The Pantheon in Rome is an example of Roman concrete construction.
The Pantheon in Rome is an example of Roman concrete construction.

Roman concrete, also called opus caementicium, is a material that was used in construction in ancient Rome. Roman concrete was based on a hydraulic-setting cement. It was durable possibly due to its incorporation of pozzolanic ash, which prevents cracks from spreading. Recent research at four universities and labs in the US, Italy and Switzerland has shown that the incorporation of lime clasts allowed the concrete to self-repair cracks.[1] By the middle of the 1st century the material was used frequently, often brick-faced, 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, the world's largest and oldest unreinforced concrete dome.[2]

Roman concrete was normally faced with stone or brick, and interiors might be further decorated by stucco, fresco paintings, or thin slabs of fancy colored marbles. Made up of aggregate and a two-part cementitious system it differs significantly from modern concrete. The aggregates were typically far larger than in modern concrete as well, often amounting to rubble, and as a result it was laid rather than poured.[3] Some Roman concretes were able to be set underwater, which was useful for bridges and other waterside construction.

It is uncertain when Roman concrete was developed,[4] but it was clearly in widespread and customary use from about 150 BC; some scholars believe it was developed a century before that.[5]

Historic references

Caesarea is the earliest known example to have used underwater Roman concrete technology on such a large scale.
Caesarea is the earliest known example to have used underwater Roman concrete technology on such a large scale.
Ruins of the so-called "Temple of Mercury" in Baiae, a Roman frigidarium pool of a bathhouse built in the 1st century BC during the late Roman Republic,[6] containing the oldest surviving concrete dome,[7] and largest one before the Pantheon.[8]
Ruins of the so-called "Temple of Mercury" in Baiae, a Roman frigidarium pool of a bathhouse built in the 1st century BC during the late Roman Republic,[6] containing the oldest surviving concrete dome,[7] and largest one before the Pantheon.[8]

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 (pulvis puteolanus in Latin), the volcanic sand from the beds of Pozzuoli, which are brownish-yellow-gray in colour in that area around Naples, and reddish-brown near Rome. Vitruvius specifies a ratio of 1 part lime to 3 parts pozzolana for cement used in buildings and a 1:2 ratio of lime to pozzolana for underwater work, essentially the same ratio mixed today for concrete used in marine locations.[9][10]

By the middle of the first century AD, 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.[9]

For rebuilding Rome after the fire in 64 AD, which destroyed large portions of the city, Nero's new building code largely called for brick-faced concrete. This appears to have encouraged the development of the brick and concrete industries.[9]

Example of opus caementicium on a tomb on the ancient Appian Way in Rome. The original covering has been removed.
Example of opus caementicium on a tomb on the ancient Appian Way in Rome. The original covering has been removed.

Material properties

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, lime clasts, and brick rubble from the remains of previously demolished buildings.

Gypsum and quicklime 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.[11] Pozzolanic mortar had a high content of alumina and silica. Tuff was often used as an aggregate.[12]

Recent research published in 2023 found that lime clasts, previously considered a sign of poor aggregation technique, react with water seeping into any cracks which develop and thus provide reactive calcium to allow new crystals to form and reseal the cracks.[13] These lime clasts had a specific structure that was most likely created in a "hot-mixing" technique with quicklime rather than traditional slaked lime and which provided the added strength.[14]

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.

The strength and longevity of Roman 'marine' concrete is understood to benefit from a reaction of seawater with a mixture of volcanic ash and quicklime to create a rare crystal called tobermorite, which may resist fracturing. As seawater percolated within the tiny cracks in the Roman concrete, it reacted with phillipsite naturally found in the volcanic rock and created aluminous tobermorite crystals. The result is a candidate for "the most durable building material in human history". In contrast, modern concrete exposed to saltwater deteriorates within decades.[15][16][17]

Crystal structure of tobermorite: elementary unit cell
Crystal structure of tobermorite: elementary unit cell

The Roman concrete at the Tomb of Caecilia Metella is another variation higher in potassium that triggered changes that "reinforce interfacial zones and potentially contribute to improved mechanical performance".[18]

Compressive strengths for modern Portland cements are typically at the 50 megapascals (7,300 psi) level and have improved almost ten-fold since 1860.[19][9] 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.[9][20][21]

Seismic technology

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.[22][9]

Another technology used to improve the strength and stability of concrete was its gradation in domes. One example is the Pantheon, where the aggregate of the upper dome region consists of alternating layers of light tuff and pumice, giving the concrete a density of 1,350 kilograms per cubic metre (84 lb/cu ft). The foundation of the structure used travertine as an aggregate, having a much higher density of 2,200 kilograms per cubic metre (140 lb/cu ft).[23][9]

