Pozzolana from Burnt Clays and Shales
The pozzolanic properties of burnt clays were well known to the Romans, who utilized ground clay bricks and tiles as a substitute for the natural volcanic ash pozzolanas. The value of burnt clay as an addition to fat lime mortars to obtain hydraulic properties has also long been known in India and Egypt, where it passes under the names Surkhi and Homra. Experiments on burnt clay-lime mortars were carried out a century ago in connection with the rebuilding of London bridge, while still earlier Smeaton had tried them, without success, as a mortar ingredient for the Eddystone lighthouse. A Swedish engineer, Bagge of Gothenburg and Count Chaptal in France, both experimented in the eighteenth century with burnt-clay pozzolana. Vicat mentioned burnt-clay pozzolanas in papers published in 1843 and 1857, and they were discussed at some length in Captain Smith’s translation of Vicat’s Mortiers et Ciment Calcaries. A pozzolanic cement consisting of ground burnt clay and portland cement was desc...