To understand the water requirement for the hydration of Ordinary Portland Cement (OPC), it is important to first understand the hydration process itself. OPC is a type of cement that reacts with water to form a hard and durable material known as concrete. The hydration process involves a series of chemical reactions between the cement particles and water molecules.

During the hydration of OPC, the water acts as a medium to facilitate the chemical reactions that transform the cement particles into a solid structure. The water molecules penetrate the cement particles and react with the compounds present in the cement, primarily tricalcium silicate (C3S) and dicalcium silicate (C2S). These reactions result in the formation of calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH).

The water requirement for OPC is typically expressed as a percentage of the weight of the cement. This percentage represents the amount of water needed to ensure complete hydration of the cement particles. The correct answer, option C, states that the water requirement is 20 to 25% of the cement weight.

Explanation:

1. Importance of water in the hydration process:
- Water is essential for the chemical reactions that convert cement particles into a solid structure.
- It provides a medium for the transport of ions and facilitates the formation of hydration products.

2. Chemical reactions during hydration:
- Tricalcium silicate (C3S) reacts with water to form calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH).
- Dicalcium silicate (C2S) also reacts with water to form C-S-H gel and CH.
- These reactions are exothermic, releasing heat.

3. Water/cement ratio:
- The water/cement ratio is the ratio of the weight of water to the weight of cement used in the concrete mix.
- It determines the workability and strength of the concrete.
- A higher water/cement ratio increases workability but decreases strength.

4. Water requirement for OPC:
- The water requirement for OPC is typically in the range of 20 to 25% of the cement weight.
- This means that for every 100 kg of cement, 20 to 25 kg of water is required for complete hydration.
- The exact water requirement may vary depending on factors such as the fineness of the cement, chemical composition, and desired properties of the concrete.

In conclusion, the correct answer is option C, which states that the water requirement for the hydration of OPC is 20 to 25% of the cement weight. This water is necessary to ensure the complete hydration of the cement particles and the formation of a strong and durable concrete structure.