Mullite and spinel sols as binder for high alumina refractory castables

Abstract

Ever-increasing demand and application of unshaped refractories replacing shaped ones have inspired the scientists and manufacturers to investigate these materials in depth for further improving the quality and performance. Among the various unshaped refractories, castables lead in all the areas of research, development, manufacturing and application. Physical, mechanical, chemical and thermo-mechanical characteristics of various castable systems as well as their processing and bonding mechanisms are the focus of such investigations. Among different bonding materials calcium aluminate (CA) cement is most common, but presence of CaO results in low melting phases with Al2O3 and SiO2 refractory systems, resulting in poor hot properties. To avoid the crisis, decrease in cement content and use of alternate bonding system were widely experimented. Among alternate bonding materials, development of sol-gel technique has opened a new horizon in bonding system for refractory technologists. The principle behind sol gel bonding is the formation of a 3-dimensional network (gel) of particles that surrounds the refractory materials and which on subsequent heating develop strength by formation of ceramic bonding through sintering. In the present work, mullite and spinel sol systems have been developed from chemical precursors and are characterized for confirmation. These sols are used as sole binder in high alumina castable system without containing any cement. High alumina castable with two different particle size distribution has been studied by using two distribution coefficients as per Dinger and Funk model. The castables are processed conventionally and evaluated at different temperatures for the various refractory properties. Phase analysis and microstructural development are also studied to investigate the reasoning for the properties development. These synthetic sol based castable compositions are further compared for properties with commercially available silica sol and high alumina cement containing compositions processed under exactly similar conditions.