Final answer:
An accelerator is an admixture that increases the concrete's rate of gaining strength by fast-tracking the hydration process.
Step-by-step explanation:
The admixture that increases the rate at which concrete gains strength is known as an accelerator. An accelerator, when added to concrete, fast-tracks the hydration process of cement, thereby speeding up the early strength development.
The use of accelerators is especially beneficial in cold weather conditions where concrete would typically take longer to set and gain enough strength to be structurally sound. An example of a commonly used accelerator is calcium chloride; however, it should be noted that excessive use of accelerators can lead to durability issues in the long term.