CLAY PRODUCTS

Clay products are one of the most important classes of structural materials. Clay bricks were
used by humans from very early dates. First it was used without burning as sundried bricks.
Nowadays, they are made from specially selected and matured brick-earth consisting chiefly
of silica (35 to 70 per cent) and alumina (10 to 20 per cent). Too much silica tends to make
the brick brittle and too much alumina makes the brick warp and crack on drying and
burning. It is also desirable to have other agents like lime, magnesia, oxide of iron which act
as colouring agent and flux to assist fusion during burning of the brick earth.
Clay when heated to lower temperatures, loses its moisture, and only physical change occurs.
Such half-burnt clay crumbles when placed in water. However, when clay is heated to high
temperatures, its constituents fuse, and chemical change takes place.
TYPES OF CLAY PRODUCTS:
1. BRICKS
-A brick is rectangular in shape and of size unit of construction that can be conveniently
handled with one hand.
-Brick may be made of burnt clay or mixture of sand and lime or of Portland cement
concrete.
-Clay bricks are commonly used since these are economical and easily available. The
length, width and height of a brick are interrelated as below:
Length of brick = 2 × width of brick + thickness of mortar
Height of brick = width of brick

A brick earth quarry should contain clay which is suitable for brickmaking or which can be
made suitable by mixing with other soils. Laboratory tests like liquid limit, plastic limit and
shrinkage limit may also be used to determine the suitability of the soil for brickmaking.
They are classified in terms of:
▪ Varieties
 Common: Suitable for general building work but having no special claim to give an
attractive appearance.
 Facing: Specially made or selected to have an attractive appearance when used
without rendering or plaster.
 Engineering: Having a dense and strong semi-vitreous body conforming to defined
limits for absorption and strength.
▪ Qualities
 Internal: Suitable for internal use only; may need protection on site during bad
weather or during the winter.
 Ordinary: Less durable than special quality but normally durable in the external face
of a building. Some types are unsuitable for exposed situations.
 Special: For use in conditions of extreme exposure where the structure may become
saturated and frozen, such as retaining walls and paving.
▪ Types
 Solid: Those in which small holes passing through or nearly through the brick do not
exceed 25% of its volume, or in which frogs do not exceed 20% of its volume. A
small hole is defined as a whole less than 20 mm wide or less than 500 mm 2 in area.
 Perforated: Those in which holes passing through the brick exceed 25% of its
volume and the holes are small as defined above.
 Hollow: Those in which the holes passing through the brick exceed 25% of its
volume and the holes are larger than those defined as small holes.
 Cellular: Those in which the holes are closed at one end and exceed 20% of the
volume of the brick.
▪ Other classifications
Bricks may also be classified by one or more of the following:
 place of origin, for example London;
 raw material, for example clay;
 manufacture, for example wire-cut;
 use, for example foundation;
 colour, for example blue;
 surface texture, for example sand-faced
 Types of bricks in terms of their shapes.

2. Blocks: these are rectangular construction units composed of a similar material as

bricks but they are larger than bricks. Their measurements can rise to as high as 600
mm in terms of length.
3. Roofing tiles: these are clay products designed to keep out rain
 4. Flooring tiles: these are clay products made mainly to produce an appealing floor
surface and also provide a corrosion resistant floor.
5. Wall tiles: they are clay products made to produce an appealing surface on walls.
6. Pipes and fittings: they are made act mainly as conduits of foul water transfer to the
outside and far away from the buildings.

MANUFACTURE OF CLAY PRODUCTS:

RAW MATERIALS
For the preparation of bricks, clay or other, suitable earth is moulded to the desired
shape after subjecting it to several processes. After drying, it should not shrink and no
crack should develop.
The clay used for brick making consists mainly of silica and alumina mixed in such a
proportion that the clay becomes plastic when water is added to it. It also consists of
small proportions of lime, iron, manganese, Sulphur, etc. The proportions of various
ingredients are as follows:

Alumina: - It is the chief constituent of every kind of clay. A good brick earth should contain
20 to 30 percent of alumina. This constituent imparts plasticity to earth so that it can be
moulded. If alumina is present in excess, raw bricks shrink and warp during drying and
burning.

