silica-gel bonded) in the temperatures up to 1620 °C, including transformation of andalusite and melt formation.The nature of observed phase transformations is verified by XRD and SEM using samples of the castables that were treated between 900 and 1600 °C at a step size of 100 K. RUL and CREEP was determined for correlating the thermal softening of the castables with the described phase evolution.