As I am sure many of us know during the second and first century BCE the Romans implemented concrete as a principal building material on a mass scale which resulted in such recognizable structures as the Pantheon, the Colosseum, and the aqueducts to name a few. However, while one may easily be able to find a treasure trove of information on the Roman use of concrete I am hard pressed finding any concrete sources on what the Romans used prior to its implementation. As I currently stand I suspect they used perhaps limestone or possibly marble, although whenever I sure for limestone it immediately directs toward the lime used in concrete mixtures. So with that I am coming to you fine historians of Reddit and hoping you can shed some light on the matter. Your help, and your time is much appreciated.
Very thin bricks, you can see them nowadays as there are many surviving buildings. Once the romans utilised concrete more, bricks were used less. However by the 4th century bricks had made a recovery and the original brick architecture made a staggering increase in the late empire.