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 Cement and concrete might be synonymous as household terms, but are by nature different. Cement, an ultra-fine gray powder, binds sand and rocks into a mass or matrix of concrete. Indeed, cement is the key ingredient of concrete. Semantics aside, concrete is the signature material in driveways, patios, basements, and a host of other American household items. It is also the world's most widely used building material. Annual global production of concrete hovers around 5 billion cubic yards, a volume approximated by yearly cement production levels of about 1.25 billion tons. Concrete's global appeal is not accidental - the ubiquitous, stone-like material is produced from some of the world's most abundant resources, as is cement. Early History and Development of Portland Cement Ever since civilizations first started to build; we've sought a material that would bind stones into a solid, formed mass. The Assyrians and Babylonians used clay for this purpose, and the Egyptians advanced to the discovery of lime and gypsum mortar as a binding agent for building such structures as the Pyramids. The Greeks made further improvements and finally the Romans developed a cement that produced structures of remarkable durability. Most of the building foundations in the Roman Forum were constructed of a form of concrete, placed in some locations to a depth of 12 feet. The great Roman baths built about 27 B.C., the Coliseum, and the huge Basilica of Constantine are examples of early Roman architecture in which cement mortar was used. Roman Formula The secret of Roman success in making cement was traced to the mixing of slaked lime with pozzolana, a volcanic ash from Mount Vesuvius. This process produced a cement capable of hardening under water. During the Middle Ages this art was lost and it was not until the scientific spirit of inquiry revived that we rediscovered the secret of hydraulic cement -- cement that will harden under water. Repeated structural failure of the Eddystone Lighthouse off the coast of Cornwall, England, led John Smeaton, a British engineer, to conduct experiments with mortars in both fresh and salt water. In 1756, these tests led to the discovery that cement made from limestone containing a considerable proportion of clay would harden under water. Making use of this discovery, he rebuilt the Eddystone Lighthouse in 1759. It stood for 126 years before replacement was necessary. Other men experimenting in the field of cement during the period from 1756 to 1830 include L. J. Vicat and Lesage in France and Joseph Parker and James Frost in England. Before portland cement was discovered and for some years after its discovery, large quantities of natural cement were used. Burning a naturally occurring mixture of lime and clay produced natural cement. Because the ingredients of natural cement were mixed by nature, its properties varied as widely as the natural resources from which it was made. |
