Admixtures are defined in ACI 116R as "a material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing".
High Range Water Reducer Admixture is defined as an admixture capable of producing large water reduction or great flowability without causing undue set retardation or entrainment of air in mortar or concrete.
Retarding Admixture is defined as an admixture that causes a decrease on the rate of hydration of the hydraulic cement and lengthens the time of setting.
Segregation is defined as separation of the constituents of a heterogeneous mixture so that their distribution is no longer uniform. The main cause of segregation in a fresh concrete mixture is the different specific gravity of the constituents.
Water Reducing Admixture is defined as an admixture that either increases slump of freshly mixed mortar or concrete without increasing water content or maintains slump with a reduced amount of water, the effect being due to factors other than air entrainment.
Workability as defined in ASTM C 125-93 is the property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity.