Retarder
Retarders are the chemicals added to cement slurry for delaying the setting time. Class G ad H are most commonly used cements for well cementing. Neat slurry prepared with these cements do not have long enough setting time or thickening time at temperatures above 100 deg F. Depending on the well depth, downhole temperature and total cementing operation time, the slurry prepared with neat cement could start thickening before the completion of cementing process and make the slurry un-pumpable. Hence cement retarders need to be added to the slurry in required proportion depending on specific application.