Ceramics and Glasses Lecture Dr.Nabaa S.
Radhi
6- Silicate Ceramics-a Kaolinite and Clays
Kaolinite
Kaolinite, Si2Al2O5(OH)4 or Al2O3,2SiO2,2H2O,
is the most common among the argillaceous
minerals used in ceramics. A projection of its
crystalline structure is represented in Figure 4. It
consists of an alternate stacking of [Si2O5]2- and
[Al2(OH)4]2+ layers, which responsible for the
lamellate character that lead to the development of
plates. The degree of crystallinity of the kaolinite
present in clays is highly variable. It depends
largely on the conditions of formation and the
content of impurities introduced into the crystalline
lattice. Figure 1:
Projected representation of the structure of kaolinite
Clays (the kaolinite can be one type or component of clays)
1- Common characteristics
i. Clays are hydrated aluminosilicate minerals whose structure is made up of a stacking of two
types of layers containing, respectively, aluminum in an octahedral environment and silicon in
tetrahedral coordination.
ii. The large surface area of clays (10–100 m2g-1), their plate-like structure, and the
physicochemical nature of their surface enable clays to form, with water, colloidal suspensions
and plastic pastes.
2- Classification
The clays are not all the same towards easy manipulation and behavior during firing.
Ceramists distinguish vitrifying plastic clays, refractory plastic clays, refractory clays, and red
clays.
7- Silicate Ceramics-
b: Feldspars and Silica
The Role of Oxides in Firing of Traditional Ceramics
Acidic Oxides: They are backbone of the ceramic and mainly improve the mechanical properties.
Formula: MO2, e.g. SiO2, GeO2, TiO2, ZrO2, CrO2 … etc
Basic Oxides: They are Fluxes, i.e. reducing firing temperature.
Formula: MO and M2O, e.g. CaO, MgO, Na2O, K2O, Li2O … etc
Amphoteric Oxides: They form bridges between acidic and basic oxides and partially substitute
them.
Formula: M2O3 and M2O5, e.g. Al2O3, B2O3, P2O5 … etc
Ceramics and Glasses Lecture Dr.Nabaa S. Radhi
Feldspars
Four feldspathic minerals are likely to enter the composition of silicate ceramic pastes. They
are:
− orthoclase, a mineral rich in potassium with the composition K2O.Al2O3.6SiO2
− albite, a mineral rich in sodium with the composition Na2O.Al2O3.6SiO2
− anorthite, a mineral rich in calcium with the composition CaO.Al2O3.2SiO2
− petalite, a mineral rich in lithium with the composition Li2O.Al2O3.8SiO2
− soda Na2O, sodash
− potash
− lucite
Orthoclase and albite, which form eutectics with silica at 990 and 1,050°C respectively, are
widely used as flux. Anorthite is rather regarded as a substitute to chalk (CaCO3). There is a
limited use of petalite. It is used mainly to improve brightness and its negative CTE.
Potassic feldspar is particularly appreciated by ceramists because its reaction with silica
leads to the formation of a liquid whose relatively high viscosity decreases slightly when the
temperature increases. This behavior is considered as a guarantee against the excessive
deformation of the pieces during the heat treatment.
Silica
Silica, SiO2, is a polymorphic raw material found in nature in an amorphous (opal, pebbles)
or crystallized form (quartz, cristobalite and tridymite). Sand contains between 95 and 100% of
quartz mass. It is the most frequently used in the ceramic industry to contribute to the mechanical
strength of the parts. In the manufacturing processes of stonewares and porcelains, it is usual to
use relatively fine sand grains (20 to 60 μm).
When a ceramic is fired, the sand can react, particularly with the fluxes. This reaction is not
often complete. The transformation of residual quartz into cristobalite can then start from
1200°C onwards. This transformation is enhanced by the rise in temperature, the use of fine-
grained sand, the presence of certain impurities, and the use of reducing atmosphere.
The form of silica determines the thermal properties of silicate ceramics. For example,
quartz and cristobalite do not have the same influence on the expansion of the product. Quartz
can also cause a deterioration of the mechanical properties of the finished product owing to the
abrupt variation in dimensions (ΔL/L ≅ –0.35%) associated, at 573°C, with the reversible
transformation β quartz → α quartz. As the crystal of cristobalite formed from the flux are
usually small, the transition β cristobalite → α cristobalite, which occurs at about 220°C often
causes less damage to the finished product.
