What type of admixture increases the rate of early strength development in concrete?

Admixtures play a critical role in modifying the properties of concrete to enhance its performance for specific applications. The type of admixture that increases the rate of early strength development is the accelerator.

Accelerators are used to hasten the hydration process of cement, which allows concrete to attain strength more quickly compared to when no accelerator is used. This can be particularly beneficial in situations where rapid curing is essential, such as in cold weather conditions where concrete may set more slowly. By accelerating the chemical reactions that lead to the hardening of concrete, these admixtures help in achieving desired strength in a shorter time frame, which can facilitate faster project completion and reduce overall construction time.

In contrast, retarders are used to delay the setting time of concrete. Coloring agents serve primarily aesthetic purposes without affecting the setting or strength development process. Plasticizers, while improving workability, do not directly enhance early strength development. Thus, the role of accelerators in promoting early strength makes them the correct choice in this question.