Silica-A good brick earth should contain about 50 to 60 percent of silica. Silica exists in clay
either as free or combined form.
As free sand, it is mechanically mixed with clay and in combined form; it exists in chemical
composition with alumina. Presence of silica prevents crackers shrinking and warping of raw
bricks. It thus imparts uniform shape to the bricks.
Durability of bricks depends on the proper proportion of silica in brick earth. Excess of silica
destroys the cohesion between particles and bricks become brittle.

Lime – A small quantity of lime is desirable in finely powdered state to prevents shrinkage of
raw bricks. Excess of lime causes the brick to melt and hence, its shape is lost due to the
splitting of bricks.
It’s added in small quantities for the following reasons:
1. Reduces the shrinkage on drying.
2. Causes silica in clay to melt on burning and thus helps to bind it.
3. In carbonated form, lime lowers the fusion point.
4. Excess of lime causes the brick to melt and the brick loses its shape.
5. Red bricks are obtained on burning at considerably high temperature (more than
800°C) and buff-burning bricks are made by increasing the lime content.
Magnesia- A small quantity of magnesia in brick earth imparts yellow tint to bricks, and
decreases shrinkage. But excess of magnesia decreases shrink leads to the decay of bricks.

Oxide of iron- A small quantity of oxide of Iron to the extent of 5 to 6 percent is desirable in
good brick to imparts red colour to bricks. Excess of oxide of iron makes the bricks dark blue
or blackish. It also Improves impermeability and durability. Tends to lower the fusion point
of the clay, especially if present as ferrous oxide. Gives strength and hardness.

The manufacturing of brick, the following operations are involved

1. Preparation of clay
2. Moulding
3. Drying
4. Burning

(i) Preparation of clay: - The preparation of clay involves following operations:

a) Un-soiling: - Top layer of 20cm depth is removed as it contains impurities.
b) Digging: - Clay dug out from ground is spread on level ground about 60cm to 120cm
heaps.
c) Cleaning: -Stones, pebbles, vegetable matter etc. removed and converted into powder
form.
d) Weathering: - Clay is exposed to atmosphere from few weeks to full season. This is done
to develop homogeneity in the mass of soil, particularly if they are from different sources,
and also to eliminate the impurities which get oxidized
Soluble salts in the clay would also be eroded by rain to some extent, which otherwise could
have caused scrumming at the time of burning of the bricks in the kiln. The soil should be
turned over at least twice and it should be ensured that the entire soil is wet throughout the
period of weathering. In order to keep it wet, water may be sprayed as often as necessary. The
plasticity and strength of the clay are improved by exposing the clay to weather.
e) Blending: - Clay is made loose and any ingredient to be added to it is spread out at top and
turning it up and down in vertical direction.
f) Tempering:- Clay is brought to a proper degree of hardness, then water is added to clay
and whole mass is kneaded or pressed under the feet of men or cattle for large scale,
tempering is usually done in pug mill as shown in the fig 2.1

Fig 2.1 Pug Mill

Process: - Clay with water is placed in pug mill from the top. When the vertical staff is
rotated by using electric pair, steam or diesel or turned by pair of bullocks. Clay is thoroughly
mixed up by the actions of horizontal arms and knives when clay has been sufficiently
pugged, the hole at the bottom of tub, is opened cut and the pugged earth is taken out from
ramp for the next operation of moulding.

(ii) Moulding: It is a process of giving a required shape to the brick from the prepared brick
earth. Moulding may be carried out by hand or by machines. The process of moulding of
bricks may be the soft-mud (hand moulding), the stiff-mud (machine moulding) or the dry
press process (moulding using maximum 10 per cent water and forming bricks at higher
pressures). Fire-brick is made by the soft mud process. Roofing, floor and wall tiles are made
by dry-press method. However, the stiff-mud process is used for making all the structural
clay products.
Clay, which is prepared form pug mill, is sent for the next operation of moulding. Following
are the two ways of moulding.
Hand Moulding: Moulds are rectangular boxes of wood or steel, which are open at top and
bottom. Steel moulds are more durable and used for manufacturing bricks on large scale as
shown in fig 2.2. Bricks prepared by hand moulding are of two types.
a) Ground moulded bricks
b) Table moulded bricks

