Waterproofing Agents in Concrete
Waterproofing agents are essential in concrete to prevent the ingress of water or moisture, which can lead to various issues such as corrosion, deterioration, and damage. Among the options given, mastic asphalt is the compound commonly used as a waterproofing agent in concrete.

Mastic Asphalt
Mastic asphalt is a type of asphalt that consists of a mixture of bitumen and fine aggregate. It is commonly used as a waterproofing agent in various construction applications, including concrete. Here's why mastic asphalt is an effective waterproofing agent:

- Water Impermeability: Mastic asphalt has excellent water impermeability properties, preventing the penetration of water or moisture into the concrete structure. Its dense and compact nature makes it highly resistant to water ingress.

- Adhesion: Mastic asphalt has excellent adhesion to various surfaces, including concrete. When applied as a waterproofing layer, it adheres tightly to the concrete surface, creating a seamless and durable barrier against water penetration.

- Durability: Mastic asphalt is known for its durability and longevity. It can withstand harsh environmental conditions, temperature fluctuations, and physical impacts, making it an ideal choice for waterproofing concrete structures.

- Flexibility: Mastic asphalt remains flexible even at low temperatures, allowing it to accommodate any movement or expansion of the concrete structure without cracking or compromising its waterproofing properties.

- Application: Mastic asphalt can be applied in various forms, such as hot-applied or cold-applied. It can be poured or sprayed onto the concrete surface, providing a seamless and uniform waterproofing layer.

In contrast, the other options mentioned, such as calcium chloride, gypsum, and silica fume, are not primarily used as waterproofing agents in concrete.

- Calcium Chloride: Calcium chloride is often used as an accelerator in concrete to speed up the curing process. It is not typically utilized as a waterproofing agent.

- Gypsum: Gypsum is a mineral commonly used in construction materials, but it is not considered a suitable waterproofing agent for concrete. Gypsum-based products are more commonly used for fire resistance or as a binder in certain construction applications.

- Silica Fume: Silica fume is a byproduct of the silicon and ferrosilicon alloy industry. While it can enhance the strength and durability of concrete, it is not primarily used as a waterproofing agent.

Therefore, the correct answer is (A) Mastic Asphalt, as it is the compound most commonly used as a waterproofing agent in concrete due to its water impermeability, adhesion, durability, flexibility, and ease of application.