The creation of bricks involves the meticulous preparation of clay or suitable earth, subjected to various processes and shaped to desired forms. Post-moulding, the bricks must undergo drying without shrinkage or crack formation.
The primary constituents of brick-making clay comprise silica and alumina, forming a plastic mixture upon the addition of water. Additionally, small proportions of lime, iron, manganese, sulphur, among others, contribute to the clay’s composition. The ideal proportions of the ingredients of brick earth are as follows:
| Ingredients | Quantity |
|---|---|
| Silica | 50–60% |
| Alumina | 20–30% |
| Lime | 10% |
| Magnesia | < 1% |
| Ferric oxide | < 7% |
| Alkalis | < 10% |
| Carbon dioxide | < Very small Percentage |
| Sulphur trioxide | < Very small Percentage |
Functions of Ingredients of Brick Earth
Silica
Silica plays a crucial role in maintaining the brick’s shape and ensuring durability while preventing shrinkage and warping. However, an excess of silica can make the brick brittle and weak upon burning. While a higher percentage of sand or uncombined silica in clay is undesirable, it’s sometimes added to reduce shrinkage during firing and enhance the refractoriness of low alumina clays.
Alumina
Alumina absorbs water and contributes to the clay’s plasticity. Excessive alumina content can lead to cracks in bricks during the drying process. Clays with high alumina content tend to be highly refractory.
Lime
Normally constituting less than 10% of clay, lime serves various purposes:
- Reduces shrinkage during drying.
- Aids in binding silica during burning.
- When in carbonated form, lowers the fusion point.
- Excessive lime content causes bricks to melt, losing their shape. Different firing temperatures yield red or buff-burning bricks, with higher lime content for the latter.
Magnesia
With a minimal presence, usually less than 1%, magnesia impacts brick color, lending a yellow hue during firing. It slows down the clay’s softening compared to lime, reducing warping.
Iron
Iron oxide, comprising less than 7% of clay, contributes in several ways:
- Gives red, brown, or black colors based on oxygen availability during burning. Excessive ferric oxide turns the brick dark blue.
- Enhances impermeability and durability.
- Reduces the clay’s fusion point, particularly when present as ferrous oxide.
- Contributes to strength and hardness of the brick.
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| Material | Function | Excess | Deficiency |
|---|---|---|---|
| Silica | Shape retention, durability | Brittle, weak bricks on burning | Shrinkage, warping |
| Alumina | Clay plasticity | Cracks in bricks on drying | Reduced plasticity |
| Lime | Shrinkage reduction, binding | Melts bricks, loss of shape | Increased shrinkage |
| Magnesia | Color impact, warping reduction | Yellowish bricks, slower softening | No significant impact |
| Iron | Color variation, durability | Dark blue bricks, lower fusion point | No significant impact |
Harmful Substances in Bricks
Lime
Optimal amounts of lime in clay result in good bricks. However, excess lime changes brick color from red to yellow. When present in lumps, lime absorbs moisture, causing brick disintegration. Finely divided lime is preferred, with any lumps removed at the outset. Particles smaller than 3mm diameter generally cause minor pockmarks, while larger ones might lead to unsightly blemishes or severe cracking.
Pebbles, Gravels, Girts
These materials hinder thorough clay mixing and spoil brick appearance. Bricks containing pebbles and gravels may crack during processing.
Iron Pyrites:
Pyrites tend to oxidize and decompose bricks during firing, potentially causing splitting and discoloration
Alkalis (Alkaline Salts)
When present in small amounts (<10% of raw clay), alkalis act as fluxes, particularly when combined with alumina silicates. However, excess alkalis make clay unsuitable for bricks, causing asymmetrical melting during firing. Efflorescence, a phenomenon where alkalis crystallize and leave gray or white powder deposits on bricks, occurs when exposed to moisture.
Organic Matter
During firing, organic matter chars and leaves pores in green bricks, increasing water absorption and reducing strength.
Carbonaceous Materials
Bituminous matter or carbon significantly affects the raw clay’s color. If not entirely removed by oxidation, bricks may have a black core.
Sulphur
Found in clay as various sulphates or iron sulphide, sulphur can cause a spongy, discolored structure in bricks if not adequately oxidized during firing.
Water
Excessive free water causes significant shrinkage during drying, while combined water leads to shrinkage during firing. Water containing traces of magnesium or calcium carbonates, along with sulphurous fuel, can have effects similar to sulphur.
| Material | Effect | Excess | Deficiency |
|---|---|---|---|
| Lime(Lumps) | Color change, disintegration | Yellow bricks, moisture absorption | No significant impact |
| Pebbles, Gravels, Girts | Disruption of mixing, appearance spoilage | Cracking in bricks | No significant impact |
| Iron Pyrites | Brick decomposition, discoloration | Brick splitting, discolouration | No significant impact |
| Alkalis (Alkaline Salts) | Efflorescence, brick spoilage | Unsymmetrical melting, spoilage | Unsuitable for bricks, spoilage |
| Organic Matter | Pore creation, reduced strength | Increased water absorption | No significant impact |
| Carbonaceous Materials | Color impact | Black core in bricks | No significant impact |
| Sulphur | Structure alteration, discolouration | Spongy structure, white blotches | No significant impact |
| Water | Shrinkage, fusion effects | Considerable shrinkage | Shrinkage during firing |
Conclusion
The meticulous balance of silica, alumina, lime, magnesia, and iron is pivotal in crafting durable bricks. Each element contributes distinct properties, impacting strength, color, and resilience. Simultaneously, excess amounts of lime, pebbles, alkalis, or water pose challenges, leading to disintegration, discoloration, and structural flaws. Understanding and managing these ingredients are fundamental in producing high-quality, long-lasting bricks for construction purposes.
