What Was Roman Concrete? The Secret of Rome's Buildings
Roman concrete — opus caementicium — was the building material of the empire. Its recipe of lime, pozzolana, and aggregate, and its mysterious ability to grow stronger in seawater, are still studied by modern engineers.
Roman concrete — Latin opus caementicium — was the building material of the Roman Empire, used in everything from the foundations of the humblest house to the dome of the Pantheon, the largest unreinforced concrete dome in the world. The recipe was simple: lime, volcanic ash (called pozzolana, from the town of Pozzuoli near Naples, where the Romans quarried it), water, and an aggregate of broken stone, brick, or rubble. The mixture was poured into wooden forms, allowed to set, and resulted in a stone-like material that was, in many respects, superior to modern Portland cement. For the broader context, see Roman engineering and architecture.
Roman concrete is the reason that the Romans were able to build on a scale that had not been seen before and would not be seen again for a thousand years. The Aqua Claudia, the great aqueduct of Rome, was built of Roman concrete and has stood for nearly two thousand years. The Colosseum was faced with stone but was built on a concrete skeleton. The Pantheon dome, 43 meters in diameter, has stood for nearly two thousand years, the largest unreinforced concrete dome in the world. The harbors of the Roman Empire, from Caesarea in Judea to the moles of Ostia, were built of Roman concrete, and the seawater structures are still intact after two thousand years.
The Recipe
The Roman recipe for concrete is described by the architect Vitruvius, who wrote his De Architectura around 25 BCE, and by the naturalist Pliny the Elder, who wrote his Naturalis Historia around 77 CE. Vitruvius recommended a mix of one part lime, three parts pozzolana, and five parts aggregate.
The lime was not the modern quicklime, but a “hot mix” lime, made by slaking quicklime with water and using it wet, often in a hot mortar. This hot-mix technique is one of the lost secrets of Roman concrete. Modern analysis has shown that the Roman hot-mix lime produced very small particles of lime that were dispersed through the concrete, and these particles could react with the pozzolana and the water to form the binding matrix.
The pozzolana was the key. Pozzolana is a volcanic ash, rich in silica and alumina, that reacts with lime and water in a pozzolanic reaction to form a hard, stable binder. The Romans obtained their pozzolana from the Bay of Naples region, where the volcanoes of Vesuvius, Campi Flegrei, and other volcanic centers had deposited enormous quantities of ash. For the transport network that carried it, see Roman roads.
The aggregate was whatever was cheap and locally available — broken travertine in Rome itself, broken brick, broken pottery, or river gravel elsewhere.
Opus Caementicium
The Romans used the concrete in two main ways. The first was opus caementicium in the strict sense: a mass of concrete poured into a form, with no facing. This was used for foundations, walls, and piers.
The second was faced concrete, in which a structural core of opus caementicium was faced with a more decorative material — stone, brick, or tile. The Romans developed a series of facing techniques, including opus quadratum (squared stone blocks), opus incertum (irregular stone pieces), opus reticulatum (small square stones set in a diagonal pattern), opus testaceum (thin bricks), and opus mixtum (a combination of bricks and stone).
The most important development was the use of light-weight aggregate for vaults and domes. For the dome of the Pantheon, built by Hadrian around 125 CE, the engineers used a graduated concrete: heavy travertine at the base, lighter tufa in the middle, and very light pumice at the top. The varying density kept the thrust of the dome within the strength of the supporting walls, and the Pantheon has stood for two thousand years.
Why Roman Concrete Gets Stronger in Seawater
The most surprising property of Roman concrete is that it gets stronger in seawater. Modern Portland cement, in seawater, suffers from a slow degradation called “sulfate attack,” in which the minerals in the cement react with the sulfates in the seawater to form expansive minerals that crack the concrete. Roman concrete, by contrast, gets stronger.
In 2017, a team of researchers led by Marie Jackson of the University of Utah published a study in the journal American Mineralogist explaining the phenomenon. The Romans used a hot-mix lime, made by slaking quicklime with water at high temperature. When the concrete was placed in seawater, the pozzolana, the lime, and the seawater reacted to form a mineral called aluminous tobermorite, which grows as long, needle-like crystals in the pores of the concrete. The aluminous tobermorite crystals reinforce the concrete and prevent the sulfate attack that destroys modern concrete.
The discovery has revived interest in Roman concrete. Roman-style concretes are increasingly being studied as a low-carbon alternative to modern Portland cement, which is responsible for about 5 to 8 percent of global carbon dioxide emissions.
The Comparison to Portland Cement
Portland cement is the modern equivalent of Roman concrete. It was patented in 1824 by the English bricklayer Joseph Aspdin, and it is made by heating limestone and clay to a high temperature, grinding the resulting clinker, and mixing it with gypsum. Portland cement is much stronger than Roman concrete in the short term, but it is also more brittle, and it does not have the self-healing properties of Roman concrete.
Modern concrete is reinforced with steel rebar, which carries the tensile loads that the concrete itself cannot carry. Roman concrete was unreinforced, and the Roman engineers worked around the tensile weakness of the material by using arches, vaults, and domes, which are mostly in compression. The result is a different aesthetic. Modern concrete is thin, light, and tensile; Roman concrete is thick, heavy, and compressive. For the buildings that this material made possible, see the Colosseum and Roman engineering and architecture.
The Survival of Roman Concrete
The durability of Roman concrete is striking. Many Roman concrete structures are still in use today, two thousand years after they were built. The Aqueduct of Segovia in Spain, built around 100 CE, is still standing. The Pont du Gard in France, the great bridge-aqueduct of Nîmes, is still standing. The harbors of Caesarea Maritima in Israel, built under Herod the Great around 22 BCE, are still under water. The Pantheon in Rome is still in use, the largest unreinforced concrete dome in the world.
The Romans’ concrete is, in a real sense, a lost technology. We have the recipe and a fairly good understanding of the chemistry, but we do not have the practical knowledge to build with it on the same scale. The hot-mix lime, in particular, is difficult and dangerous to work with, and the Roman masons learned their craft through a long apprenticeship.