How did the Romans make concrete?

Dr. dig responds:
The Roman writer Vitruvius in his Ten Books of Architecture, gives us the Roman recipe for concrete, which is for every one part chalk add 2 to 3 parts sand. Mix with 15-20% water (Vitruvius, Book 2, chapter 5). Vitruvius goes on to say that concrete used in the construction of buildings and structures exposed to lots of water, such as bath houses, aqueducts, and harbor construction, should be made with a particular kind of volcanic sand, called pouzzolanic sand, because it was harder and more durable.