Fig 2.2 Wooden mould & Steel mould

(a) Ground moulded bricks: ground is first made level and fine sand is sprinkled over
it. Mould is dipped in water and placed over the ground to fill the clay. Extra clay is
removed by wooden or metal strike after the mould is filled forced mould is then
lifted up and raw brick is left on the ground. Mould is then dipped in water every
time; lower faces of ground moulded bricks are rough and it is not possible to place
frog on such bricks.
Ground moulded bricks of better quality and with frogs on their surface are made by
using a pair of pallet boards and a wooden block.

(b) Table-moulded bricks: Process of moulding these bricks is just similar to ground
bricks on a table of size about 2m x 1m.
The bricks are moulded on stock boards nailed on the moulding table. Stock boards
have the projection for forming the frog. The process of filling clay in the mould is
the same as explained above. After this, a thin board called pallet is placed over the
mould. The mould containing the brick is then smartly lifted off the stock board and
 inverted so that the moulded clay along with the mould rests on the pallet. The mould
is then removed as explained before and the brick is carried to the drying site.

(1) Machine moulding: This method proves to be economical when bricks in huge quantity
are to be manufactured at the same spot. It is also helpful for moulding hard and string clay.
These machines are broadly classified in two categories:
(a) Plastic clay machines
(b) Dry clay machines

a) Plastic clay machines: This machine containing rectangular opening of size equal to
length and width of a brick. Pugged clay is placed in the machine and as it comes out through
the opening, it is cut into strips by wires fixed in frames, so these bricks are called wire cut
bricks.

b) Dry clay machines: In these machines, strong clay is first converted into powder form and
then water is added to form a stiff plastic paste. Such paste is placed in mould and pressed by
machine to form hard and well-shaped bricks. These bricks are behaviour than ordinary hand
moulded bricks. They carry distinct frogs and exhibit uniform texture.

(iii) Drying: Green bricks contain about 7–30% moisture depending upon the method of
manufacture. The object of drying is to remove the moisture to control the shrinkage and save
fuel and time during burning. The damp bricks, if burnt, are likely to be cracked and
distorted. The drying shrinkage is dependent upon pore spaces within the clay and the mixing
water. The addition of sand or ground burnt clay reduces shrinkage, increases porosity and
facilities drying. The moisture content is brought down to about 3 per cent under exposed
conditions within three to four days. Thus, the strength of the green bricks is increased and
the bricks can be handled safely.
Hence moulded bricks are dried before they are taken for the next operation of burning.
Bricks are laid along and across the stock in alternate layers. The drying of brick is by the
following means:

(i) Artificial drying – drying by tunnels usually 1200C about 1 to 3 days

(ii) Circulation of air- Stacks are arranged in such a way that sufficient air space is left
between them free circulation of air.
(iii)Drying yard- special yards should be prepared slightly higher level prevent the
accumulation of rain water.
(iv)Period for frying – usually about 3 to 10 days to bricks to become dry.
(v) Screens – screens are necessary, may be provided to avoid direct exposure to wind or sun.

Natural drying

(iv) Burning: This is very important operation in the manufacturing of bricks to impart
hardness, strength and makes them dense and durable. Burning of bricks is done either in
clamps or in kilns.
Clamps are temporary structures and they are adopted to manufacture bricks on small scale.
Kilns are permanent structures and they are adopted to manufacture bricks on a large scale. A
typical clamp is as shown in fig 2.3

