About 2,000 years later, long after the Egyptian civilization had died, the men of Carthage discovered concrete for themselves and built a marvellous aqueduct 70 miles long, through which water was brought to their city. It was carried across a great valley over about 1,000 arches, many of which are still standing in good condition.
To the Romans, however, we are indebted for some of the best examples of ancient concrete work. They used this material in their wonderful city for buildings, bridges, sewers, aqueducts, water mains, and in fact in a great many of the ways that we have seen it is used to-day. The great Coliseum and the Pantheon at Rome are relics of the skill of the ancient architects in the use of concrete.
Although many historians think that the secret of making cementious building material was lost from the fall of Rome until the middle of the eighteenth century, there are ruins of ancient castles which stood in mediæval times in Europe which indicate at least some use of concrete.
The real discoverer of natural cement in our modern times though, was John Smeaton, who will be remembered by the readers of "The Boy's Second Book of Inventions" as the man who built the first rock lighthouse at Eddystone, England, in 1756. In his great work he discovered a kind of limestone with which he could make a cement that would set, or harden, under water. His discovery was hailed as the recovery of the secret of the ancient Romans of making hydraulic cement. It was so called because it would harden under water.
In 1796, Joseph Parker, another Englishman, made what he called Roman cement. Several others followed, and in 1818 natural cement was first made in the United States by Canvass White near Fayetteville, N. Y. The material was made from natural rock and was used in the construction of the Erie Canal.
All of these early cements are called natural cements by engineers nowadays, because they were made from natural rock. It was only necessary to find a clayey limestone which contained a certain percentage of iron oxide and two other minerals known as silica and alumina. The limestone was crushed to a convenient size and was burned in a kiln. The heat turned the stuff into cinders which, when ground to a fine powder and mixed with water, would make a cement that would harden under either air or water very quickly, and last for practically all time. Just for the sake of those who have studied chemistry we will say that in this process the heat drives off the carbon dioxide in the limestone, and the lime, combining with the silica alumina and iron oxide, forms a mass containing mineral properties called silicates, aluminates, and ferrites of lime. These properties mixed with the water make natural cement. In the United States, natural cement was called Rosendale cement, because it was first made commercially in a town of New York State by that name.
The supply of natural cement, however, is limited, because the proper kind of limestone is only found in a few places. Consequently, when an artificial mortar called Portland cement was invented in 1824, the world took a step forward that could not be measured in those days.
Most authorities give the credit for the invention to Joseph Aspdin, a bricklayer of Leeds, England. He took out a patent on the material and in 1825 set up a large factory. In 1828 Portland cement was used in the Thames tunnel, making the first time that the material figured in any big engineering work. In those days even the most enthusiastic supporters of cement little dreamed that in this modern age it would be the material that would make possible such tremendous victories over the obstacles of nature as the Panama Canal, the tunnels under the rivers that surround New York and the great dams that hold back the waters all over the country.
Aspdin, however, is not given the credit for the invention of Portland cement by all authorities, as some claim that Isaac Johnson, also an Englishman, who early in 1912 died at the age of 104, was really the first man to invent a practical, commercial, artificial cement.
The advantage in Portland cement is that it can be made of a number of different kinds of earth, to be found in many different parts of the world, and makes a far stronger rock. It sets more slowly than natural or hydraulic cement, but is more satisfactory for use in reinforced concrete work. In the Lehigh Valley, where about two thirds of the Portland cement used in the United States is made, the raw material is a rock, called cement rock, and limestone. In New York State they make Portland cement of limestone and clay; in the Middle West they make it of marl and clay, while in other Western States they make it of chalk and clay. In Europe slag is sometimes used. The artificial product contains lime oxide, silica, alumina, iron oxide, and other minerals in varying quantities, but the necessary ones are silica, alumina, and lime. In making Portland cement the raw material is ground into a fine powder and poured into one end of a long cylindrical kiln which looks like a smokestack lying on its side. Powdered coal is shot into the kiln, where it is kept burning, at a heat of about 2,500 to 3,000 degrees Fahrenheit. After the raw material has been burned thoroughly and is taken from the kiln it looks like little cinders or clinkers about the size of marbles. The cement clinker is then cooled and ground to a powder, after which it is stored away for a little while to season.