Pozzolanic materials are essentially siliceous or aluminous compounds that do not possess any cementitious property but when in finely divided form mixed in the presence of water, react with calcium hydroxide to form cementitious property. 
The reaction of pozzolana and its effect: 
The silica in the pozzolana reacts with the lime produced during the hydration of cement and contributes to the development of strength. 
Slowly and gradually additional calcium silicate is formed, which is a binder and fills up the space. This gives impermeable, durable cement with high final strength. 
In addition to this, it also makes concrete more workable and reduces bleeding and segregation, and is better resistant to freezing and thawing. 
Examples: fly ash, silica fumes, metakaolin, slag, VCAS (vitrified calcium alumino-silicate).