Complex sentence: The earliest large-scale users of concrete technology were the ancient Romans and concrete was widely used in the Roman Empire--the Colosseum was built largely of concrete and the concrete dome of the Pantheon is the world's largest.
Split into simpler sentences: The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest.