Gas Forming Agents:

A gas forming agent is a chemical admixture such as aluminium powder which produces minute bubbles of hydrogen gas throughout the matrix. These agents are used to make concrete lightweight, to stop bleeding in concrete, to make it denser, etc. These are used in very small quantities.

Air-entraining Agents:

This admixture is used for inducing minute air bubbles uniformly throughout the concrete mass. These are used to increase the workability of concrete and also to increase resistance to freezing and thawing. These agents also reduces the bleeding of concrete. These agents entrap more air during the preparation of concrete and that air acts as a sort of ball bearing between the ingredients of concrete.