Roman concrete, also called opus caementicium, was a material used in construction during the late Roman Republic through the whole history of the Roman Empire. Roman concrete was based on a hydraulic-setting cement with many material qualities similar to modern portland cement. By the middle of the 1st century, the material was used frequently as brick-faced concrete, although variations in aggregate allowed different arrangements of materials. Further innovative developments in the material, called the Concrete Revolution, contributed to structurally complicated forms, such as the Pantheon dome. Roman concrete was also used to make roads.
Vitruvius, writing around 25 BC in his Ten Books on Architecture, distinguished types of aggregate appropriate for the preparation of lime mortars. For structural mortars, he recommended pozzolana, which are volcanic sands from the sandlike beds of Pozzuoli brownish-yellow-gray in color near Naples and reddish-brown at Rome. Vitruvius specifies a ratio of 1 part lime to 3 parts pozzolana for cements used in buildings and a 1:2 ratio of lime to pulvis Puteolanus for underwater work, essentially the same ratio mixed today for concrete used at sea.
By the middle of the 1st century, the principles of underwater construction in concrete were well known to Roman builders. The City of Caesarea was the earliest known example to have made use of underwater Roman concrete technology on such a large scale.
Rebuilding Rome after the fire in 64 AD, which destroyed large portions of the city, the new building code by Nero consisted of largely brick-faced concrete. This appears to have encouraged the development of the brick and concrete industries.
In most usage, the raw concrete surface was considered unsightly and some sort of facing was applied. Different techniques were characteristic of different periods and included:
- Opus incertum: small irregular stones.
- Opus reticulatum: small squared tuff blocks laid in a diamond pattern.
- Opus quadratum: regularly laid courses of ashlars.
- Opus latericium: regularly laid courses of brick.
- Opus spicatum: brick laid in a herringbone pattern.
- Opus vittatum: square tuff blocks intersected by brick bands at regular and irregular distances.
- Opus africanum: vertical chains of upright blocks with alternating horizontal blocks.
- Opus testaceum: thick horizontal brick work.
Roman concrete, like any concrete, consists of an aggregate and hydraulic mortar – a binder mixed with water that hardens over time. The aggregate varied, and included pieces of rock, ceramic tile, and brick rubble from the remains of previously demolished buildings. Reinforcing elements, such as steel rebar, were not used.
Gypsum and lime were used as binders. Volcanic dusts, called pozzolana or "pit sand", were favored where they could be obtained. Pozzolana makes the concrete more resistant to salt water than modern-day concrete. The pozzolanic mortar used had a high content of alumina and silica.
Concrete, and in particular, the hydraulic mortar responsible for its cohesion, was a type of structural ceramic whose utility derived largely from its rheological plasticity in the paste state. The setting and hardening of hydraulic cements derived from hydration of materials and the subsequent chemical and physical interaction of these hydration products. This differed from the setting of slaked lime mortars, the most common cements of the pre-Roman world. Once set, Roman concrete exhibited little plasticity, although it retained some resistance to tensile stresses.
The setting of pozzolanic cements has much in common with setting of their modern counterpart, portland cement. The high silica composition of Roman pozzolana cements is very close to that of modern cement to which blast furnace slag, fly ash, or silica fume have been added.
Compressive strengths for modern portland cements are typically at the 50 MPa level and have improved almost ten-fold since 1860. There are no comparable mechanical data for ancient mortars, although some information about tensile strength may be inferred from the cracking of Roman concrete domes. These tensile strengths vary substantially from the water/cement ratio used in the initial mix. At present, there is no way of ascertaining what water/cement ratios the Romans used, nor are there extensive data for the effects of this ratio on the strengths of pozzolanic cements.
For an environment as prone to earthquakes as the Italian peninsula, interruptions and internal constructions within walls and domes created discontinuities in the concrete mass. Portions of the building could then shift slightly when there was movement of the earth to accommodate such stresses, enhancing the overall strength of the structure. It was in this sense that bricks and concrete were flexible. It may have been precisely for this reason that, although many buildings sustained serious cracking from a variety of causes, they continue to stand to this day.
Another technology used to improve the strength and stability of concrete was its gradation in domes. One example included the Pantheon, where the aggregate of the upper dome region consisted of alternating layers of light tuff and pumice, giving the concrete a density of 1,350 kg/m3. The foundation of the structure used travertine as an aggregate, having a much higher density of 2,200 kg/m3.
Use by other ancients
In the Philippines, large monuments and burial tombs were constructed using limestone blocks. They were cemented together with a form of ancient cement. After completing a structure or sculpture, the entire work was coated. Within Tabon Caves sculptures of elephants may have been made in the same manner. Research to answer questions this raises will be conducted at a new college in Palawan. 
- Jean-Pierre Adam, Anthony Mathews, Roman Building, 1994
- Lynne C. Lancaster, Concrete Vaulted Construction in Imperial Rome, Cambridge University Press, 2005
- Heather N. Lechtman & Linn W. Hobbs, “Roman Concrete and the Roman Architectural Revolution,” Ceramics and Civilization Volume 3: High Technology Ceramics: Past, Present, Future, edited by W.D. Kingery and published by the American Ceramics Society, 1986
- W. L. MacDonald, The Architecture of the Roman Empire, rev. ed. Yale University Press, New Haven, 1982
- Moore, David (February 1993). "The Riddle of Ancient Roman Concrete". S Dept. of the Interior, Bureau of Reclamation, Upper Colorado Region. www.romanconcrete.com. Retrieved 20 May 2013.
- Heather Lechtman and Linn Hobbs "Roman Concrete and the Roman Architectural Revolution", Ceramics and Civilization Volume 3: High Technology Ceramics: Past, Present, Future, edited by W.D. Kingery and published by the American Ceramics Society, 1986; and Vitruvius, Book II:v,1; Book V:xii2
- Lechtman and Hobbs "Roman Concrete and the Roman Architectural Revolution"
- Wayman, Erin. “The Secrets of Ancient Rome’s Buildings.” Smithsonian.com. 16 November 2011. Retrieved 24 April 2012.
- N. B. Eden and J.E. Bailey, "Mechanical Properties and Tensile Failure Mechanism of a High Strength Polymer Modified Portland Cement," J. Mater. Sci., 19, 2677–85 (1984); and Lechtman and Hobbs "Roman Concrete and the Roman Architectural Revolution"
- Lechtman and Hobbs "Roman Concrete and the Roman Architectural Revolution"; see also: C. A. Langton and D. M. Roy, "Longevity of Borehole and Shaft Sealing Materials: Characterization of Ancient Cement Based Building Materials," Mat. Res. Soc. SYmp. Proc. 26, 543–49 (1984); and Topical Report ONWI-202, Battelle Memorial Institute, Office of Nuclear Waste Isolation, Distribution Category UC-70, National Technical Information Service, U.S. Department of Commerce, 1982.)
- W. L. MacDonald, The Architecture of the Roman Empire, rev. ed. Yale University Press, New Haven, 1982, fig. 131B; Lechtman and Hobbs "Roman Concrete and the Roman Architectural Revolution"
- K. de Fine Licht, The Rotunda in Rome: A Study of Hadrian's Pantheon. Jutland Archaeological Society, Copenhagen, 1968, pp. 89–94, 134–35; and Lechtman and Hobbs "Roman Concrete and the Roman Architectural Revolution"
- Joy Tabuada (16 July 2015). "HTU named as research center for ancient Palawan man". Palawan News. Retrieved 17 July 2015.