The refractory materials used in the wall of the furnaces for glass melting can be prepared from mullite-zirconia composite material.
The composite of mullite/zirconia was synthesized from Iraqi kaolin, ?-alumina, and zirconia using thermal decomposition with
reaction sintering at 1600 °C. Several batches were prepared with various ratios of kaolin, ?-alumina, and zirconia, and the composite
compositions were selected from the Al2O3-SiO2-ZrO2 phase diagram. The mullite-zirconia composite was prepared with different
steps beginning with milling the starting materials, semi-dry uniaxial pressing, and then reactive sintering at various temperatures
(1200, 1400, and 1600 °C). The predicted phases ZrO2 and Al6Si2O13 were identified by X-ray diffraction patterns according to the
phase diagram for all the batches. The lower amount of the zirconia added to mullite reduced porosity and improved the bulk density
of the mullite/zirconia composite. The thermal expansion coefficient slightly increased with the addition of zirconia. It also enhanced
the thermal shock resistance of the composite. Finally, the mechanical properties were improved by increasing the amount of zirconia
particles in a matrix of mullite due to the phase transformation of zirconia from tetragonal to monoclinic phase.

CONCLUSIONS
Pure mullite material was synthesized from Iraqi kaolin and
?-alumina using thermal decomposition and reaction sintering.
Then the mullite was reinforced by zirconia particle for improving
the physical, mechanical, and thermal properties. The bulk
density was enhanced by increasing the content of zirconia; the
glassy silica phase filled the pores and increased the densification
of the composite materials that reduced the composite porosity.
The zirconia particles improved the mechanical properties due
to the phase transformation of the partially stabilized zirconia.
The microhardness decreased with the increase of zirconia
from 3 to 10 wt%; the hardness values were 40.3 and 22.7 GPa,
respectively. The increase of the zirconia content from 3 to 10
wt% caused the increase of the compressive strength from 83.2
to 163.2 MPa. Also, the increase in temperature from 50 to
800 ?C led to an increase in the thermal expansion coefficient
of mullite-zirconia composite up to 0.000074 ?C-1. Finally, the
physical and mechanical properties deteriorated during the
thermal shock when the temperature difference was increased
from 200 to 600 ?C.