Burnt Clay Bricks: Identifying Incorrect Statements

This solution identifies statements that are factually incorrect regarding the composition and properties of burnt clay bricks.

Analysis of Statements

• Statement 1 (Correct): Lime ($<10\%$) acts as a flux, lowering the fusion point. This aids in binding silica particles, contributing to brick strength.
• Statement 2 (Incorrect): The red colour of bricks typically comes from iron oxides, not magnesia. Excess magnesia ($>1\%$) can negatively impact brick quality, causing issues like black coring or crumbling, but it does not impart the characteristic red colour.
• Statement 3 (Correct): Iron pyrites ($FeS_2$) can oxidize during the burning process, producing sulfur oxides ($SO_2$) and iron oxides. This decomposition can lead to brick swelling and structural damage.
• Statement 4 (Incorrect): Alkalis (alkaline salts), particularly when present in excess (like $>10\%$), significantly increase the likelihood of efflorescence. These soluble salts migrate to the surface with moisture and crystallize, causing white powdery deposits. They do not decrease the probability of efflorescence.

Conclusion

The statements identified as NOT correct are:

• Magnesia ($>1\%$ of clay) imparts red colour to the bricks.
• Alkalis (alkaline salts) when present in excess ($>10\%$ of clay) decrease the probability of efflorescence.

Therefore, the incorrect statements correspond to options B and D.