A set retarding admixture is an admixture that delays the hydration of concrete and thus prevents the premature stiffening of the mix. This is accomplished by either slowing the growth of hydration products or by reducing water penetration to the cement particles. In both cases, the retarding admixture prolongs the period of time that concrete remains plastic and workable. This is often necessary to offset the accelerating effect of hot weather on hydration and/or to allow successive lifts of concrete to be placed without developing cold joints.

Retarding admixtures should be used with care as they can have negative effects on the development of concrete strength. These admixtures can have detrimental effects on the final strengths of concrete if the dosage is not controlled or the temperature conditions are not properly monitored. It is important to test the admixture for its efficacy under the expected field conditions and the intended use in order to determine the proper dosage rate.

Generally speaking, conventional set retarders work by forming an adsorbent barrier around the hydrates that temporarily stall further hydration. These admixtures can also be effective in the presence of high humidity, low water content or even abrasive aggregates. In addition to retarders, there are also advanced hydration stabilizers that provide more consistent and predictable results. These admixtures are typically more water-reducing than traditional set retarders and can help to achieve better concrete quality under difficult conditions. They also tend to be less cement-specific than retarders and can provide a more uniform result on a wide range of different types of concrete.