Building Materials MCQ Questions & Answers | Civil Engineering

Gypsum is a mineral that is composed of:

Calcium sulfate dihydrate (CaSO₄ · 2H₂O)

This means gypsum consists of:

Calcium sulfate (CaSO₄), which is the primary component.

Water (H₂O), which is combined with calcium sulfate to form gypsum.

Gypsum is widely used in construction and building materials, particularly for plaster and drywall.

Initial Setting Time refers to the period after water is added to cement during which the mixture remains workable before it begins to stiffen.

For asbestos cement products such as roofing sheets and pipes, a longer setting time is essential to allow for proper mixing, molding, and compaction.

According to standard requirements, the minimum initial setting time should be 90 minutes to ensure sufficient workability.

Other options like 30, 50, and 75 minutes are too short and do not provide adequate time for the manufacturing process of asbestos cement products.

Permanent magnets must be made from materials that retain magnetism for long periods.

High carbon steel is a good base because it can be hardened and magnetized effectively.

Adding cobalt to high carbon steel improves its magnetic strength and stability by increasing the material’s ability to resist demagnetization.

For conventional permanent magnets, the optimal composition includes around 15% cobalt alloyed with high carbon steel.

Higher percentages, like 35%, are not standard in steel-based magnets and are more applicable to specialized magnet alloys such as Alnico, but even then, typical cobalt content does not reach 35%.

Therefore, Option A (15% of cobalt) is the correct choice for permanent magnets made from high carbon steel.