Why Has Roman Concrete Lasted 2,000 Years?

Lime, ash and rubble

The Romans called it opus caementicium. It was made of three things: lime, a volcanic ash now called pozzolana (after Pozzuoli, near Naples, where it was first found), and an aggregate of rubble, broken tuff, brick or travertine. Water started the reaction, and the mix hardened into a mass that behaved like a single stone. By the third century BCE Roman builders were using the ash in place of ordinary sand, about two parts of ash to one of lime.

Why does the ash matter so much? Ground with lime, it turns the mortar into a hydraulic cement, that is, one that hardens through a reaction with water, so the mixture sets even where air cannot reach it. The architect Vitruvius saw the effect two thousand years before anyone could explain it: the powder found “about Baiæ and the territory in the neighbourhood of Mount Vesuvius”, mixed with lime and rubble, “hardens as well under water as in ordinary buildings”. That is what made harbours, bridge piers and aqueduct channels possible.

Poured? Not quite

Roman concrete was not poured into moulds like the concrete of a modern pillar. Builders laid it in layers of mortar and rubble between two outer faces of small stones or bricks. The faces gave the wall its finish; the concrete core gave it its strength. For a vault, the mixture was laid on a temporary wooden frame that was taken away once the concrete had set.

The method also let builders control weight. In the Pantheon dome, the aggregate changes from bottom to top: heavy basalt in the foundations and lower walls, then brick and tufa, and finally pumice, a volcanic glass so full of air that it floats on water, near the opening at the centre. The dome gets lighter as it rises, so less weight bears down on the walls. Nearly nineteen centuries later, it is still the largest vault ever built in masonry.

What the labs found

Look closely at a broken piece of Roman concrete and you’ll often see small white lumps. For a long time they were taken as a sign of careless mixing. In 2023 a team led by Admir Masic at MIT published a different reading in Science Advances, based on samples from the city wall of Privernum (Priverno, south of Rome). The lumps, called lime clasts, formed because the Romans mixed the ash with quicklime instead of lime already slaked in water. Quicklime heats up violently when it meets water, a method now called hot mixing, and the heat leaves behind brittle pockets of reactive lime. When a crack forms and water seeps in, it dissolves some of that lime and redeposits it as calcium carbonate, which fills the crack. In practice, the concrete repairs itself.

In 2025 the same group found direct evidence on a Roman building site in Pompeii, left unfinished by the eruption of 79 CE: piles of dry raw material with lumps of quicklime already mixed in, ready for the water. The study appeared in Nature Communications.

Seawater has its own role. A study of cores drilled from Roman harbour structures, published in American Mineralogist in 2017, found that seawater seeping through the concrete dissolves part of the volcanic ash and lets two rare minerals, aluminous tobermorite and phillipsite, crystallise in the pores. Their plate-like crystals interlock and make the material tougher with time. In modern concrete made with Portland cement, the same contact with seawater is one of the causes of decay.

Is Roman concrete better than ours?

It depends on the job. Plain concrete, ancient or modern, resists squeezing well and bending badly, which is why the Romans used it in thick walls, vaults and arches, shapes that keep the material in compression. Modern concrete is usually reinforced with steel bars that take the bending, so it can carry flat floors and long spans that Roman builders never attempted. What the Romans had was durability in the right conditions, especially in water, and that is why engineers are now studying their recipes.

Why it changed architecture

Concrete freed Roman building from the cut-stone block and the straight beam. A wall could rise as one solid mass, a vault could be cast over a bath hall, and a dome could cover a room as wide as the Pantheon dome. It lasted two thousand years because the chemistry of the ash keeps working, and because in some mixtures the material can still mend its own cracks.

Where to see it

The Pantheon in Rome is the best place to stand under Roman concrete. At Ostia and in the Roman Forum, broken walls show the concrete core behind its facing. The unfinished building site studied in 2025 is inside the archaeological park of Pompeii.

Sources

• Vitruvius, On Architecture 2.6.1 (trans. Gwilt, LacusCurtius).
• Encyclopaedia Britannica, “Pozzolana”; “Pantheon”.
• Ministero della Cultura, “Pantheon”.
• MIT News, “Riddle solved: Why was Roman concrete so durable?” (6 January 2023), on Seymour, Masic et al., Science Advances 2023.
• MIT News, “Pompeii offers insights into ancient Roman building technology” (9 December 2025), on Nature Communications 2025.
• University of Utah News, “Roman concrete” (2017), on Jackson et al., American Mineralogist 2017.