Retard the rate of setting of concrete.
Most retarders act as water reducers (water-reducing retarders). Retarders may also entrain some air.
- Lignin, Borax
- Tartaric acid and salts
Reducers of solubility ex. Ca(OH)2
- Coaters: they form an anhydrous compound, insoluble, and thus impermeable on surface of cement grains in contact with mixing water.