CHARACTERISTICS OF GOOD BRICK
The essential requirements for building bricks are sufficient strength in crushing, regularity in size, a proper suction rate, and a pleasing appearance when exposed to view.
Size and Shape : The bricks should have uniform size and plane, rectangular surfaces with parallel sides and sharp straight edges.
Colour : The 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.
Texture and Compactness : The surfaces should not be too smooth to cause slipping of mortar. The brick should have precompact and uniform texture. A fractured surface should not show fissures, holes grits or lumps of lime.
Hardness and Soundness : The brick should be so hard that when scratched by a finger nail no impression is made. When two bricks are struck together, a metallic sound should be produced.
Water Absorption : should not exceed 20 per cent of its dry weight when kept immersed in water for 24 hours.
Crushing Strength : should not be less than 10 N/mm2.
Brick Earth : should be free from stones, kankars, organic matter, saltpetre, etc.
INGREDIENT OF GOOD BRICK EARTH
For the preparation of bricks, clay or other suitable earth is moulded to the desired shape after subjecting it to several processes. After drying, it should not shrink and no crack should develop. The clay used for brick making consists mainly of silica and alumina mixed in such a proportion that the clay becomes plastic when water is added to it. It also consists of small proportions of lime, iron, manganese, sulphur, etc. The proportions of various ingredients are as follows:
Silica 50–60%
Alumina 20–30%
Lime 10%
Magnesia < 1%
Ferric oxide < 7% } Less than 20%
Alkalis < 10%
Carbon dioxide Very small percentage
Sulphur trioxide Very small percentage
Water Very small percentage
FUNCTIONS OF VARIOUS INGREDIENT
Silica : It enables the brick to retain its shape and imparts durability, prevents shrinkage and warping. Excess of silica makes the brick brittle and weak on burning. A large percentage of sand or uncombined silica in clay is undesirable. However, it is added to decrease shrinkage in burning and to increase the refractoriness of low alumina clays.
Alumina : Absorbs water and renders the clay plastic. If alumina is present in excess of the specified quantity, it produces cracks in brick on drying. Clays having exceedingly high alumina content are likely to be very refractory.
Lime : Normally constitutes less than 10 per cent of clay. Lime in brick clay has the following effects:
1. Reduces the shrinkage on drying.
2. Causes silica in clay to melt on burning and thus helps to bind it.
3. In carbonated form, lime lowers the fusion point.
4. Excess of lime causes the brick to melt and the brick looses its shape.
5. Red bricks are obtained on burning at considerably high temperature (more than 800°C) and buff burning bricks are made by increasing the lime content.
Magnesia : Rarely exceeding 1 per cent, affects the colour and makes the brick yellow, in burning; it causes the clay to soften at slower rate than in most case is lime and reduces warping.
Iron : Iron oxide constituting less than 7 per cent of clay, imparts the following properties:
1. Gives red colour on burning when excess of oxygen is available and dark brown or even black colour when oxygen available is insufficient, however, excess of ferric oxide makes the brick dark blue.
2. Improves impermeability and durability.
3. Tends to lower the fusion point of the clay, especially if present as ferrous oxide.
4. Gives strength and hardness.
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