Fig 2.3 Clamp

(1) A trapezoidal shape in plan with shorter end is slightly inclined in excavation and wider
end raised at an angle of 150 from ground level.
(2) A brick wall with mud is constructed on the short end and a layer of 70cm to 80cm thick
fuel (grass, cow dung, ground nuts, wood or coal) laid on the floor.
(3) A layer consists of 4 or 5 courses of raw bricks laid on edges with small spaces between
them for circulation of air.
(4) A second layer of fuel is then placed, and over it another layer of raw bricks is put up. The
total height of clamp in alternate layers of brick is about 3 to 4 m.
(5) When clamp is completely constructed, it is plastered with mud on sides and top and
filled with earth to prevent the escape of heat.
(6) The period of burning is about one to two months and allow the same time for cooling.
(7) Burnt bricks are taken out from the clamp.
Advantages:
(i) The bricks produced are tough and strong because burning and cooling are gradual.
(ii) Burning in clamps proves to be cheap and economical.
(iii) No skilled labour and supervision are required for the construction of clamps.
(iv) There is considerable saving of clamps fuel.
Disadvantages:
(i) Bricks are not of required shape
(ii) It is very slow process
(iii) It is not possible to regulate fire in a clamp
(iv) Quality of brick is not uniform

Kilns: A kiln is a large oven, which is used to burnt bricks by

1) Intermittent kilns
2) Continuous kilns
These may be rectangular, circular or oval in shape. When the process of burning bricks is
continuous, the kiln is known as continuous kiln. On the other hand, if the process of burning
bricks is discontinuous, the kiln is known as intermittent kiln.
1) Intermittent kilns: These intermittent in operation, which means that they are loaded,
fired, cooled and unloaded a) Intermittent up-draught kilns
b) Intermittent down-draught kilns
a) Intermittent up-draught kiln: This is in the form of rectangular with thick outside walls,
wide doors are provided at each end for loading and unloading of kilns. A temporary roof
may be installed to protect from rain and it is removed after kiln is fired. Flues are provided
to carry flames or hot gases through the body of kiln.
(i) Raw bricks are laid in row of thickness equal to 2 to 3 bricks and height 6 to 8 bricks with
2 bricks spacing between rows
(ii) Fuels are filled with brush wood which takes up a free easily
(iii) Loading of kiln with raw bricks with top course is finished with flat bricks and other
courses are formed by placing bricks on edges
(iv) Each door is built up with dry bricks and are covered with mud or clay
(v) The kiln is then fired for a period of 48 to 60 hours, draught rises in the upward direction
from bottom of kiln and brings about the burning of bricks.
(vi) Kiln is allowed to cool down and bricks are then token out
(vi) Same procedure is repeated for the next burning Bricks manufactured by intermittent up
drought kilns are better than those prepared by clamps but bricks burnt by this process is not
uniform, supply of bricks is not continuous and there’s wastage of fuel heat.

(b) Intermittent down-draught kilns:

These kilns are rectangular or circular in shape. They are provided with permanent walls and
closed tight roof. Floor of the kiln has opening which are connected to a common chimney
stack through flues. Working is same as up-draught kiln. But it is so arranged in this kiln that
hot gases are carried through vertical flues up to the level of roof and they are then released.
These hot gases move down ward by the chimney draught and in doing so, they burn the
bricks.

Advantages:
(i) Bricks are evenly burnt.
(ii) Performance of this kiln is better than that of up-draught kiln.
(iii) This kiln is suitable for burning of structural clay tiles, terra-cota because of close control
of heat.

2. Continuous kilns:
These kilns are continuous in operations. This means that loading, firing, cooling and
unloading are carried out simultaneously in these kilns.

Classification:
Bricks can broadly be divided into two categories.
(i) Unburnt or sundried bricks
(ii) Burnt bricks

1.)Unburnt or Sun dried bricks- UN burn or sun dried with the help of heat received from
sun after the process of moulding. These bricks can only be used in the constructions of
temporary and cheap structures. Such bricks should not be used at places exposed to heavy
rains.
2.)Burnt Bricks: The bricks used in construction works are burnt bricks and they are
classified into the following four categories.

a. First Class bricks: These bricks are table moulded and of standard shape. The surface and
edges of the bricks are sharp, square, smooth and straight. The comply all the qualities of
good bricks and used for superior work of permanent nature.
b. Second class bricks: These bricks are ground moulded and they are burnt in kilns. The
surface of bricks is somewhat rough and shape is also slightly irregular. These bricks are
commonly used at places where brick work is to be provided with a coat of plaster.
c. Third class bricks: These bricks are ground moulded and they burnt in clamps. These
bricks are not hard and they have rough surfaces with irregular and distorted edges. These
bricks give dull sound when struck together. They are used for unimportant and temporary
structures and at places where rainfall is not heavy.
d. Fourth class bricks: These are over burnt bricks with irregular shape and dark colour.
These bricks are used as aggregate for concrete in foundation, floors, roads, etc. because of
the fact that the over burnt bricks have compacted structure and hence, they are some times
found stronger than even first class bricks.

