Introduction:
In the field of civil engineering, concrete is one of the most widely used construction materials. It is a composite material composed of cement, aggregates, water, and various admixtures. Admixtures are substances added to the concrete mix in order to modify its properties and enhance its performance. One common type of admixture used in concrete is an accelerator, which helps to speed up the initial setting time of the concrete.

Explanation:
Accelerating the initial set of concrete:
When concrete is mixed, it undergoes a chemical reaction known as hydration, in which the cement particles react with water to form a solid matrix. This process leads to the hardening and setting of the concrete. The initial setting time refers to the time it takes for the concrete to begin to harden after the mixing process. Accelerators are admixtures that are added to the concrete mix to reduce the initial setting time and allow the concrete to gain strength more quickly.

Types of accelerators:
There are several types of accelerators available for concrete, including calcium chloride, calcium nitrate, and triethanolamine. Among these, calcium chloride is the most commonly used accelerator due to its effectiveness and cost-efficiency.

Why calcium chloride is used:
Calcium chloride is a common accelerator for concrete due to its ability to increase the rate of hydration and shorten the setting time. It works by reacting with the cement particles and increasing the solubility of the calcium silicates, which are the primary compounds responsible for the hardening of concrete. This increased solubility leads to a faster rate of hydration and setting.

Advantages of using calcium chloride:
- Faster setting time: The use of calcium chloride as an accelerator helps to speed up the initial setting time of concrete, allowing for faster construction and shorter project timelines.
- Cold weather applications: Calcium chloride is particularly beneficial in cold weather conditions, as it can help prevent delays in construction due to freezing temperatures. It lowers the freezing point of water in the concrete mix, enabling the hydration process to occur even in sub-zero temperatures.
- Increased early strength: Accelerated setting time also translates to increased early strength development. This can be advantageous in situations where early load-bearing capacity is required.

Conclusion:
In conclusion, calcium chloride is the most common admixture used to accelerate the initial set of concrete. Its ability to speed up the setting time and enhance early strength development make it a preferred choice for construction projects. However, it is important to use calcium chloride in the recommended dosage to avoid any potential negative effects on the long-term durability of the concrete.