Bricks: its Constituents and qualities of good bricks

 

Bricks 

Bricks can be defined as artificial stones which are usually made of clay. These are obtained by moulding clay in rectangular blocks and then by drying and burning these blocks.

One of the oldest building material brick continues to be a most popular and leading construction material because of being cheap, durable and easy to handle and work with. Clay bricks are used for building-up exterior and interior walls, partitions, piers, footings and other load bearing structures. 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.

Size of a standard brick (also known as modular brick) should be 190 mm × 90 mm × 90 mm (19 × 9 × 9 cm). When placed in masonry the 19 × 9 × 9 cm brick with mortar becomes 20 × 10 × 10 cm. Weight of such a brick is 3.0 kg. An indent called frog, 1–2 cm deep, is provided for 9 cm high bricks. The size of frog should be 10 × 4 × 1 cm. The purpose of providing frog is to form a key for holding the mortar and therefore, the bricks are laid with frogs on top.

• 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."

Fig: Brick and it's faces

Constituents of Good Brick Earth

The main constituents of good brick earth are alumina, silica, lime, oxide of iron and magnesia.

1.Alumina -

• This is the main constituent of every kind of brick earth. 
• A good brick earth should contain about 20% to 30% of alumina. 
• This imparts plasticity to the earth so that it can be moulded. 
• If alumina is present in excess, it leads to shrinking and warping during drying and burning. This can be prevented by the addition of sand.

 

2. Silica -

• It exists in brick earth as free sand or in a combined form as silicate of alumina. 
• A good brick earth should contain about 50% to 60% of silica. 
• The presence of silica prevents cracking, shrinking and warping of raw bricks. Thus, it imparts uniform shape to bricks. 
• The addition of silica also increases hardness, durability and resistance to heat. 
• Excess of silica removes the cohesion between particles and makes the brick brittle.

 

3. Lime -

A small quantity of lime not exceeding 5 % is desirable in good brick earth.

The silica(sand) alone is infusible. But it slightly fuses at kiln temperature in presence of lime means It enables the silica to melt during burning and bind the particles together. 

Lime should be in the form of powder because when lime is added to brick earth in a coarse or lumpy form, it does not mix well with the other ingredients and this can result in uneven distribution of the lime, which can lead to weak spots in the bricks. 

Excess of lime causes the brick to melt too readily, and the shape will be lost.

 4. Iron Oxide -

• A small quantity of iron oxide to the extent of about 5 - 6 % is desirable in good brick.
• It acts as a flux and helps the grains of sand to melt and bind the particles of clay together.
• It is responsible for imparting a red colour to the brick. 
• It improves the durability of the brick. 
• Excess of iron oxide makes the bricks dark blue or blackish, and less quantity of iron oxide makes the brick yellowish in colour.

5. Magnesia -

• A small quantity of magnesia(about 1%) in brick earth imparts a yellow tint and decreases shrinkage. 
• Excess of magnesia causes decay of bricks.

 Harmful Ingredients/Constituents in Brick Earth

Naturally available soil may contain ingredients other than those required for making quality bricks, which are discussed below.

1.Alkalis -

• Alkalis in the form of soda and potash lower the fusion point of clay, and cause bricks to fuse, twist and warp during burning. As a result, the bricks are melted and they loose their shape.
• Alkalies present in bricks absorb moisture from the atmosphere, when bricks are used in masonry. Such dampness (i.e, moisture) when dries, leaves behind grey or white deposits on the surface of the wall. The appearance of the building as a whole is then seriously spoiled.

2.Pebbles -

The presence of pebbles or grits of any kind is undesirable in brick earth because it will not allow the clay to be mixed uniformly and thoroughly which will result in weak and porous bricks. Regular breaking of bricks during masonry work is not possible because of the presence of pebbles. 

Fig: Pebbles in brick

3. Iron Pyrites -

The presence of iron pyrites in brick earth, the bricks are crystallized and disintegrated during burning because of the oxidation of the iron pyrites.
 

4. Vegetation and Organic Matter -

The presence of vegetation and organic matter in brick earth assists in burning. Incomplete burning of these materials causes the bricks to become porous.

5. Lime -

When a desirable amount of lime is present in the clay, it results in good bricks, but if in excess, it changes the colour of the brick from red to yellow. When lime is present in lumps, it absorbs moisture, swells and causes disintegration of the bricks. Therefore, lime should be present in finely divided state and lumps, if any, should be removed in the beginning itself.

QUALITIES OF GOOD BRICKS

The qualities/Properties of good bricks are as follows:

1.  Bricks should have perfect edges, must be adequately burnt, should be uniform red or copper in colour and should be free from cracks.
2.  Bricks should have rectangular plane surfaces with parallel sides and sharp right-angled edges. The size of a standard brick is 190 mm × 90 mm × 90 mm.
3.  It should be hard enough such that no impression is left when scratched with one’s fingernails.
4.  Bricks when struck with each other should produce a ringing sound.
5.  Bricks should not break when dropped flat from a height of 1 m.
6.  Bricks should be homogeneous and compact throughout, and the brick should not have any voids or grit.
7.  Bricks should have a percentage of absorption of water by weight less than 20%.
8.  Bricks should not show deposits of salts when immersed in water and dried.
9.  Bricks should have less thermal conductivity and must be sound-proof.
 
10. Crushing strength should not be less than 10 N/mm².