Which of the following options is not a type of admixture?

In the context of concrete and construction, an admixture refers to any material added to the concrete mix that modifies its properties. Compressive strength is a measurement of the ability of a material to withstand axial loads without failure, but it is not an admixture itself; rather, it is a characteristic of the finished concrete product.

On the other hand, a water-reducing agent, a retarder, and an accelerator are all types of admixtures. A water-reducing agent is added to decrease the amount of water needed in the concrete mix while maintaining workability, resulting in improved strength and durability. A retarder slows down the setting time of concrete, which is helpful in hot weather or for large pours. An accelerator, conversely, speeds up the curing process, allowing for faster finishing of concrete applications.

Understanding the definition and function of admixtures helps clarify why compressive strength does not fit into this category, as it describes a property of concrete rather than a substance that modifies its behavior during mixing and curing.