Clay products are one of the most
important classes of structural materials. The raw materials used in their
manufacture are clay blended with quartz, sand, chamatte (refractory clay
burned at 1000-1400 o C and crushed), slag, sawdust and
pulverized coal. Structural clay products or building ceramics* are basically
fabricated by moulding, drying and burning a clay mass. Higher the bulk
specific gravity, the stronger is the clay product. This rule does not hold
good for vitrified products since the specific gravity of clay decreases as
specific gravity of clay brick ranges from 1.6 to 2.5.
According to the method of
manufacture and structure, bricks, tiles, pipes, terracotta, earthenwares,
stonewares, porcelain, and majolica are well recognized and employed in
building construction. Clay bricks have pleasing appearance, strength and
durability whereas clay tiles used for light-weight partition walls and floors
possess high strength and resistance to fire. Clay pipes on account of their
durability, strength, lightness and cheapness are successfully used in sewers,
drains and conduits.
materials and products formed by baking natural clays and mineral admixtures at
a high temperature and also by sintering the oxides of various metals and other
high melting-point inorganic substances.
1 Clay And Its Classifications
Clay is the most important raw
material used for making bricks. It is an earthen mineral mass or fragmentary
rock capable of mixing with water and forming a plastic viscous mass which has
a property of retaining its shape when moulded and dried. When such masses are
heated to redness, they acquire hardness and strength. This is a result of
micro-structural changes in clay and as such is a chemical property. Purest
clays consist mainly of kaolinite (2SiO2.Al2O3.2H2O)
with small quantities of minerals such as quartz, mica, felspar, calcite,
magnesite, etc. By their origin, clays are subdivided as residual and
transported clays. Residual clays, known as Kaolin or China clay, are formed
from the decay of underlying rocks and are used for making pottery. The
transported or sedimentary clays result from the action of weathering agencies.
These are more disperse, contain impurities, and free from large particles of
On the basis of resistance to
high temperatures (more than 1580 o C), clays are classified as refractory, high
melting and low melting clays. The refractory clays are highly disperse and
very plastic. These have high content of alumina and low content of impurities,
such as Fe2O3, tending to lower the refractoriness. High
melting clays have high refractoriness (1350-1580 o C)
and contain small amount of impurities such as quartz, felspar, mica, calcium
carbonate and magnesium carbonate. These are used for manufacturing facing
bricks, floor tiles, sewer pipes, etc. Low melting clays have refractoriness
less than 1350 o C and have varying compositions. These are used to manufacture
bricks, blocks, tiles, etc.
Admixtures are added to clay to
improve its properties, if desired. Highly plastic clays which require mixing
water up to 28 per cent, give high drying and burning shrinkage, call for
addition of lean admixtures or non-plastic substances such as quartz sand,
chamottee, ash, etc. Items of lower bulk density and high porosity are obtained
by addition of admixture that burn out. The examples of burning out admixtures
are sawdust, coal fines, pulverized coal. etc. Acid resistance items and facing
tiles are manufactured from clay by addition of water-glass or alkalis.
Burning temperature of clay items
can be reduced by blending clay with fluxes such as felspar, iron bearing ores,
etc. Plasticity of moulding mass may be increased by adding surfactants such as
sulphite-sodium vinasse (0.1-0.3%).
2 Physical Properties Of Clays
Plasticity, tensile strength,
texture, shrinkage, porosity, fusibility and colour after burning are the
physical properties which are the most important in determining the value of
clay. Knowledge of these properties is of more benefit in judging the quality
of the raw material than a chemical analysis.
By plasticity is meant the
property which wetted clay has of being permanently deformed without cracking.
The amount of water required by different clays to produce the most plastic
condition varies from 15 to 35 per cent. Although plasticity is the most
important physical property of clay, yet there are no methods of measuring it
which are entirely satisfactory. The simplest and the most used test is
afforded by feeling of the wetted clay with the fingers. Personal equation
necessarily plays a large part in such determination.
Since clay ware is subjected to
considerable stress in moulding, handling and drying, a high tensile strength
is desirable. The test is made by determining the stregth of specimens which
have been moulded into briquette form and very carefully dried.
The texture of clay is measured
by the fineness of its grains. In rough work the per cent passing a No. 100
sieve is determined. No numerical limit to the grain size or desired relation
between sizes has been established. Very fine grained clays free from sand are
more plastic and shrink more than those containing coarser material.
Knowledge of shrinkage both in
drying and in burning is required in order to produce a product of required
size. Also the amount of shrinkage forms an index of the degree of burning. The
shrinkage in drying is dependent upon pore space within the clay and upon the
amount of mixing water. The addition of sand or ground burnt clay lowers
shrinkage, increases the porosity and facilitates drying. Fire shrinkage is
dependent upon the proportion of volatile elements, upon texture and the way
that clay burns.
By porosity of clay is meant the
ratio if the volume of pore space to the dry volume. Since porosity affects the
proportion of water required to make clay plastic, it will indirectly influence
air shrinkage. Large pores allow the water to evaporate more easily and
consequently permit a higher rate of drying than do small pores. In as much as
the rate at which the clay may be safely dried is of great importance in
manufacturing clay products, the effect of porosity on the rate of drying
should be considered.
The temperature at which clay fuses is determined by the
proportion of fluxes, texture, homogeneity of the material, character of the
flame and its mineral constitution. Owing to non-uniformity in composition,
parts of the clay body melt at different rates so that the softening period
extends over a considerable range both of time and temperature. This period is
divided into incipient vitrification and viscous vitrification.
Experiments roughly indicate that
the higher the proportion of fluxes the lower the melting point. Fine textured
clays fuse more easily than those of coarser texture and the same mineral
composition. The uniformity of the clay mass determines very largely the influence
of various elements; the carbonate of lime in large lumps may cause popping
when present in small percentages, but when finely ground 15 per cent of it may
be allowed in making brick or tile. Lime combined with silicate of alumina
(feldspar) forms a desirable flux. Iron in the ferrous form, found in
carbonates and in magnetite, fuses more easily than when present as ferric
iron. If the kiln atmosphere is insufficiently oxidizing in character during
the early stages of burning, the removal of carbon and sulphur will be
prevented until the mass has shrunk to such an extent as to prevent their
expulsion and the oxidation of iron. When this happens, a product with a
discoloured core or swollen body is likely to result.
determination of the fusibility of a clay is of much importance both in judging
of the cost of burning it and in estimating its refractoriness.