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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. Many buildings and structures still standing today, such as bridges, reservoirs and aqueducts, were built with this material, which attests to both its versatility and its durability. It was previously thought that its strength was enhanced by the incorporation of pozzolanic ash where available (particularly in the Bay of Naples). The addition of ash prevented cracks from spreading. However, research in 2023 has shown that the incorporation of mixtures of different types of lime, forming conglomerate "clasts" allowed the concrete to self-repair cracks, which was more likely the case. Roman concrete was in widespread use from about 150 BC; some scholars believe it was developed a century before that. Further innovative developments in the material, part of the so-called concrete revolution, contributed to structurally complicated forms. The most prominent example of these is the Pantheon dome, the world's largest and oldest unreinforced concrete dome. Roman concrete differs from modern concrete in that the aggregates often included larger components; hence, it was laid rather than poured. Roman concretes, like any hydraulic concrete, were usually able to set underwater, which was useful for bridges and other waterside construction.

The Romans first used hydraulic concrete in coastal underwater structures, probably in the harbours around Baiae before the end of the 2nd century BC. The harbour of Caesarea is an example (22-15 BC) of the use of underwater Roman concrete technology on a large scale, for which enormous quantities of pozzolana were imported from Puteoli. 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. A building site from 79 CE was uncovered at Pompeii in 2025, which contained unmixed Roman concrete building materials as well as the concreted structures.

Roman concrete, like any concrete, consists of an aggregate and hydraulic mortar, a binder mixed with water that hardens over time. Pozzolana makes the concrete more resistant to salt water than modern-day concrete. 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 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".

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).

Scientific studies of Roman concrete since 2010 have attracted both media and industry attention. Usable examples of Roman concrete exposed to harsh marine environments have been found to be 2000 years old with little or no wear. However, Roman concrete was still gaining its strength for several decades after construction had been completed. Roman brick – Style of brick used in Ancient Roman architecture Roman cement – Cement made by burning septaria, unrelated to ancient Rome "Roman Seawater Concrete Holds the Secret to Cutting Carbon Emissions".

Quick Facts

• Like its modern equivalent, Roman concrete was based on a hydraulic-setting cement added to an aggregate.
• Roman concrete differs from modern concrete in that the aggregates often included larger components; hence, it was laid rather than poured.
• Roman concrete, like any concrete, consists of an aggregate and hydraulic mortar, a binder mixed with water that hardens over time.
• Roman concretes, like any hydraulic concrete, were usually able to set underwater, which was useful for bridges and other waterside construction.
• However, research in 2023 has shown that the incorporation of mixtures of different types of lime, forming conglomerate "clasts" allowed the concrete to self-repair cracks, which was more likely the case.

Source material: Wikipedia - "Roman concrete". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.