Answer:
The constituent which imparts strength to cement is silica (option B). Silica, also known as silicon dioxide (SiO2), is a key ingredient in the production of cement and plays a crucial role in imparting strength to the final product. Here is a detailed explanation:

Silica in Cement:
1. Silica is one of the main compounds found in cement. It is present in the form of silicon dioxide (SiO2) and is derived from various sources such as sand, clay, and shale.
2. Silica reacts with the calcium hydroxide (Ca(OH)2) formed during the hydration process of cement to produce calcium silicate hydrate (C-S-H) gel, which is responsible for the strength and durability of cement.
3. The C-S-H gel forms a matrix that binds the particles of cement together, providing strength and stability to the hardened cement paste.
4. The presence of silica in cement also contributes to the reduction of porosity and permeability, making the cement less susceptible to water penetration and chemical attack.

Role of Silica in Cement:
1. Strength Development: Silica plays a crucial role in the early and long-term strength development of cement. The reaction between silica and calcium hydroxide generates calcium silicate hydrate gel, which is responsible for the strength gain.
2. Binding Agent: Silica acts as a binding agent in cement, holding the particles together and creating a solid and cohesive structure.
3. Chemical Resistance: Silica improves the chemical resistance of cement, making it more resistant to aggressive substances such as sulfates and acids.
4. Durability: The presence of silica enhances the durability of cement by reducing the permeability and increasing the resistance to cracking and deformation.

Conclusion:
Silica is a vital constituent in cement that imparts strength and durability to the final product. It reacts with calcium hydroxide to produce calcium silicate hydrate gel, which acts as a binding agent and forms a robust matrix within the cement. The presence of silica also improves the chemical resistance and durability of cement.