retarder

(materials) A material that inhibits the action of another substance, such as flameproofing agents or substances added to cement to retard setting time. Also known as retardant.
(mechanical engineering) A braking device used to control the speed of railroad cars moving along the classification tracks in a hump yard. A strip inserted in a tube of a fire-tube boiler to increase agitation of the hot gases flowing therein.

A chemical added to a substance to slow a chemical reaction, prolong the set of the material, and provide more working time.