CHAPTER - Five - CLAY AND CLAY PRODUCTS
CHAPTER - Five - CLAY AND CLAY PRODUCTS
CHAPTER - Five - CLAY AND CLAY PRODUCTS
Clays: - are finely grained soils resulted from decay of rocks. (They could be residual clays
formed from the decay of the underlying rocks, or sedimentary if removed from the parent rock,
transported and deposited somewhere else by water or wind.
Chemical constituents of clay
- The formation of hydrate of alumina silicate (A1 2 O3 . 2Sio2 .2H2 O) is, in fact, one of
the commonest minerals in clay.
- Silica (Si O 2 )
- Ferric Oxide (Fe2 O 3 )
- Lime (Cao)
- Magnesia (Mgo)
- Carbon dioxide (Co2 )
- Alkalies, water, etc.
On account of the different phases they might have gone through and their history of formation,
clays are generally found mixed with other materials (impurities) which influence their
properties.
BRICKS
Are building materials manufactured from clays (raw material)
Are most extensively used materials of construction because of their strength, durability,
insulating property against heat and sound, & their being handy to work with and most
availability near building sites at relatively cheap price.
Raw material
A clay soil for brick making should be such that when prepared with water, it can be molded,
dried and burnt without cracking or changing its shape or wrapping. Such material should
preferably have the following composition:
Clay 20 - 40%
Sand 30 - 50%
Others (lime, silt, cement, etc.) - 20 - 35%
Each of the components and their constituents play different rolls in the manufacture of bricks
and influence the characteristics of the final product.
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- Thus form hard material when burnt & mixed with other constituents.
b) Silica: - course grained mineral, which can be present either in the form of pure sand or
compound of silicate of alumina.
- is useful in reducing shrinkage & wrapping in burning.
- its presence in bricks produce hardness and durability; however, a large
percentage of uncombined silica is undesirable because it leads to brittleness of
the product.
- Silica fuses only at very high temperature (which is lowered in the presence
of alumina and iron oxide) and hence increases the refractoriness of low
alumina clay and makes bricks resistant to heat. In fire bricks silica content
may rise to 98%.
c) Lime: - When present in small quantities, lime acts as a flux and lower the fusion point of
silica.
It also acts as a binder to the clay & silica particles leading to greater
strength.
Excess of lime may cause the bricks to melt & lose their shape.
d) Iron Oxide:- Lowers fusion point of silica & the clay and hence helps the
- Imparts the colour of the clay and the burnt product. (Light yellow to red depending on
its percentage) .Higher % make bricks dark blue.
- The appropriate iron type should be used -eg. If iron present in the form of
pyrites (sulphides of iron), it can get oxidized, crystallize and split the bricks
to pieces.
e) Magnesia: - Usually present in small quantity (1%) in clay and together with the iron
oxide, it gives the brick darker or even black colour.
- It also lessens wrapping of bricks in burning.
MANUFACTURE OF BRICKS
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these T0 hard - burnt bricks are produced which contain melted components acting
as binder to the non-melted minerals.
Hard burnt bricks: - are stronger and harder.
-more durable
- Less absorptive on wetting with water.
- Cooling: two to three days for kiln burnt bricks
- Then we have the brick.
CLASSIFICATION OF BRICKS
a) First Class
1. These are thoroughly burnt and are of deep red, cherry or copper colour.
2. The surface should be smooth and rectangular, with parallel, sharp and straight edges and square
corners.
3. These should be free from flaws, cracks and stones.
4. These should have uniform texture.
5. No impression should be left on the brick when a scratch is made by a finger nail.
6. The fractured surface of the brick should not show lumps of lime.
7. A metallic or ringing sound should come when two bricks are struck against each other.
8. Water absorption should be 12–15% of its dry weight when immersed in cold water for 24 hours.
b) Second Class: are supposed to have the same requirements as the first class ones except that
1. Small cracks and distortions are permitted.
2. A little higher water absorption of about 16–20% of its dry weight is allowed.
3. The crushing strength should not be less than 7.0 N/mm 2 .
Uses: Second class bricks are recommended for all important or unimportant hidden masonry works and
centering of reinforced brick and reinforced cement concrete (RCC) structures.
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c). Third Class: are under burnt. They are soft and light-coloured producing a dull sound when struck
against each other. Water absorption is about 25 per cent of dry weight.
Uses: It is used for building temporary structures.
d). Fourth Class: are over burnt and badly distorted in shape and size and are brittle in nature.
Uses: The ballast of such bricks is used for foundation and floors in lime concrete and road metal.
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pressed into moulds; moulded bricks—when bricks are moulded by machines imitating
hand mixing.
3. ENGINEERING BRICKS: are strong, impermeable, smooth, table moulded, hard and
conform to defined limits of absorption and strength. These are used for all load bearing
structures.
Classification of Bricks on the Basis Of Type
1) SOLID: Small holes not exceeding 25 per cent of the volume of the brick are permitted;
alternatively, frogs not exceeding 20 per cent of the total volume are permitted.
2) PERFORETED: Small holes may exceed 25 per cent of the total volume of the brick.
3) HALLOW: The total of holes, which need not be small, may exceed 25 per cent of the
volume of the brick.
4) CELLULAR: Holes closed at one end exceed 20 per cent of the volume.
DEFECTS OF BRICKS
1. Over burning bricks
Bricks should be burned at temperatures at which incipient, complete and viscous vitrification
occur. However, if the bricks are overburnt, a soft molten mass is produced and the bricks loose
their shape. Such bricks are not used for construction works.
2. Under burning brick
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When bricks are not burnt to cause complete vitrification, the clay is not softened because of
insufficient heat and the pores are not closed. This result is higher degree of water absorption and
less compressive strength. Such bricks are not recommended for construction works.
3. Bloating
This defect observed as spongy swollen mass over the surface of burned bricks is caused due to
the presence of excess carbonaceous matter and sulphur in brick-clay.
4. Black core
When brick-clay contains bituminous matter or carbon and they are not completely removed by
oxidation, the brick results in black core mainly because of improper burning.
5. Efflorescence
This defect is caused because of alkalies present in bricks. When bricks come in contact with
moisture, water is absorbed and the alkalis crystalise. On drying grey or white powder patches
appear on the brick surface. This can be minimised by selecting proper clay materials for brick
manufacturing, preventing moisture to come in contact with the masonry, by providing
waterproof coping and by using water repellent materials in mortar and by providing damp proof
course.
USES OF BRICK
In metalurgy industry , glass industry for lining furnaces.
Use as a refractory (silica, magnesia bricks)
To make walls, fences etc..
CLASSES OF BRICKS
Adobe brick - brick that is not burnt in a kiln at all.
- Traditional
- adding lime increases bonding
Sand-lime brick - not burnt at high temperature.
- Cement can sometimes be used.
Kiln burned brick - brick burnt at high T0 .
Concrete bricks - is like a concrete block
- Relatively coarser agg. (6mm ) is used.
- are lighter in wt than concrete blocks - lees density.
- are fire resistant.
- less sound transmission.
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Testing and Classification Of Bricks
Tests - visual inspection - as shape, colour, blister, checks, cracks.
- Checking of dimension and plainness, compressive strength.
- Compressive strength.
- Water absorption
- Saturation coefficient
- Efflorescence - assed by repeated wetting and drying of test bricks.
- paint.