Admixture
A chemical added in small doses to change how concrete behaves: reducing water demand, speeding or slowing set, entraining air, or improving pumpability.
What does admixture mean?
Admixtures are dosed in ounces per hundredweight of cement, so batching accuracy matters enormously. They are grouped by function into standardized types, and products from different manufacturers can interact unpredictably when combined. Modern concrete is essentially impossible without them, since the workability contractors expect at the strengths engineers require depends on chemistry rather than water.
In practice
The admixtures in your load are listed on the batch ticket, and an unexpected set time is usually traced there first.