The sinterability of two industrial mullite powders, in the presence of MgO
as a sintering aid, was investigated. A glassy phase, which was generated
during preparation, was present in both powders; this glassy phase had a st
rong influence on sintering, depending on its content, composition, and spa
tial distribution. MgO promoted sintering in the presence of a liquid phase
, both in the as-received materials and in samples washed with HF, in which
most of the pre-existing glassy phase was eliminated. Investigations using
transmission electron microscopy, coupled with energy-dispersive spectrosc
opy, as well as dilatometric measurements and X-ray diffraction data, on wa
shed and unwashed materials and on quenched and slow-cooled samples allowed
a better understanding of the influence of MgO and the glassy phase on the
sintering behavior and the formation of new phases. Most of the phases, in
fact, can be explained by using the MgO-Al2O3-SiO2 phase diagram, even in
such complex systems.