Qualities of Good Brick:

(i) Bricks should be table moulded, well burnt in kilns, copper coloured, free from cracks and
with sharp and square edges.
(ii) Bricks should be uniform shape and should be of standard size.
(iii) Bricks should give clear ringing sound when struck each other.
(iv) Bricks when broken should show a bright homogeneous and compact structure free from
voids.
(v) Bricks should not absorb water more than 20 percent by weight for first class bricks and
22 percent by weight for second class bricks, when soaked in cold water for a period of 24
hours.
(vi) Bricks should be sufficiently hard no impression, should be left on brick surface, when it
is scratched with finger nail.
(vii) Bricks should be low thermal conductivity and they should be sound proof.
(viii) Bricks should not break when dropped flat on hard ground from a height of about one
meter.
(ix) Bricks, when soaked in water for 24hours, should not show deposits of white salts when
allowed to dry in shade.
(x) No brick should have crushing strength below 55kg/cm2

USES OF CLAY PRODUCT

1. Wall materials
The examples are common clay brick, perforated clay brick, porous and perforated stiff-mud
brick, hollow clay dry-press brick. Perforated plastic moulded ceramic stones and light
weight building brick. Clay brick accounts for half of the total output of wall materials.
Structural properties of hollow clay products and low heat losses through air-filled voids
(particularly at subzero temperatures) provide great possibilities for reducing the thickness
and the weight of exterior walls. Ceramic facing tiles remain the chief finishing material for
sanitary and many other purposes and are still in great use for external facing of buildings.
2. Brick for special purposes.
The example are curved clay brick, stones for sewage installations (underground sewer pipes)
brick for road surface (clinker).
3. Hollow clay products for floors.
The examples are stones for close-ribbed floors (prefabricated or monolithic), stones for
reinforced ceramic beams, sub flooring stones (fillers between beams).
4. Facade decoration.
The examples are glazed or non-glazed varieties subdivided in to facing brick and ceramic stones,
floor ceramics, small-size ceramic tiles, ceramic plates for facades and window-sill drip stones.
5.Clay products for interior decoration.
The examples are tiles for facing walls, built-in parts, large floor tiles and mosaic floor tiles.
6. Roof materials.
The examples are common clay roof tiles for covering slopes of roofs, ridge tiles for covering
ridges and ribs, valley tiles for covering valleys, end tiles ("halves" and "jambs") for closing
row of tiles, special tiles.
7. Acid-resistant lining items.
The examples are common acid-resistant brick, acid-resistant and heat-and-acid-resistant
ceramic shaped tiles for special purposes, ceramic acidresistant pipes and companion shapes.
8. Sanitary clay items.
Sanitary ware items are manufactured mainly form white-burning refractory clay, kaolins,
quartz and feldspar. There are three groups of sanitary ceramics: faience, semi-porcelain and
porcelain, which differ in degree of caking and, as a consequence, in porosity. Items from
faience have a porous shell, and items from porcelain, a solid shell, while those from semi-
porcelain are of intermediate densities. The various degrees of caking of faience, porcelain and
semi-porcelain, made of the same raw materials, are due to the latter's different proportions in
the working mass.
Solid faience is used mainly to manufacture toilet bowls, wash basins, toilet tanks and bath
tubs. Items are glazed, since unglazed faience is water permeable. Semi-porcelain items feature
excellent hygienic and mechanical properties being intermediate between those of faience and
porcelain. Porcelain outer shell is impervious to water and gases and possesses high mechanical
strength and resistance to heat and chemical agent. Porcelain is used to manufacture insulators
for power transmission lines, chemical laboratory vessels, etc.
9. Aggregate for concrete.
Creamiste (manufactured from low-heat clay), a light weight porous material forms excellent
aggregate for light weight concrete.

