Formation and influence of magnesium-alumina spinel on the properties of refractory forsterite-spinel ceramics

Forsterite refractory ceramic is mostly used in the cement industry as the lining of a rotary kiln and as the lining of metallurgical furnaces due to its high refractoriness of up to 1850 °C.Another significant property of forsterite is its coefficient of linear thermal expansion used in the electrotechnical industry for ceramic-metal joints.An addition of aluminium oxide to a raw-material mixture results in the creation of a magnesium-alumina spinel (MgO•Al2O3), which improves the sintering and mechanical properties of forsterite ceramics.An inexpensive source of aluminium oxide is fly ash.The utilization of fly ash, a secondary energy product of coal-burning power plants, is important for the environment and sustainable development.This paper evaluates the transformation of mullite (3Al2O3•2SiO2) from fly ash into a magnesium-alumina spinel, its influence during the synthesis and the resulting properties of a fired forsterite refractory ceramic body.Forsterite-spinel ceramics were synthesized from olivine, calcined magnesite and 0-20 % of fly-ash powders.XRD analyses were used to determine the mineralogical composition, thermal analyses were used to determine the formation of spinel and its behaviour during the firing, and scanning electron microscopy (SEM) was used to determine the morphology of crystal phases.The refractoriness of pyrometric cones, refractoriness under load, thermal-shock resistance, water absorption, porosity and mechanical properties of the fired test samples were also determined.The transformation of mullite resulted in small amounts of magnesium-alumina spinel in the forsterite ceramics.Test results showed that the presence of magnesium-alumina spinel improved the sintering, microstructure, thermal-shock resistance and mechanical properties in comparison with pure-forsterite refractory ceramics.