Study Of Utilization Of Andalusite To Improve High Temperature Strength Of Refractory Castables

At present, aluminum silicate low cement castables are widely used in manyindustries, however, its application performance when suffered high mechanical stress isnot satisfying due to the low strength at high temperature. How to improve the strength ofaluminum silicate castables at high temperature is always an urgent and significativeresearch subject.The effect of addition of andalusite into aluminum silicate low cement castables wereinvestigated by SEM, XRD, HMOR tester and other testing equipment for refractory. Theeffect of the microstructure of andalusite at high temperature, particle size of andalusite onaluminum silicate low cement castables and the optimized combination of andalusite andother aluminum silicate aggregates were also evaluated. Through the systematic research,some conclusions are got as below:1.Particle of andalusite is ready to decompose at the temperature of1200℃¹300℃.Andalusite is decomposed into mullite and SiO₂when the temperature reaches to1300℃or higher. At this temperature, SiO₂exists in the surface of the particles, which is liquidphase with high viscosity. Viscosity of the liquid phase in the surface of particlesdecreases with the increasing of temperature, meanwhile, SiO₂penetrates into the innersurface the particle again through the cracks and porosities which are generated during thedecomposing.2. SiO₂generated during the decomposing of andalusite couldn’t react with the Al₂O₃existing in the matrix to form secondary mullite, however, it acts as lubrication for theparticles that causes the decreasing of strength of the castables at high temperature.3. When andalusite is added into aluminum silicate low cement casables, SiO₂generated during the decomposing of adalusie has larger distribution, which can react withAl₂O₃existing in the matrix to form secondary mullite more easily and increase thestrength of castables by reducing the glassy phase and jointing the aggregates togethereffectively.4. In the premise of invariant proportion of andalusite fine particles, andalusitepowder, homo bauxite and alumina powder, the sequence of high temperature strength ofthe castables with good quality bauxite, brown fused alumina, fused mullite and tabularalumina as aggregates is: good quality bauxite＞fused mullite＞brown fused alumina≥homo bauxite≥tabular alumina. The mechanism is that Al₂O₃existing in the surface ofgood quality bauxite is more active to react with SiO₂to form dentate secondary mullite.5. Andalusite doesn’t make a contribution to the castalbes when the temperature isbelow1300℃,because the mullitization doesn’t start. At the temperature of1300℃to1550℃, the strength of the castables increases with the formation of secondary mullite byreaction of Al₂O₃in the matrix with SiO₂generated by decomposing of andalusite. Whenthe temperature exceeds1550℃, the liquid phase formed by impurities in andalusitedecreases the strength that surpasses the enhancement of secondary mullite. So aluminum silicate castables with andalusite is suitable to be used at the temperature of1300℃to1550℃.