Cement is the glue that holds mortar and concrete together and may have a range of different chemical compositions. The earliest cements were simply quicklime, which when mixed with water and stone would set to form a type of concrete.
The ancient Egyptians used a cement based on burnt gypsum and the Greeks also used lime cements. The Romans discovered that if a particular type of volcanic ash (found near Pozzuoli) was added to the lime, the cement has stronger and set both in air and under water. This material acquired the name ‘pozzolana’. A similar material could be obtained by adding ground up tiles or pottery to the lime, if the volcanic ash was not available. Lime cement continued to be used throughout the middle ages, sometimes including pozzolana or ‘trass’.
John Smeaton discovered in 1756 that if the limestone used to prepare the lime contained a high proportion of clay material, the cement would harden better under water. This was hydraulic lime. A successful hydraulic cement known as ‘Roman Cement’ was also made by calcining ‘Septarian Nodules’ (a limestone containing clay impurities) found along the Thames Estuary. Other natural cements were made in France and America from calcining similar rock compositions.
Louis Vicat conducted experiments on mixtures of chalk and clay ground together in a wet paste and then calcined , which can be considered as the forerunner of today’s process for making Portland cement.