retarder

noun

1. a person or thing that retards.
2. Chemistry.
   1. any substance added to rubber to delay or prevent vulcanization.
   2. any substance added to delay a process.
3. Building Trades. an admixture of concrete or plaster that retards its set.

noun

1. a person or thing that retards
2. a substance added to slow down the rate of a chemical change, such as one added to cement to delay its setting