Short answers, pulled from the story.
The Roman architectural revolution is also known as the concrete revolution. The name refers to the widespread adoption of the arch, vault, and dome in Roman architecture, made possible by the invention of Roman concrete, called opus caementicium.
The first use of arches in Roman architecture was for civic structures such as drains and city gates. Arches were later applied to major civic buildings, bridges, and aqueducts, with outstanding 1st century AD examples including the Colosseum, the Pont du Gard, and the aqueduct of Segovia.
Roman concrete, known as opus caementicium, was the crucial factor in the architectural revolution because it freed builders from the constraints of stone and brick. It allowed the construction of curved forms such as barrel vaults and domes and shifted architectural interest toward shaping interior space.
During the Age of Augustus, the Mausoleum in Campus Martius was built almost entirely of concrete using concentric rings as structural support. The Theatre of Marcellus, dedicated to Augustus's nephew, was also completed during his reign and used volcanic stones such as Tuscolo tuff and Tufo Lionato as aggregates in pozzolanic concretes.
The peak of Hadrian's architectural program includes Hadrian's Villa, the Pantheon in Rome, and the so-called temples and the villa of Pisoni at Baiae. These buildings are considered masterpieces of architecture and are noted for complex ground plans, striking curved shapes, and innovations such as segmented domes raised on drums with windows.
The Roman architectural revolution spread across the entire Roman Empire, from Syria to Scotland. Engineers throughout this range erected structures using semicircular arches as a consistent building form.