What material revolutionized Roman architecture by enabling large domes and vaults?

The material that revolutionized Roman architecture by enabling large domes and vaults was Roman concrete.

Roman concrete, often called opus caementicium, combined a lime-based mortar with aggregate such as rubble, brick, tufa, or volcanic material. The addition of pozzolanic volcanic ash allowed the mixture to set effectively and helped Roman builders create massive structural forms that were difficult to achieve with cut stone alone.

Concrete made it practical to build thick walls, barrel vaults, groin vaults, and enormous domes. Instead of relying only on columns and beams, Roman architects could enclose broad interior spaces with continuous curved structures. The Pantheon’s dome, spanning about 43.4 metres, remains the most famous surviving example of Roman concrete construction.

Roman concrete was not modern reinforced concrete: it had poor tensile strength and often depended on compression, geometry, and supporting masonry. Tufa, brick, and marble were important Roman building materials, but they did not provide the same transformative structural flexibility.