The plasticity to mould bricks in suitable shape is contributed by
🧱 Understanding Plasticity in Brick Earth
Plasticity is arguably the most critical physical property of brick earth for manufacturing. It refers to the ability of the clay, when mixed with water, to be deformed into any shape without cracking and to retain that shape after the deforming force is removed. This property is what allows raw clay to be pressed into moulds to form sharp, well-defined bricks.
🔬 Detailed Analysis of the Options
A. Alumina
This is the correct answer. Alumina (Al₂O₃) is the chief constituent in brick earth responsible for imparting plasticity. It absorbs water and makes the entire clay mass workable, allowing it to be easily moulded. A content of 20% to 30% is ideal. However, if alumina is present in excess, the raw bricks will shrink and warp during drying and become too hard when burned.
D. Silica
This is incorrect. Silica (SiO₂) is the main structural component, but it is non-plastic. Its role is to prevent the brick from shrinking and warping during drying and firing. An excess of silica would actually destroy the plasticity of the clay, making it brittle.
B. Lime
This is incorrect. Lime (CaO) acts as a flux, helping the components fuse together at a lower temperature during burning. It does not contribute to the plasticity of the raw clay.
C. Magnesia
This is incorrect. Similar to lime, magnesia (MgO) acts as a flux. It also helps reduce shrinkage and can impart a yellow tint to the brick, but it plays no role in providing plasticity for moulding.
📊 Summary: Role of Key Ingredients in Plasticity
| Ingredient | Primary Role | Effect on Plasticity |
|---|---|---|
| Alumina | Makes clay workable | The Primary Contributor. Excess causes shrinkage, warping, and excessive hardness. |
| Silica | Provides structure, prevents shrinkage | Reduces plasticity; excess makes brick brittle. |
| Lime | Acts as a flux during firing | No direct effect. |
| Magnesia | Acts as a flux during firing | No direct effect. |
💡 Study Tips
- Alumina = Adaptable: A simple way to remember is that Alumina makes the clay Adaptable, allowing it to be moulded.
- Silica = Skeleton: Think of Silica as the rigid Skeleton of the brick. Skeletons provide structure but are not plastic.
- Fluxes are for Firing: Remember that ingredients like Lime and Magnesia are primarily fluxes, which means their main job happens during the high temperatures of firing, not during the initial cold moulding stage.
- Balance is Key: Understand that even the correct ingredient in excess can be harmful. Too much alumina leads to warping and cracking.