Retarders, typically phosphates or hydroxycarboxylic acids, decrease the rate of set, thus extending the time between initial mixing and final compaction, but they do not adversely affect 28-day strength. Retarders may be applied to formwork, to retard the surface concrete where an exposed aggregate finish is required by washing after the formwork is struck. Retarders are also frequently used in ready-mixed mortars to extend their workable life up to 36 hours. The mortars are usually delivered on site in date marked containers of 0.3 m3 capacity.

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