Answer:

Introduction:
For the construction of dams and foundations, the type of cement generally used is portland blast furnace cement. This type of cement offers several advantages that make it suitable for such applications. In this response, we will discuss the properties and benefits of portland blast furnace cement in detail.

Properties of Portland Blast Furnace Cement:
1. Composition: Portland blast furnace cement is a blend of ordinary Portland cement clinker, gypsum, and granulated blast furnace slag. The slag is a byproduct of the iron and steel industry and is used as a supplementary cementitious material.

2. Strength: This type of cement exhibits high early strength, which is beneficial for construction projects that require quick setting and early strength development.

3. Durability: Portland blast furnace cement offers excellent durability due to its low permeability and resistance to chemical attacks. It is particularly suitable for structures exposed to aggressive environments, such as dams.

4. Heat of Hydration: The heat of hydration of portland blast furnace cement is lower compared to ordinary Portland cement. This is advantageous in large-scale construction projects, as it reduces the risk of thermal cracking.

5. Workability: The inclusion of blast furnace slag in the cement mixture enhances workability, making it easier to handle and place during construction.

Benefits of Portland Blast Furnace Cement for Dams and Foundations:
1. Strength and Stability: The high early strength of portland blast furnace cement ensures the stability and structural integrity of dams and foundations. It can withstand the heavy loads and pressures exerted by water and soil.

2. Durability: The low permeability and resistance to chemical attacks make portland blast furnace cement an ideal choice for structures exposed to water and aggressive environments. It helps in preventing deterioration and prolongs the lifespan of dams and foundations.

3. Reduced Risk of Cracking: The lower heat of hydration of this cement type reduces the risk of thermal cracking, which is crucial for large-scale concrete structures like dams.

4. Workability: The enhanced workability of portland blast furnace cement allows for better placement and consolidation, ensuring proper compaction and minimizing the chances of voids or weak points in the concrete.

5. Economic and Environmental Benefits: The use of blast furnace slag as a supplementary cementitious material reduces the demand for clinker production, which leads to lower greenhouse gas emissions. Additionally, the use of slag helps in utilizing a byproduct of the iron and steel industry, promoting sustainable practices.

In conclusion, portland blast furnace cement is the preferred type of cement for constructing dams and foundations due to its strength, durability, reduced risk of cracking, workability, and environmental benefits. It provides the necessary properties to withstand the challenging conditions and loads imposed on such structures.