Question

Select the statement(s) which are NOT correct with respect to burnt clay bricks.

• A. Lime (<10% of clay) in carbonated form lowers the fusion point of bricks.
• B. Magnesia (>1% of clay) imparts red colour to the bricks.
• C. Iron Pyrites tend to oxidize and decompose the brick during burning.
• D. Alkalis (alkaline salts) when present in excess (>10% of clay) decrease the probability of efflorescence.

Hint:

Magnesia → yellow tint, Iron → red colour, Excess alkalis → efflorescence.

Answer 1

The Correct Option is B, D

Step 1: Effect of lime.
Small amounts of lime (<10%) lower the fusion point and help in vitrification. Correct. ✓

Step 2: Effect of magnesia.
Magnesia provides a yellowish tint and imparts durability, not red colour. Wrong statement. ✗

Step 3: Effect of iron pyrites.
Pyrites oxidize during burning, causing disintegration and spots. Correct. ✓

Step 4: Effect of alkalis.
Alkalis promote efflorescence by leaving behind salts when water evaporates. Excess alkalis increase efflorescence, not decrease. Wrong statement. ✗

Final Answer: \[ \boxed{(B) \; \text{and} \; (D)} \]