The exact method for making Roman concrete was lost, but has been reconstructed and demonstrated in recent years. It’s analogous to, but very different from, the method Davidovits has reconstructed for Old Kingdom Egypt. Check out this one:
http://www.monolithic.com/thedome/pantheon/index.html
“...when Michelangelo first saw the Pantheon in the early 1500s, he proclaimed it of ‘angelic and not human design.’ Surprisingly, at that point, this classic Roman temple, converted into a Christian church, was already more than 1350 years old... Though the Romans had been building with concrete since about 200 BC, work on the Pantheon was difficult and proceeded in gradual stages... Vitruvius (cir. 20 BC), a noted Roman architect, recorded the process followed in his day, that was probably still used by the Pantheon’s builders. The ancients hand mixed wet lime and volcanic ash in a mortar box, adding very little water so that they got a nearly dry composition. They carried this mixture to the job site in baskets and poured it over a prepared layer of rock pieces. They then tamped the mortar into the rock layer. The tamping packed the mortar, reduced the need for excess water, but, at the same time, stimulated bonding... Eventually, work began on the concrete dome, constructed in tapering courses or steps that are thickest at the base (20 feet) and thinnest at the oculus (7.5 feet). The Romans used the heaviest aggregate, mostly basalt, at the bottom and lighter materials, such as pumice, at the top. They embedded empty clay jugs into the dome’s upper courses to further lighten the structure and facilitate the concrete’s curing.”