Kiln Construction and Brick Firing
It is during the firing that the bricks receive their strength. In the presence of high heat, the alkalies in the clay, together with small amounts of oxides of iron and other metals, are joined in chemical union with the alumina and silica in the clay to form a dense and durable mass.
A kiln is a furnace or oven in which bricks are fired or heat treated to develop hardness. Where brickmaking is done on a large scale, the firing operation is performed in a continuous-process kiln referred to as a tunnel kiln. In making brick on a small scale, firing is a periodic operation wherein the bricks are placed in the kiln, the fire started and heat developed, and then, after several days of firing, the fuel is cut off from the fire and the entire kiln and its load are allowed to cool down naturally.
The kiln is filled with well-dried bricks, stacked in the same manner as during the drying. The top of the stack in the kiln is then sealed with mud. Some openings are left through which combustion gases can escape. Pieces of sheet metal are provided to slide over the openings to control the rate at which the fire burns.
Although a range of fuels can be used in this kiln, wood or charcoal are the most common. When the kiln is at the prime heat for firing, a cherry-red hue develops (corresponding to a temperature range of 875 to 900°C). This condition is held for about 6 hours. Sufficient fuel must be available when the burning starts since the entire load of bricks might be lost if the fires were allowed to die down during the operation. Firing with wood will require four to five days.
During the firing the bricks will shrink as much as 10%. As they are taken out of the kiln they should be sorted to different grades, the main criteria being strength, irregular dimensions, cracks and sometimes discoloration and stain.
Binders
When binders are mixed with sand, gravel and water, they make for a strong and long lasting mortar or concrete.
Binders can be broadly classified as non-hydraulic or hydraulic. The hydraulic binders harden through a chemical reaction with water making them impervious to water and therefore able to harden under water. Portland cement, blast-furnance cement (super sulphated), pozzolanas and high- alumina cement belong to the hydraulic binders. High-calcium limes (fat or pure limes) are nonhydraulic since they harden by reaction with the carbon dioxide in the air. If, however lime is produced from limestone containing clay, compounds similar to those in portland cement will be formed, i.e., hydraulic lime.
Lime
Non-hydraulic lime is high-calcium limes that are produced by burning fairly pure limestone, essentially calcium carbonate, so as to drive off the carbon dioxide leaving calcium oxide or quicklime. The burning process requires a temperature of 900 to 1 100° C. Quicklime must be handled with great care because it reacts with moisture on the skin and the heat produced may cause burns. When water is added to quicklime considerable heat is evolved, expansion takes place breaking down the quick lime pieces to a fine powder and the resulting product is calcium hydroxide, also called hydrated lime, or slaked lime.
After drying the powder is passed through a 3mm sieve, and poured into bags for storage (in dry conditions) and distribution.