•  Air-entraining agents are liquid chemicals added during mixing to produce microscopic air bubbles in concrete.

•  Water-reducing chemicals are used for two separate purposes: to lower the water content which increases the strength; or to obtain higher slump using the same water content for pumping concrete or in hot weather.

•  Retarding chemicals delay the initial set of concrete and are used primarily in hot weather.

•  Accelerating chemicals reduce the initial set time of concrete and are recommended in cold weather.

•  Plasticizers or high-range water reducers (HRWR) constitute a special class of water-reducers.

All other varieties of admixtures fall into the specialty category whose functions include corrosion inhibition, shrinkage reduction, alkali-silica reactivity reduction, workability enhancement, bonding, damp proofing, and coloring.

Supplementary Cementing Materials
Supplementary cementing materials, also called mineral admixtures, contribute to the properties of hardened concrete through hydraulic or pozzolanic activity. Typical examples are natural pozzolans, fly ash, ground granulated blast-furnace slag, and silica fume, which can be used individually with portland or blended cement or in different combinations.

These materials react chemically with calcium hydroxide released from the hydration of portland cement to form cement compounds. These materials are often added to concrete to make concrete mixtures more economical, reduce permeability, increase strength, or influence other concrete properties. Fly ash, the most commonly used pozzolan in concrete, is a finely divided residue that results from the combustion of pulverized coal and is carried from the combustion chamber of the furnace by exhaust gases. Commercially available fly ash is a by-product of thermal power generating stations.

Blast-furnace slag, or iron blast-furnace slag, is a nonmetallic product consisting essentially of silicates, aluminosilicates of calcium, and other compounds that are developed in a molten condition simultaneously with the iron in the blast-furnace. Silica fume, also called condensed silica fume and microsilica, is a finely divided residue resulting from the production of elemental silicon or ferro-silicon alloys that is carried from the furnace by the exhaust gases. Silica fume, with or without fly ash or slag, is often used to make high-strength concrete.

(Some content provided by Portland Cement Association )