MICROSTRUCTURE AND PHASE ANALYSIS
OF ETHYL SILICATE-BONDED ALUMINA-ZIRCON REFRACTORIES
and N. Kushkonmaz
Translated from Novye Ogneupory, No. 4, pp. 82 – 86, April, 2005.
Original article submitted November 18, 2004.
Alumina-zircon refractories are prepared using an aqueous ethyl silicate solution as the gel-polymerized bond.
Test specimens are heat-treated by reaction sintering at different temperature (1450 – 1600°C) and holding
times (1–4h)toyield new refractory phases zirconium dioxide and mullite. The specimens are tested for
strength, porosity, sintering shrinkage, apparent density, and chemical and phase composition using x-ray
diffractometry and electron scanning microscopy. Reactive sintering is shown to be a effective route towards
obtaining ethyl-silicate-bonded alumina-zircon refractories with tailored microscopic and physical properties.
The shape and size of a component are factors to be pri-
marily considered in deciding on a technique for the compo-
nent’s fabrication. Conventional pressure molding is best
suited for the commercial production of simple-shaped com-
ponents. The casting method may be a better choice for fabri-
cation of unique or irregularly shaped parts. Still, the latter
technique, based on the use of ethyl silicate as a bonding
agent (a binder), is not the most efficient economical method
for the fabrication of thick-walled components .
Alkali silicates, especially ethyl silicate (normally a li
quid with a vaporization temperature of 168°C and a freezing
point of – 77°C), have found wide application as binders in
the steel and refractory industries .
For ethyl silicate to be suited for use as a binder, it is
treated with water to convert it to a gel through hydrolysis.
The gel thus prepared can be used as a bonding agent.
Normally, the hydrolysis of ethyl silicate is accelerated when
carried out in an alkaline or acid medium. Water and ethyl
silicate are poorly miscible components; to overcome this
obstacle, a proper solvent (acetone or an alcohol) should be
used. Under alkaline hydrolysis conditions, gelation is usu
ally a sufficiently fast process. Alkaline hydrolysis is in most
cases recommended for ethyl silicate to be used in the pro
duction of refractories. Acid hydrolysis is preferably used in
casting technologies and in the production of dyes [3, 4].
Hydrolysis of ethyl silicate under normal conditions is a
slow process; to accelerate it, basic or acid catalysts are used.
Hydrolysis proceeds as shown by the reaction
O ® Si(OH)
The hydrolyzed ethyl silicate and four water molecules
enter into a reaction to form a SiO
network by reaction
Ethyl silicate while subjected to hydrolysis and gelation
remains in close contact with the refractory material.
Gelation does not interfere with the direct contact between
the finely dispersed particles and the matrix. Despite the fact
that the volatiles were removed during the drying, and the gel
lost moisture, the matrix suffered no change. It is important
to emphasize that, for the assumed quartz equivalent, the
method in question provides a higher strength for the precur
sor raw material. When sintered, the residual silica gel enters
into a reaction with refractory granular material to form a ce
ramic binder .
Refractories based on ethyl silicate binder have found
wide application in the glass industry and metallurgy, espe
cially in steel-making technologies. Net shape casting using
ethyl silicate has become at present a conventional technique
Refractories and Industrial Ceramics Vol. 46, No. 3, 2005
1083-4877/05/4603-0212 © 2005 Springer Science+Business Media, Inc.
YildizTechnical University, Istanbul, Turkey.
U.S. Patent No. 2795022.