Roman concrete was not a lost secret, and it was not stronger than modern concrete in every sense. It was a carefully made mixture of lime, volcanic material and rubble that suited the arches, vaults and walls Roman builders favoured, and some of it, especially in the sea, has aged remarkably well. The story is partly one of good materials and partly one of survivorship: the structures that remain are the ones that were well built and well placed.
What it was made of
The Roman building term is opus caementicium, and the mixture had three main parts: a binder, an aggregate, and water. The binder was lime, made by burning limestone and then slaking it with water. For the aggregate, builders used fist-sized pieces of stone, broken brick, tile or volcanic rock, the caementa. The ingredient that sets many Roman mixes apart was a volcanic sand, known as pozzolana after the town of Pozzuoli on the Bay of Naples, though similar ash was found in other volcanic regions of Italy. The Vesuvius eruption of 79 CE is the best-known event in that region, but pozzolana deposits there come from far older volcanic activity.
The Roman architect Vitruvius, writing in the first century BCE, praises this volcanic powder as giving buildings strength, including those built in the water. Pliny the Elder, in the first century CE, describes how the material hardened in the sea. Concrete of this type was used from about the second century BCE onward.
How it was used
Roman builders did not pour concrete as a smooth liquid the way a modern crew might. Instead they built a retaining wall or form, usually of brick, stone or timber, then packed in layers of mortar and rubble, sometimes ramming them down. In many buildings the visible facing of brick or cut stone hides a concrete core. The facing could be decorative, and in some cases helped hold the mass while it set.
As Roman expansion created demand for forts, harbours, roads and city buildings in every province, concrete offered a way to build quickly with unskilled labour supervised by a few experts. The material was cheap, could be made by large labour gangs, and could take shapes that stone could not. Concrete vaults and domes opened up wide interior spaces in baths, markets, palaces and basilicas. Many of the large vaulted halls of the Roman public baths owed their form to this technique, and the architecture of Roman apartment blocks relied on concrete and brick to build tall and cheaply, though structures like those had their own risks of collapse and fire.
The Pantheon dome
The dome of the Pantheon in Rome, built under the emperor Hadrian in the early second century CE, is the most famous demonstration of the material. Its interior span is about 43 metres, and it is still described as the largest unreinforced concrete dome in the world. The builders used heavier aggregate low in the structure and lighter material, such as volcanic scoria and pumice, towards the top, and they made the dome thinner and lighter as it rose, with coffers cut into the inner surface and an open oculus at the crown.
The key to its survival is shape and compression. A dome loads its material mostly in compression, which concrete handles well, and it has no steel to rust. Concrete is weaker when pulled or bent, and some cracks have indeed appeared in the dome over time, but the structure has remained serviceable for nearly two thousand years.
Concrete in the sea
Roman harbours offer the strongest evidence for something special in the mix. Engineers built piers, breakwaters and moles by pouring or lowering concrete into forms set in the water, as at Caesarea Maritima in the time of Herod and at Portus near Rome. Core samples from several harbour structures have been studied in recent decades. Researchers have reported rare minerals formed over time as seawater reacted with the volcanic ash and lime, and have suggested that these crystals help bind the concrete together and resist cracking.
A study published in 2023 proposed another explanation for a self-healing quality in some Roman concrete: small white lumps of lime in the mix, which earlier researchers had thought were signs of poor mixing, might have been the result of a hot mixing process using quicklime, and might have filled new cracks when water seeped in. This idea is promising, but it is a hypothesis under discussion and may not apply to every site, so it is best treated as one possible part of the explanation.
Why so much still stands
Several reasons work together, and none alone explains everything.
- Good raw materials. Volcanic ash reacts chemically with lime and water to form durable compounds.
- Compression-friendly design. Arches, vaults and thick walls keep the material squeezed together, which suits its strengths.
- No steel to corrode. Modern reinforced concrete can fail when the embedded steel rusts and expands. Roman concrete contained none.
- Survivorship. Poorly made or poorly sited structures were lost, rebuilt or quarried for stone, and what remains is a selected sample.
- Continuous use and repair. Buildings such as the Pantheon survived because they were kept in use, often as churches.
Not a single recipe
Roman concrete varied by place and period. Builders in regions without volcanic ash used crushed brick or tile to make a similar reactive mortar, and recipes changed over the centuries. Earlier monumental builders, such as those of the Giza pyramid complex, had relied on cut stone instead, while the lime-and-rubble concept was not new either: earlier builders in Egypt and the Near East had used lime and gypsum mortars, as the study of early Egyptian construction shows.
Modern Portland cement was developed in the nineteenth century and is a different material, designed for rapid strength and for use with steel reinforcement. Comparing it with Roman concrete is useful, but the two were made for different purposes, and researchers are interested in the Roman material mainly as a source of ideas for more durable and lower-carbon building.