GLAZING
This is a process of forming some film over the surfaces of clay products to protect the
surface from chemical attack and other weathering agencies. A glaze is a glassy coat
(vitreous) applied on the surface of a clay item. The different types of glazing in use are as to
follow.
Transparent glazing: There are many methods for imparting transparent glazing, but salt
glazing is most commonly used, since this makes the items impermeable. It consists of
throwing sodium chloride in the kiln when burning is at peak (1200°–1300°C). The heat of
the kiln volatizes the salt, which enters into the pores of the burning item and combines with
the silica in clay to make soda silicate.
The soda silicate so formed combines with alumina, lime and iron in the clay to form a
permanent thin, transparent surface coating.
Lead glazing
Clay items are burned thoroughly and then dipped in a solution of lead oxide and tin oxide.
The particles of lead and tin adhere to the surface of clay items. After this, the articles are
returned in potter’s kiln where these adhered particles melt and form a thin transparent layer
on the outer surface. This method of glazing is used for items of inferior clay which cannot
withstand high temperature required for salt glazing.

Opaque glazing: Borax, kaolin, chalk and colouring matter is fired with total or a part of
feldspar, flint, and lead oxide. The resulting molten glass is poured into water to give
shattered frit. The frit is then ground with remaining materials and water and is made of the
consistency of cream known as slip. Fully burnt earthenware’s known as biscuits are dipped
in the slip. The biscuits absorb water and form thin layer of glaze on the surfaces. After
drying the products, these are once again fired to a lower temperature so as to fuse the glaze.

Porcelain enamelling: this is a glaze acquired by fusing powdered glass to a clay item. The
powder melts, flows and then hardens to a smooth durable vitreous coating on a clay article.
PROPERTIES OF CLAY PRODUCTS:
The essential requirements for building bricks are sufficient strength in crushing, regularity in
size, a proper suction rate, and a pleasing appearance when exposed to view.
➢ Size and shape; The bricks should have uniform size and plane, rectangular surfaces
with parallel sides and sharp straight edges
➢ Colour: The brick should have a uniform deep red or cherry colour as indicative of
uniformity in chemical composition and thoroughness in the burning of the brick.
➢ Texture and compactness: The surfaces should not be too smooth to cause slipping of
mortar. The brick should have pre-compact and uniform texture. A fractured surface
should not show fissures, holes grits or lumps of lime.
➢ Hardness and soundness: The brick should be so hard that when scratched by a finger
nail no impression is made. When two bricks are struck together, a metallic sound
should be produced.
➢ Water absorption: should not exceed 20 per cent of its dry weight when kept
immersed in water for 24 hours.
➢ Crushing strength: should not be less than 10 N/mm2

Tests for bricks :

A brick is generally subjected to following tests to find out its suitability of the construction
work.
ii. Absorption
iii. Crushing strength or compression strength
iv. Hardness
v. Presence soluble salts
vi. Shape and size
vii. Soundness
viii. Structure
1) Absorption: A good should not absorb not more than 20 percent of weight of dry brick.

(The specimen is first weighed when dry. It’s then immersed in water for a period of 16-24
hrs. Its then removed from the water and weighed again. The difference in weight indicates
the amount of water absorbed by the brick. It should not in any case, exceed 20 % of weight
of the dry brick.)
2) Compressive strength: crushing or compressive strength of brick is found out by placing
it in compression testing machine. It is pressed till it breaks. Minimum crushing strength of
brick is 35kg/cm2 and for superior bricks, it may vary from 70 to 140 kg/cm2.
3) Hardness: No impression is left on the surface the brick is treated to be sufficiently hard
4) Presence of soluble salts: The bricks should not show any grey or white deposits after
immersed in water for 24 hours
(it’s also known as an efflorescence test. The ends of the brick are kept in a 150 mm diameter
porcelain or glass dish containing 25 mm depth of water at room temperature (20°–30°C) till
the entire water is absorbed or evaporated. The water is again filled to 25 mm depth in the
dish and allowed to be absorbed by the brick or evaporated. Presence of efflorescence is
classified as below;

5) Shape and size: It should be standard size and shape with sharp edges
6) Soundness: The brick should give clear ringing sound struck each other
7) Structure: The structure should be homogeneous, compact and free from any defects