How did Roman architects use concrete to expand the possibilities of arches, vaults, and domes?

Roman architects revolutionized Western architecture by utilizing opus caementicium, a durable concrete made from a mixture of lime, sand, water, and pozzolana volcanic ash. Unlike traditional stone construction, which relied on trabeated (post-and-lintel) systems that limited interior spans, the liquid nature of concrete allowed architects to create complex, curved forms in both plan and section.12

This material enabled the construction of massive, uncluttered interior spaces spanned by arches, vaults, and domes. Because concrete could be poured into wooden forms, it freed builders from the structural constraints of stone blocks, allowing for unprecedented architectural scale. Notable examples include:21

• The Pantheon: Featuring a massive concrete dome 43.2 meters in diameter, supported by thick, brick-faced concrete walls.2
• Large-scale vaults: Structures like the Baths of Diocletian and the Basilica of Constantine utilized concrete to achieve spans of 26 meters, far exceeding the capabilities of stone-frame construction.2

By enabling these expansive, column-free interiors, concrete became the primary tool for expressing the power of the imperial state through monumental architecture.2