JNTU B.Tech II BUILDING MATERIALS, CONSTRUCTION AND PLANNING Previous Question Paper (September - 2021 - R16)

Bricks are primarily classified based on their quality, manufacturing process, and usage. In Indian standards, bricks are broadly categorized into first-class, second-class, third-class, and fourth-class bricks.

1. First-Class Bricks: These are table-molded, burnt in kilns, and possess a deep red, cherry, or copper color. They are uniform in shape, size, and texture with sharp, square edges. They have high compressive strength and low water absorption (less than 15% of their dry weight). They produce a metallic ringing sound when struck together and are used for permanent structures and load-bearing walls.

2. Second-Class Bricks: These are ground-molded and kiln-burnt. They may have slight variations in shape and size. They exhibit small cracks or distortions but remain suitable for general masonry work. Their water absorption limit is up to 20%. They are commonly used for internal walls and hidden masonry.

3. Third-Class Bricks: These are ground-molded and burnt in clamps. They are soft, light-colored, and have irregular shapes with distorted edges. They produce a dull thud when struck and have high water absorption (up to 25%). They are used for temporary structures or as filling material.

4. Fourth-Class Bricks: These are over-burnt or under-burnt bricks. Over-burnt bricks are dark, brittle, and distorted, while under-burnt bricks are soft and yellow. They are generally used as aggregate for road foundations or concrete base layers.

A standard brick sketch typically shows a rectangular prism with dimensions 190mm x 90mm x 90mm. Key features to label in a sketch include:
1. Stretcher: The long narrow face of the brick.
2. Header: The short narrow face of the brick.
3. Frog: A depression on the top face of the brick, typically 10mm to 20mm deep, used to form a key for mortar and to reduce the weight of the brick.

The manufacturing of aluminum is a two-stage process: the extraction of alumina from bauxite ore via the Bayer process, followed by the reduction of alumina to aluminum metal via the Hall-Héroult process.

1. The Bayer Process (Extraction of Alumina):

1. Crushing and Grinding: Bauxite ore is crushed and ground into a fine powder.
2. Digestion: The ore is mixed with a hot concentrated solution of sodium hydroxide (caustic soda) under high pressure. This dissolves the aluminum-bearing minerals to form a sodium aluminate solution, while impurities like iron oxide remain solid.
3. Clarification: The mixture is filtered to remove the solid impurities, known as 'red mud'.
4. Precipitation: The clear sodium aluminate solution is cooled and seeded with aluminum hydroxide crystals, causing the aluminum to precipitate out as aluminum hydroxide.
5. Calcination: The aluminum hydroxide is washed and heated in rotary kilns at temperatures exceeding 1000°C to remove chemically combined water, resulting in pure white aluminum oxide powder (alumina).

2. The Hall-Héroult Process (Electrolytic Reduction):

1. Electrolysis: Alumina is dissolved in a molten bath of cryolite (sodium aluminum fluoride) at approximately 950°C in a carbon-lined steel cell (the electrolytic cell).
2. Reduction: A high-intensity electric current is passed through the bath using carbon anodes. The current causes the aluminum ions to migrate to the carbon cathode (the lining of the cell), where they are reduced to form molten aluminum metal.
3. Collection: The molten aluminum, being denser than the electrolyte, settles at the bottom of the cell and is periodically siphoned off.
4. Refining and Casting: The molten aluminum is transferred to holding furnaces for further purification, alloying (if required), and casting into ingots, billets, or slabs for industrial use.