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 described in 1909 by Potter who stated that over 5000 tons of it, known as Potter’s Red Cement were used in fresh water and sea water construction at that time. For a period of many years upto about 1915 the Lafarge Company in France manufactured an ‘undecomposable’ cement composed of a mixture of burnt clay and the French grappiers cement. The use of a lime-surkhi mortar, prepared by grinding lime and crushed bricks or burnt clay together in a mortar mill, was long been common practice in India Sub-continent, where it was formed the cementing material in dams and structures under water. In the construction of the Sennar Dam on the Blue Nile in 1919-1925 the masonry was set in cement composed of 70 percent portland cement and 30 percent burnt clay.