Modern use

Scientific studies of Roman concrete since 2010 have been gathering media and industry attention.[24] Because of its unusual durability, longevity and lessened environmental footprint, corporations and municipalities are starting to explore the use of Roman-style concrete in North America, replacing the volcanic ash with coal fly ash that has similar properties. Proponents say that concrete made with fly ash can cost up to 60% less because it requires less cement, and that it has a smaller environmental footprint due to its lower cooking temperature and much longer lifespan.[25] Usable examples of Roman concrete exposed to harsh marine environments have been found to be 2000 years old with little or no wear.[26]

In 2013, the University of California Berkeley published an article that described for the first time the mechanism by which the suprastable calcium-aluminium-silicate-hydrate compound binds the material together.[27] During its production, less carbon dioxide is released into the atmosphere than any modern concrete production process.[28] Its disadvantages include the longer drying time and somewhat lower strength than modern concrete, despite its greater durability. It is no coincidence that the walls of Roman buildings are thicker than those of modern buildings. However, Roman concrete was still gaining its strength for several decades after construction had been completed, which is not the case with modern concretes.[29]

See also


  • Adam, Jean-Pierre; Mathews, Anthony (2014). Roman Building. Florence: Taylor & Francis. ISBN 9780203984369.
  • Lancaster, Lynne C. (2009). Concrete Vaulted Construction in Imperial Rome: innovations in context. Cambridge University Press. ISBN 9780521842020.
  • Lechtman, Heather; Hobbs, Linn (1986). "Roman Concrete and the Roman Architectural Revolution". In W.D. Kingery (ed.). Ceramics and Civilization. Vol. 3: High Technology Ceramics: Past, Present, Future. American Ceramics Society. ISBN 091609488X.
  • MacDonald, William Lloyd (1982). The Architecture of the Roman Empire, v.2, an Urban Appraisal. New Haven: Yale University Press. ISBN 9780300034561.


  1. ^ Chandler, David (January 2023). "Riddle solved: Why was Roman concrete so durable?". Massachusetts Institute of Technology.
  2. ^ Moore, David (February 1993). "The Riddle of Ancient Roman Concrete". S Dept. of the Interior, Bureau of Reclamation, Upper Colorado Region. Retrieved 20 May 2013.
  3. ^ Henig, Martin, ed. (1983). A Handbook of Roman Art. Phaidon. p. 30. ISBN 0714822140.
  4. ^ "National Pozzolan Association: The History of Natural Pozzolans". Retrieved 2021-02-21.
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  7. ^ Lancaster 2009, p. 40.
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  10. ^ Vitruvius. De Architectura, Book II:v,1; Book V:xii2.
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  14. ^ Hunt, Katie. "Mystery of why Roman buildings have survived so long has been unraveled, scientists say". CNN. CNN. Retrieved 7 January 2023.
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  16. ^ Jackson, Marie D.; Mulcahy, Sean R.; Chen, Heng; Li, Yao; Li, Qinfei; Cappelletti, Piergiulio; Wenk, Hans-Rudolf (2017). "Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete". American Mineralogist. 102 (7): 1435–1450. Bibcode:2017AmMin.102.1435J. doi:10.2138/am-2017-5993CCBY. ISSN 0003-004X.
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  21. ^ Topical Report ONWI-202 (Report). Battelle Memorial Institute, Office of Nuclear Waste Isolation, Distribution Category UC-70, National Technical Information Service, U.S. Department of Commerce. 1982.
  22. ^ MacDonald 1982, fig. 131B.
  23. ^ K. de Fine Licht, The Rotunda in Rome: A Study of Hadrian's Pantheon. Jutland Archaeological Society, Copenhagen, 1968, pp. 89–94, 134–35
  24. ^ "Fixing Canada's Infrastructure with Volcanoes". Trebuchet Capital Partners Research. 15 October 2015. Retrieved 19 August 2016.
  25. ^ "By 25 BC, ancient Romans developed a recipe for concrete specifically used for underwater work which is essentially the same formula used today". 6 September 2016.
  26. ^ M. D. Jackson, S. R. Chae, R. Taylor, C. Meral, J. Moon, S. Yoon, P. Li, A. M. Emwas, G. Vola, H.-R. Wenk, and P. J. M. Monteiro, "Unlocking the secrets of Al-tobermorite in Roman seawater concrete", American Mineralogist, Volume 98, pp. 1669–1687, 2013.
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