Retarder
An admixture that delays setting, buying time to place and finish concrete in hot weather or on long placements where a cold joint would otherwise form.
What does retarder mean?
Retarders extend the working window without reducing final strength, and in some cases slightly increase it. They are essential on large pours where the first concrete would otherwise set before the last truck arrives. The tradeoff is a longer wait before finishing can start, which can push a crew into the night, and increased bleeding while the mix stays plastic.
In practice
On a hot summer commercial pour, retarder is what keeps the far end of the slab workable until the crew gets there.