Which of the following is NOT a common admixture in concrete?

Water expanders are not commonly recognized as admixtures in concrete. Admixtures are materials added to concrete to modify its properties, and they generally fall into specific categories based on their intended function.

Plasticizers, for example, are used to improve the workability of concrete without adding more water, allowing for a smoother finish and less risk of segregation. Air-entraining agents introduce tiny air bubbles into the concrete mix, which can help improve its resistance to freeze-thaw cycles and enhance durability. Retarders are utilized to slow down the setting time of concrete, allowing for longer working times in warm weather or during large pours.

Each of these admixtures has recognized roles in augmenting concrete's performance according to specific environmental or application needs. In contrast, ‘water expanders’ is not a standard term or recognized category in the field of concrete technology, making it the correct choice in identifying what is not a common admixture.