Concrete
![Exterior of the Roman Pantheon, finished 128 AD, still the largest unreinforced solid concrete dome.[1]](/Images/godic/202501/07/Pantheon_dome1650.jpg")
![Interior of the Pantheon dome, seen from beneath. The concrete for the coffered dome was laid on moulds, probably mounted on temporary scaffolding.](/Images/godic/202501/07/Pantheon_cupola1650.jpg")
![Opus caementicium exposed in a characteristic Roman arch. In contrast to modern concrete structures, the concrete used in Roman buildings was usually covered with brick or stone.](/Images/godic/202501/07/Museo_Foro_Caesaragusta_-_Cloaca_del_foro_031650.JPG")
![Smeaton's Tower](/Images/godic/202501/07/Smeaton's_Lighthouse001650.jpg")
Concrete is a composite material composed of coarse aggregate bonded together with a fluid cement which hardens over time. Most concretes used are lime-based concretes such as Portland cement concrete or concretes made with other hydraulic cements, such as ciment fondu. However, road surfaces are also a type of concrete, asphalt concrete, where the cement material is bitumen, and polymer concretes are sometimes used where the cementing material is a polymer.