Brick

Brick

Composition of Good Brick Earth

Following are the constituents of brick earth:

1. Alumina

It is the chief constituent of every kind of clay. A good brick earth should contain about 20 to 30 per cent of alumina. This constituent imparts plasticity to earth so that it can be moulded.

2. Silica

• A good brick earth should contain about 50 to 60 per cent of silica. Presence of this constituent prevents cracking, shrinking and warping of raw bricks. It thus imparts uniform shape to the bricks.
• Excess of silica destroys the cohesion between particles and bricks become brittle.

3. Lime

• It should be present in a finely powdered state and not in lump.
• Lime prevents shrinkage of raw bricks. Sand alone is infusible. But it slightly fuses at kiln temperature in presence of lime.
• Excess of lime causes the brick to melt and hence, its shape is lost. Lumps of lime are converted into quick lime after burning and this quicklime slakes and expands in presence of moisture.

4. Oxide of Iron

• About 5 to 5 per cent is desirable in good brick earth. It helps lime to fuse sand. It also imparts red color to bricks.
• Excess of oxide of iron makes the bricks dark blue or blackish

5. Magnesia

A small quantity of magnesia in brick earth imparts yellow tint color to bricks and decreases shrinkage. But excess of magnesia leads to the decay of bricks.

Harmful Ingredients in Brick Earth

1. Lime

• It causes unsoundness in brick if present in excess amounts.

2. Iron pyrites

• If iron pyrites are present in brick earth, bricks are crystallized and disintegrated during burning.

3. Alkalies

These are mainly in the form of soda and potash

4. 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.

5. Organic Matter

Presence of organic matter in brick earth assists in burning. But if such matter is not completely burnt, bricks become porous.

Manufacture of bricks

1. Preparation of clay

Clay of bricks is prepared in the following order:

1. Unsoiling
2. Digging
3. Cleaning
4. Weathering
5. Blending
6. Tempering

2. Moulding

Hand Moulding

• Machine Moulding

• Plastic Clay Machine
• Dry Clay Machine

3. Drying

4. Burning: Burning of bricks is done either in clamps or in kilns.

(a) Clamps: Its shape in plan is generally trapezoidal. Floor of clamp is prepared in such a way that short end is slightly in the excavation and wider end is raised at an angle of about 15° from ground level.

(b) Kilns: A kiln is a large over which is used to burn bricks. The kilns which are used in the manufacture of bricks are of the following two types.

(i) Intermittent kilns (ii) Continuous kilns

(i) Intermittent Kiln: This may be over ground or underground they are classified in two ways: (a) intermittent up-drought kilns, (b) Intermittent down-drought kilns

Comparison between bull's trench kiln and Hoffman's kiln

Comparison between clamp Burning and kiln Burning

Tests For Bricks

1. Absorption

• A brick is taken and it is weighted dry. It is then immersed in water for a period of 16 hours.
• Then weight again and the difference in weight should not, in any case, exceed
  (a) 20 per cent of weight of dry brick for first class bricks.
  (b) 22.5 per cent for second class bricks.
  (c) 25 per cent for third class bricks.

2. Crushing strength

• Minimum crushing strength for first class bricks  and for second class bricks 

3. Hardness

In this test, a scratch is made on brick surface with the help of a finger nail. If no impression is left on the surface, brick is treated to be sufficiently hard.

4. Presence of soluble salts

• Soluble salts, if present in bricks, will cause efflorescence on the surface of bricks.
• It is immersed in water for 24 hours. It is then taken out and allowed to dry in shade. Absence of grey or white deposits on its surface indicates absence of soluble salts.
• If the white deposits cover about 10% surface, the efflorescence is said to be slight.
• When white deposits cover about 50% of surface thin it is said to be moderate.
• If grey or white deposits are found on more than 50% of surface, the efflorescence becomes heavy and it is treated as serious.

5. Shape and Size

• Its shape should be truly rectangular with sharp edges.
• 20 bricks are randomly selected of standard (19 × 9 × 9 cm) for good quality bricks, the results should be within the following permissible limits:
  Length – 368 cm to 392 cm
  Width – 174 cm to 186 cm
  Height – 174 to 186 cm

6. Soundness

• In this test, two bricks are taken and they are struck with each other.
• Bricks should not break and a clear ringing sound should be produced.

7. Structure

• It should be homogenous compact and free from any defects such as holes, lumps, etc.
• High duty fire-clays can resist temperature range of 1482°C to 1648°C; medium duty fire-clays can resist temperature range of 1315°C to 1482°C and low duty fire-clays can resist temperature up to 870°C only.

Quality of Good Bricks

• The bricks should be table-moulded, well burnt in kilns, copper-coloured free from cracks and with sharp and square edges.
• The bricks should be uniform in shape and should be of standard size.
• The bricks should give a clear metallic ringing sound when struck with each other.
• The bricks when broken or fractured should show a bright homogeneous and uniform compact structure free from voids.
• The brick should be sufficiently hard. No impression should be left on brick surface, when it is scratched with finger hail.
• The bricks should not break into pieces when dropped flat on hard ground from a height of about one meter.
• The bricks, when soaked in water for 24 hour should not show deposits of white salts when allowed to dry in shade.
• No brick should have the crushing strength below 5.50 N/MM².