Pozzolan
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A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition (the technical term is "admixture") to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete. Pozzolans are primarily vitreous siliceous materials which react with calcium hydroxide to form calcium silicates; other cementitious materials may also be formed depending on the constituents of the pozzolan.
The pozzolanic reaction may be slower than the rest of the reactions which occur during cement hydration, and thus the short-term strength of concrete made with pozzolans may not be as high as concrete made with purely cementitious materials. On the other hand, highly reactive pozzolans, such as silica fume and high reactivity metakaolin can produce "high early strength" concrete that increase the rate at which concrete gains strength.
The first known pozzolan was pozzolana, a volcanic ash, for which the category of materials was named. The most commonly-used pozzolan today is fly ash, though silica fume, high reactivity metakaolin, ground granulated blast furnace slag and other materials are also used as pozzolans.
A pozzolan is a silicious or aluminosilicious material, which is highly vitreous. This material independently has little/less cementitious properties, but in the presence of a lime rich media like calcium hydroxide, shows better cementitious properties towards the later day strength (> 28 day).The mechanism for this display of strength is the reaction of silicates with lime to form secondary cementitiuos phases (Calcium Silicate Hydrates with a lower C/S ratio) which display gradual strenthening properties usually after 7 days.
The extent of the strength development depends upon the chemical composition of the pozzolan, greater the composition of alumina ans silica along with vitreous phase in the material, the better is the pozzolanic reaction and strength display.
