2. Roman Cement
Modern concrete was developed in the 1700s, and today the simple mixture of cement, water, sand, and rocks is the most widely used building material in the world. But the recipe developed in the 18th century wasn’t the first time concrete was invented. In fact, concrete was widely used throughout antiquity by the Persians, Egyptians, Assyrians, and Romans. The Romans in particular made extensive use of concrete, and they were responsible for first perfecting the recipe by mixing burnt lime with crushed rocks and water. Their mastery of its use allowed them to build many of their most famous structures, among them the Pantheon, the Colosseum, the aqueducts, and the Roman Baths.
How was it Lost?
Like so many technologies of the Greeks and Romans, the recipe for concrete was lost during the descent into the Dark Ages, but just why remains a mystery. The most popular theory is that the recipe was something of a trade secret among stonemasons, and that the method for making cement and concrete died along with those who knew it. Perhaps even more interesting than the disappearance of Roman cement are the particular qualities that separate it from more modern Portland cement, which is the most common type of cement used today. Structures built with Roman cement, like the Colosseum, have managed to weather thousands of years of punishment from the elements and remain standing, but buildings constructed with Portland cement have been known to wear down much faster. This has been theorized to be the result of different chemicals that the Romans added to their cement, among them milk and even blood. These were said to create air bubbles within the concrete that helped the material to expand and contract in the heat and cold without damaging itself.