Pilae
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Pilae is the Latin word the ancient Romans used for "piers", "piles" or "pylons", vertical pillars often used to support structures such as hypocausts.
Wall painting from Stabiae showing harbour with pilae, 1st century
Pilae from Portus Julius, Baiae
Roman concrete
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Roman concrete, also called opus caementicium, was used in construction in ancient Rome. Like its modern equivalent, Roman concrete was based on a hydraulic-setting cement added to an aggregate.
The Pantheon in Rome is an example of Roman concrete construction.
Caesarea harbour in Roman Judaea, an example of underwater Roman concrete technology on a large scale
The "Temple of Mercury" in Baiae, a Roman frigidarium pool of a bathhouse built in the 1st century BC containing the oldest surviving concrete dome, and largest one before the Pantheon.
Example of opus caementicium on a tomb on the ancient Appian Way in Rome. The original covering has been removed.