Which of the following constituents, when present in excess quantity, changes the colour of the brick from red to yellow?

Explanation:

Effect of Excessive Limestone on Brick Colour

Definition: Bricks are one of the most widely used construction materials, and their quality and aesthetic appearance depend significantly on the raw materials used during their manufacturing process. One of the primary constituents that can influence the colour of bricks is limestone (calcium carbonate, CaCO₃). When limestone is present in excess, it reacts during the firing process, causing a distinct change in the brick's colour from red to yellow.

Working Principle: During the firing of bricks, the raw materials undergo various chemical reactions due to the high temperatures in the kiln. Limestone, which is a calcium-based compound, plays an important role in this process. When present in small quantities, limestone contributes positively to the strength and durability of bricks. However, when present in excess, it interferes with the natural chemical reactions of other constituents, such as iron oxide, which is responsible for the red coloration of bricks.

Iron oxide imparts the characteristic red colour to bricks during the firing process. Excessive limestone alters the chemical environment in the kiln by forming compounds such as calcium silicates and calcium aluminates. These compounds suppress the formation of the red iron oxide, resulting in a yellowish colour in the final product.

Chemical Reaction:

The key reactions involved when limestone is present in excess are as follows:

• Decomposition of limestone during heating:

  CaCO₃ → CaO + CO₂ (calcium oxide and carbon dioxide)

• Formation of calcium silicates and aluminates:

  CaO + SiO₂ → CaSiO₃ (calcium silicate)

  CaO + Al₂O₃ → Ca(AlO₂)₂ (calcium aluminate)

These reactions consume the calcium oxide (CaO) generated by the decomposition of limestone. The presence of these newly formed compounds inhibits the normal reactions of iron oxide, leading to the yellowish hue in bricks.

Advantages:

• In controlled amounts, limestone can improve the workability of raw brick material and enhance its durability.
• It contributes to a smoother surface finish in fired bricks.

Disadvantages:

• Excessive limestone significantly alters the natural red colour of bricks, which may be undesirable for aesthetic purposes.
• It can lead to a brittle and weaker brick structure if not carefully controlled during the manufacturing process.

Applications: The use of bricks with a yellowish hue due to excess limestone might be acceptable for specific architectural styles or where the colour variation is not a concern. However, for standard construction practices, controlling the limestone content is crucial to maintain the desired red colour and structural integrity of bricks.