Study on The Effect of Different Sols on High Alumina Castable Refractory

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 the different bonding materials, calcium aluminate cement (CAC) is most common, but the introduction of CaO in the refractory castable system by the use of CAC produces low melting phases on reaction with Al2O3 and SiO2 of the refractory systems and finally resulting in poor high temperature 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, four different precursor sol systems have been synthesized, namely alumina, boehmite, mullite, and spinel by wet chemical synthesis. The sol systems were chosen in such a way that they generate high temperature withstanding refractory oxides on firing (alumina, mullite, and spinel). These synthesized sols are used as a sole binder in high alumina castable systems, with two different particle size distributions, containing no cement. The castables were 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 development of the properties. These synthesized sol based castable compositions were further compared with commercially available silica sol and cement-containing compositions using similar raw materials, particle size distributions and processed under exactly similar conditions. The synthesized sol bonded castables showed well-sintered density and strength compared to traditional cement and silica sol bonded castables. The densities (3.0-3.2 g/cc) obtained signifies a well compact microstructure, as also observed in the microstructural analysis. Considerably high hot strength and higher thermal shock and corrosion resistances were observed for the developed sol containing compositions, higher than the commercial binders, mainly due to the absence of any impurity phase. Among the developed sol bonding systems, mullite and spinel sols were found to have improved hot properties and greater resistances against corrosion and thermal shock. All the developed sol containing compositions showed no deformation or any considerable shrinkage even on firing at 16500C, indicating its application at and above this temperature without any difficulty. However, conventional cement and silica containing compositions resulted in poor properties due to the formation of low melting compounds in the system.