Roman concrete was an early, very similar form of what is used today. It was used in construction during the late Roman Republic through the history of the Roman Empire.
The concrete was based on a hydraulic setting cement with most qualities the same as modern cement. By the time the 1st century AD came around, it was already used as frequently as brick. It became so big there was even an event known as the Concrete Revolution.
Roman concrete, like modern concrete, consists of a mix of hydraulic mortar, which is a binder mixed with water. The Roman mix had some variations, and included, rock, ceramic, and brick.
Gypsum and lime often made up the binders, as were volcanic dusts. This is all that’s really known of the Roman concrete today. Things like the water/cement ratios are lost to time.