Hydraulic Cement: Lost and Found
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HISTORICAL NOTE
Hydraulic Cement: Lost and Found In 1756, John Smeaton, a British engineer, was presented with the challenge of building a sturdy lighthouse on a site where two earlier lighthouses had fallen. The location was Eddystone, off the coast of Cornwall, where navigating conditions were treacherous and shipwrecks frequent. The previous lighthouses, each made of wood, had collapsed in stormy weather, and Smeaton was determined to use stone this time. His problem was finding a cement that would endure under water—a hydraulic cement. Since the method of making such a material was not well understood, Smeaton undertook a systematic study of hydraulic cements. The first hydraulic cements are attributed to the Greeks, whose addition of a volcanic tuff from the Island of Santorini (so-called Santorin earth) to burned limestone produced cement that could harden under water. The ancient Romans added a tuff called “pozzolana” from the region of Pozzuoli near Naples at the base of Mt. Vesuvius. This “pozzolana cement” was used to hold up structures that are still standing such as the Coliseum in Rome and the Pont du Gard aqueduct at Nîmes in France. In general, the use of a strong mortar made the construction of thinner walls possible. Cement can thus be credited with inspiring some of the graceful architecture of Roman civilization. But no one knew why pozzolana cement worked as it did. Clearly, some material in the volcanic-earth additive turned a nonhydraulic cement into a hydraulic one, but chemistry had not progressed far enough to provide an analysis. A Roman writer named Vitruvius described the Roman methods of making cement, but his speculations on the mechanisms at work were burdened by the alchemical theories popular at the time. Nonetheless, his surviving writings are credited by some for the later revival of hydraulic cements. When the Roman Empire fell, the “secret” of hydraulic cement disappeared with it. Cements continued to be used, but the omission of volcanic earth, and the general lack of care in burning the lime to a sufficiently high temperature, rendered these mortars nonhydraulic. The quality of the cement generally depended on the chemistry of the limestone in the local quarry. If by chance the limestone contained some of the silica and alumina compounds found in the volcanic tuff, the cement might be of high quality, and at least semihydraulic. But those with little of these cementitious materials would not hold up under water. This state of affairs 726
continued until the 18th century. So Smeaton knew of the ancient hydraulic cements at the time of his Eddystone assignment, but was generally surrounded by inferior nonhydraulic varieties. The only contemporary example he could follow was that of Holland, a notoriously water-logged country that had succeeded in building sea defenses by combining “tarras” (a local type of pozzolana) with lime to form a hydraulic cement widely known as “tarras mortar.” Before beginning his experiments, Smeaton toured Holland in 1754–1755. He saw that the Dutch
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