Introduction:
Rapid hardening cement is a type of cement that sets and hardens quickly, allowing for faster construction processes. It is commonly used in situations where early strength gain is required, such as in cold weather conditions or when quick repairs are needed. Rapid hardening cement can be obtained through various methods, but the correct option is fine grinding of clinker.

Explanation:
1. Clinker:
- Clinker is the main component of cement, produced by heating limestone and other materials at high temperatures in a kiln.
- The clinker is then ground into a fine powder to produce cement.

2. Fine Grinding:
- Fine grinding of clinker involves reducing the particle size of the clinker to increase its reactivity.
- This is typically done by using a ball mill, a rotating cylindrical device with steel balls that crush and grind the clinker into a fine powder.
- Fine grinding increases the surface area of the clinker, allowing for more efficient hydration and faster setting and hardening of the cement.

3. Hydration Process:
- When water is added to cement, a chemical reaction called hydration occurs.
- Hydration is the process by which cement particles react with water molecules to form new compounds, such as calcium silicate hydrate (C-S-H) and calcium hydroxide (CH).
- The hydration process is responsible for the setting and hardening of cement.

4. Increased Reactivity:
- Fine grinding of clinker increases the reactivity of the cement particles, leading to faster hydration and setting.
- The smaller particle size allows for more efficient contact between cement and water, promoting the formation of hydration products.
- As a result, rapid hardening cement achieves higher early strength development compared to conventional cement.

Conclusion:
Rapid hardening cement is obtained by fine grinding of clinker, which increases the reactivity of the cement particles and promotes faster hydration and setting. The smaller particle size achieved through fine grinding allows for more efficient contact between cement and water, leading to higher early strength development.