A water reducer is a chemical compound added to concrete before or after it is mixed in order to produce concrete using less water while maintaining the material's workability, meaning it will not harden or set too quickly or too slowly.
Some advantages of using water reducers are increased strength of the hardened concrete, a less porous concrete that is also less water permeable, and often also a better finish.
A water reducer used as an admixture in concrete functions as a surface active agent, making it more difficult for the cement particles in the concrete mix to stick together.
What type of water reducer is added to the concrete depends on what the concrete will be used for. For example, different properties are needed when pouring tall columns of concrete than when making a concrete floor.