Compostion of Good Brick Earth and Compostion unsuitable for Brick Earth

Composition of good brick material

Following are the constituents of good brick material:

Silica-50-60%

Alumina-20-30%

Lime-5-10%

Iron Oxide-5-6%

Magnesia-3%

• Alumina: It is the chief constituent of every kind of clay. A good brick should contain 20% to 30% of alumina. This constituent imparts plasticity to the clay so that it can be moulded. If alumina is present in excess, with inadequate quantity of sand, the raw bricks shrink and warp during drying /burning and become too hard when burnt.
• Silica: It exists in clay either as free or combined. As free sand, it is mechanically mixed with clay. In combine form, it exists in chemical composition with alumina. A good brick material should contain about 50% to 60% of silica. The presence of this constituent prevents cracking, shrinking and warping of raw bricks. It thus imparts uniform shape to the bricks. The durability of bricks depends on the proper proportion of silica in brick material. The excess of silica destroys the cohesion between particles and the bricks become brittle.
• Lime: A small quantity of lime not exceeding 5 percent is desirable in good brick material. It should be present in a very finely powdered state because even small particles of the size of a pin-head cause flaking of the bricks. The lime prevents shrinkage of raw bricks. The sand alone is infusible. But it slightly fuses at kiln temperature in presence of lime. Such fused sand works as a hard cementing material for brick particles. The excess of lime causes the brick to melt and hence its shape is lost. The lumps of lime are converted into quick lime after burning and this quick lime slakes and expands in presence of moisture. Such an action results in splitting of bricks into pieces.
• Oxide of iron: A small quantity of oxide of iron to the extent of about 5 to 6 percent is desirable in good brick material. It helps as lime to fuse sand. It also imparts red colour to the bricks. The excess of oxide of iron makes the bricks dark blue or blackish. If, on the other hand, the quantity of iron oxide is comparatively less, the bricks will be yellowish in colour.
• Magnesia: A small quantity of magnesia in brick material imparts yellow tint to the bricks and decreases shrinkage. However, excess of magnesia leads to the decay of bricks.

Following are the ingredients which are undesirable in the brick material:

• Lime: The excess of lime is undesirable in brick material.
• Iron pyrites: If iron pyrites are present in brick material, the bricks are crystallized and disintegrated during burning because of the oxidation of the iron pyrites.
• Alkalies: These are mainly in the form of soda and potash. The alkalies act as a flux in the kiln during burning and they cause bricks to fuse, twist and warp. As a result, the bricks are melted and they loose their shape. Further, the alkalies remaining in bricks will absorb moisture from the atmosphere, when bricks are used in masonry. Such moisture, when evaporated, leaves behind grey or white deposits on the wall surface. The appearance of the building as a whole is then seriously spoiled.
• Pebbles: The presence of pebbles or grits of any kind is undesirable in brick material because it will not allow the clay to be mixed uniformly and thoroughly which will result in weak and porous bricks. Also, the brick containing pebbles will not break regularly as desired.
• Vegetation and organic matter: The presence of vegetation and organic matter in brick material assists in burning. But if such matter is not completely burnt, the bricks become porous. This is due to the fact that the gases will be evolved during the burning of the carbonaceous matter and it will result in the formation of small pores. Hence, it is necessary to see that all these gases are removed during the process of burning for getting bricks of good quality.