- Pozzolan
A pozzolan is a material which, when combined with
calcium hydroxide , exhibitscement itious properties. Pozzolans are commonly used as an addition (the technical term is "admixture") toPortland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete. Pozzolans are primarilyvitreous siliceous materials which react with calcium hydroxide to formcalcium silicate s; 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 that 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 assilica fume and high reactivitymetakaolin 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 isfly ash , thoughsilica fume , high-reactivitymetakaolin ,ground granulated blast furnace slag , and other materials are also used as pozzolans.A pozzolan is a siliceous or aluminosiliceous material, which is highly vitreous. This material independently has few/fewer cementitious properties, but in the presence of a lime-rich medium like
calcium hydroxide , shows better cementitious properties towards the later day strength (> 28 days).The mechanism for this display of strength is the reaction of silicates withlime to form secondary cementitious phases (calcium silicate hydrates with a lower C/S ratio) which display gradual strengthening properties usually after 7 days.The extent of the strength development depends upon the chemical composition of the pozzolan: the greater the composition of
alumina andsilica along with the vitreous phase in the material, the better the pozzolanic reaction and strength display.ee also
*
Pozzolana
*Portland cement
*Sodium silicate
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