The concrete was re-discovered in the year 1757. The Romans used it long before.
Concrete is a building material made of a cement and fillers or aggregates, mixtures of sand and rock giving the material substance. Cement, in the form of limestone mortar, was used through history. Concrete, however, has gone in and out of history.
The differences between Roman and modern concrete aren’t primarily a consequence of different formulas. They’re more a product of technique. In Roman concrete buildings, the concrete was poured in smaller quantities (they didn’t have mechanized pumps and concrete mixers, after all), and they spent a significant amount of labor compressing it.
The Romans produced concrete which is denser and therefore somewhat more durable than what we make today.
As the authors of the 2017 study that analyzed Roman concrete point out, the Romans "spent a tremendous amount of work [on developing Roman concrete] – they were very, very intelligent people.”
As Pliny the Elder wrote, Roman concrete was "impregnable to the waves." No wonder then that the Ancient Romans were pioneers in the principles of underwater construction.
By the first century, Roman engineers started to build constructions, such as breakwaters, that could withstand the power of the sea — many of which still stand today. The city of Caesarea (built approx. 25 BCE) is the earliest known example of a large-scale underwater Roman concrete building site. Today it is part of a natural park in Israel.
When they had developed the concrete to the required strength, they were perfectly aware of their accomplishments. As the study explains, Pliny the Elder wrote in his Natural History that it is “impregnable to the waves and every day stronger”.
The strength of Roman concrete will likely have been made through a culmination of trial and error, experimentation, and luck. Romans were fortunate in the type of rock they had to work with. They observed that volcanic ash grew cement to produce the tuff. We don’t have those rocks in a lot of the world, so there would have to be substitutions made.
The use of concrete never quite entirely vanished, though it did fall mostly into disuse after the collapse of the Roman empire in the west.
Demand for expensive stone buildings dropped to nearly nothing and at any rate the political fragmentation caused by the fall of the empire meant it was difficult to ship one of the key ingredients, pozzolano stone from certain locations in Italy, to anywhere else.
Since most masons stopped using concrete, this meant that they were no longer able to pass that knowledge on to their apprentices, since on-the-job training and learning by experience were how technical trades worked at the time. There are, however, scattered examples of people working with concrete here and there through the Middle Ages and early modern period.
Smeaton’s Tower in the 1750s is one of the first major post-Roman uses of concrete, but the Canal du Midi, opened in 1670, predates it by a considerable margin, and there are a few others here and there.