Clay Bricks

General composition of good clay is as follows :-

• Silica -Good quality bricks generally contain 50-60% silica. Silica prevents raw bricks from cracking, shrinking and warping. The higher the proportion of sand, the more shapely and uniform in texture will be the brick. A large percentage of sand or uncombined silica in clay is undesirable since it destroys cohesion between the brick clay particles and makes brick brittle and weak. However, it is added to decrease shrinkage in burning and to increase the refractoriness of low alumina clays.
• Alumina – The proportion of alumina varies between 20-30 percent. Alumina is the main constituent of clay. It acts as a cementing material in raw brick. Brick clay is plastic due to the presence of alumina. This plasticity ensures that bricks can be molded. An excess amount of alumina in clay may cause the bricks to shrink, warp or crack on drying and burning.
• Lime – Bricks should contain a little amount (around 10%) of finely powdered lime. It enables silica (of a required portion) to melt at the furnace temperature of 1650^oC and binds the particles of brick together resulting in strong and durable bricks. At about 1100^o C, lime acts as a catalyst to elevate the furnace temperature to 1650^oC at which silica fuses. This slightly fused silica works as a strong cementing material. Excess lime in brick clay will cause vitrification of bricks. It causes bricks to melt, as more than the required amount of silica will fuse. The bricks then lose their shape and become disfigured.
• Oxide of Iron – Bricks contain a small quantity of Iron Oxide. Iron Oxide acts a flux like lime, thus helps silica to fuse at low temperature. It imparts a red color to bricks upon burning. Iron also increases the durability and impermeability of the bricks thus, providing strength and hardness.
• Magnesia – A small proportion of magnesium (rarely exceeding 1%) decreases shrinkage and gives a yellow tint to the bricks. An excess amount of it causes bricks to decay.