Synthesis and mechanical properties of mullite from beach sand sillimanite: Effect of TiO2

Reaction sintering of beach sand sillimanite and calcined alumina is an innovative as well as cost effective method of mullite formation. In the present investigation, beach sand sillimanite and calcined alumina in microfine state were used as starting materials to develop high alumina aggregates for refractories applications using titania additive (0-6 wt%). The raw, materials and additive (TiO2) were mixed in definite proportion and sintered in compacted form at 1400-1550 degrees C with 2 h soaking. It was determined that, alumina/silica ratio of the batch controls densification, flexural strength and microstructural development of the aggregates. Aggregates with almost zero porosity were developed at a sintering temperature of 1500 degrees C with the help of titania additive. Gradual addition of TiO2 up to 4 wt% promotes the densification process, however, TiO2 affects the high temperature-flexural strength of the aggregates. Aggregates developed from sillimanite sand at 1500 degrees C indicates that the mullites formed are not ideal 3:2 type, but are 1:1 type. Mullite formed in the silica rich composition is long and elongated in nature. At the densification temperature of 1500 degrees C up to 3.06 wt% TiO2 enters into the mullite structure as solid solution. (C) 1998 Elsevier Science Limited. All rights reserved