Properties and Composition of Good Bricks
1. Shape and Size
- Bricks should have uniform size and plane, rectangular surfaces with Parallel sides. and sharp straight edges. Standard Size = 19x9x9 cm or 190x90x90 mm.
2. Colour
- 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.
3. Texture and compactness
- Surfaces should not be too smooth to cause slipping of mortar. The brick should have a precompact and uniform texture.
- A fractured surface should not show fissures, holes grits or lumps of lime.
4. Hardness
- Brick should be so hard that when Scratched by a fingernail, no impression occurs.
5. Soundness
- When two bricks are struck together, a metallic sound should be produced.
6. Water absorption
- It should not be more than 20% of its dry weight when kept immersed in water for 24 hrs.
7. Crushing strength
- It should not be less than 10 N/mm2.
Brick earth should be free from stones. grits, organic matter etc.
The Composition of Good Brick Earth
- Properties of various ingredients in Bricks are as below.
Silica 50-60%
Alumina 20-30%
Lime <5%
Ferric oxide 5-6%
Magnesia <1%
Alkalis, Carbon dioxide, Sulphur trioxide, water in a very small percentage
1. Silica (50-60%)
- It is responsible for the strength of cement.
- Up to a certain proportion the more silica there is the better the shape and more even the texture of the resulting brick.
- In the presence of lime and oxide of iron, silica fuses at a lower temperature and provides the brick with its hardness and durability.
- It prevents cracking, shrinking and warping of green bricks.
- Excess of silica destroys the cohesion between the particles and the bricks become brittle.
2. Alumina (20-30%)
- It is the principal and most important constituent of good clay, as it absorbs water and imparts plastic qualities so that the brick can be moulded.
- If alumina is present in excess, it causes cracks in bricks on drying and becomes too hard when burnt.
- Clays having high alumina are found to be very refractory.
3. Lime (<1. 5%)
- It acts as a flux.
- Prevents shrinkage on drying.
- It causes silica to fuse during the burning process and bind the brick particles to together.
- In carbonated form, lime lowers the fusion point.
- Excess of lime in form of lumps causes the brick to disintegrate and hence brick shape is lost.
4. Magnesia (<1%)
- A small quantity of magnesia in brick earth imparts the yellow tint to the bricks and decreases shrinkage.
- It slows down the rate of softening of clay during burning and reduces warping.
- Excess of magnesia leads to the decay of bricks.
5. Iron Oxide (5-6%)
- Like lime, it acts as a flux helping silica to fuse during the burning process and bind the brick particles together, and thus providing the brick with its strength and hardness.
- The colour of the brick ranges from light yellow to red depending upon the quantity of oxide of iron in the clay. The intensity of colour increases with the quantity of oxide of iron.
- It improves impermeability and durability.
- It lowers the fusion point of the clay, especially if present as ferrous oxide.
- lt lower the fusion point of the clay, especially if present as ferrous oxide.