Natural Pozzolans are an ancient form of cementious material and have been used for thousands of years. They have been occurring naturally since, well...practically forever, at least since there was life on earth. Called Pozzolan for the village of Pozzuoli where great volcanic eruption from Mount Vesuvius had deposited a large volume of volcanic ash in 79 AD. Pozzuoli, located near Naples, made the most of this misfortune and beginning mining this material which was an already proven material with cementious properties. Pozzolan was likely the first cementious material being used in the ancient world. Timelines show uses of such materials dating back to 3000 BC when both the Eqyptians and Chinese were incorporating them inmajor infrastructure projects and beyond. I have heard, but can not yet verify, that when the very first civilizations began taking shape, around 6500 BC, in the area of what is now Syria and Jordan, it is likely that they were crafting their buildings, which can still be found today, with mortars consisting of Natural Pozzolans.
Today Natural Pozzolans have a use in concrete that is very similar to that of fly-ash.
They can be used as a partial replacement for the cement often in the range of 15% to 35% (per PCA Concrete Manual). They can improve sulfate resistance an can help manage the reactivity to alkali-silica. The physical nature of Pozzolans helps to reduce permeability.
Natural Pozzolans that you are likely to come across are usually processed materials. They are heat treated in a kiln and are then pulverised into a fine powder. These Pozzolans include Calcined Clay, Calcined Shale, and Metakaolin
Natural Pozzolans are classified by ASTM C 618
I have worked with a few companies that were both developing and using Natural Pozzolans. They seemed to have a Pozz-itive effect.
For more information on Natural Pozzolans and other Cementious Materials, Look Here:
http://www.ce.memphis.edu/1101/notes/concrete/PCA_manual/
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