# Roman Concrete

> Lime, volcanic ash and rubble

- Date: c. 200 BCE
- Era: Antiquity
- Domain: [Building & Cities](https://lastcodex.org/en/domains/building/)
- Rebuild stage: III (Crafts & cities)
- Difficulty: 3/5
- URL: https://lastcodex.org/en/inventions/roman-concrete/

Roman concrete is lime mortar packed around lumps of stone or broken brick; in its best-known form the mortar contains volcanic ash. The ash makes it set hard even under water, and it carried the vaults of Rome, its harbours and the unreinforced dome of the Pantheon, still the largest of its kind.

![Roman Concrete · Lime, volcanic ash and rubble](https://lastcodex.org/_astro/plate.wFcookS-_Z2jfA7N.webp)

## Legend

1. Brick facing
2. Core of rubble in mortar
3. Bonding course of large flat tiles
4. Heavy travertine in the foundations
5. Light pumice and tuff in the upper dome
6. Oculus

## What it is

Roman concrete, opus caementicium, is a mass of stone fragments bound by a lime mortar, which in many Roman works contained volcanic ash. The Romans called the fragments caementa, from which the word cement comes. From the second century BCE it let builders raise walls, vaults and domes of a size that cut stone alone could not reach, at lower cost and with less skilled labour.

Two things set it apart from a plain lime mortar: volcanic ash, which lets it harden even under water, and a way of building in layers of rubble between faces of brick or small stones.

## How it works

![Cutaway of a lime kiln built into a slope, limestone stacked over a wood fire, with a heap of white quicklime beside it](https://lastcodex.org/_astro/v1.D7Jl8mFe_Z1Ee5mr.webp)
_A lime kiln: the limestone is stacked on an arch above the fire and comes out as white quicklime._

The binder starts as limestone. Burnt in a kiln, it gives off carbon dioxide and becomes quicklime; mixed with water, the quicklime swells and heats up and becomes slaked lime. A mortar of pure air lime hardens slowly, as it takes back carbon dioxide from the air, and it stays soft wherever air cannot reach.

Volcanic ash changes that. The ash of the Bay of Naples, named pozzolana after the town of Pozzuoli, is rich in silica and alumina in a reactive, glassy form. It reacts with the lime and water and forms new binding minerals, so the mortar hardens by reacting with water rather than with air, even when it is submerged. Vitruvius, writing about 25 BCE, noted that this volcanic sand from around Baiae and Vesuvius, mixed with lime and rubble, hardened under water as well as on land.

![A wooden caisson standing in the sea, cut away to show rubble and mortar being laid under water](https://lastcodex.org/_astro/v2.TEKzgkRi_1bGL2P.webp)
_Marine concrete: builders filled timber boxes standing in the sea with ash mortar and rubble._

For harbours, builders drove timber piles into the sea bed, tied them together into an enclosure and filled it with mortar and lumps of tuff straight into the water; Vitruvius also describes enclosures that were pumped dry before filling. Pliny the Elder described such piers as a single stone mass "every day stronger". In 2017 a team led by the geologist Marie Jackson found that seawater slowly dissolves parts of the ash and that new minerals, among them the rare aluminous tobermorite and phillipsite, grow in its pores. The team argues that these interlocking crystals help the concrete resist cracking.

Workers built Roman concrete up in layers. They spread a layer of hand-sized stones or broken brick, rammed mortar into it, and repeated the process course after course; baskets of mortar were carried to the wall by hand. The faces of brick or small squared stones served as permanent formwork. Courses of large flat tiles ran through the wall at intervals and tied the two faces to the core.

![A broken lump of Roman concrete showing pieces of tuff and brick and small white lime lumps, a hairline crack partly filled with white deposits](https://lastcodex.org/_astro/v3.Cy1MASJb_1sGImF.webp)
_The white lumps are lime clasts. Where a crack crosses one, water can dissolve the lime and refill the crack with calcite._

In 2023 a team at MIT led by Admir Masic studied the small white lumps of lime found in Roman concrete. They concluded that the builders mixed some of the lime as quicklime, which heats the mix as it reacts. The lumps act as a reserve: when water enters a crack, it dissolves lime from them and deposits calcium carbonate that seals the gap. The team then made modern test mortars with added quicklime, cracked them and ran water through them; according to MIT's report the cracks sealed within about two weeks, while a control made without quicklime did not heal.

Roman concrete is weaker in compression than modern Portland cement concrete and gains strength slowly. Builders allowed for this: in the Pantheon, brick arches buried in the walls carried loads while the concrete cured.

## How to build it

A small test block, about 30 cm long, shows how the material behaves. Treat it as a demonstration only, not as a load-bearing part of a building. Use materials sold for lime work and restoration.

**Gather the materials.** You need hydrated lime powder, a natural pozzolana or another pozzolan sold for lime mortars, and aggregate: clean broken brick or stones from walnut to egg size. Wear gloves and eye protection from the start, work in a well-ventilated place and keep dust down.

**Mix the mortar.** Use the proportions by volume that the pozzolan supplier gives for hydrated lime. Put on a fitted particulate respirator, as the product data sheets specify, and mix the powders dry. Add water little by little until the mortar is stiff and sticky, not runny.

**Build the faces.** Dampen the bricks and the broken-brick aggregate beforehand, without leaving water standing on them, so that they do not suck the water out of the fresh mortar. Lay two short, thin skins of brick in lime mortar, a hand's height at a time. They act as the formwork and stay in place.

**Fill the core.** Spread a layer of mortar between the skins, press the aggregate into it, then ram more mortar into the gaps with a wooden tamper until no voids remain. Work in layers about 6 to 8 cm deep, so that mortar surrounds every piece, and finish with a course of flat tiles laid right across the block.

**Cure it slowly.** Keep the work damp and shaded for at least two weeks, covered with wet sacking, and protect it from frost. Lime and pozzolan go on gaining strength for months; break the block open after a few months to see how far it has set.

> **Safety.** Lime is caustic. It burns skin and can blind: wear gloves, long sleeves and sealed goggles, and rinse any splash with plenty of clean water at once, then get medical help for the eyes. Wear a fitted particulate respirator when tipping or mixing dry lime and pozzolan. Do not slake quicklime or try the Roman "hot mixing" yourself: quicklime and water react violently, boil and spit. This home mix is not tested for strength: do not use it for steps, walls or anything that must carry a load.

## History and evolution

Lime mortars were used around the Mediterranean long before Rome; Roman builders turned them into a structural material. Concrete was in wide use by about 150 BCE, and some scholars place its beginning a century earlier. Concrete that sets under water was probably developed around the Bay of Naples, where the ash is found, by the end of the second century BCE; which surviving harbour works are the oldest is debated.

Under the empire it carried most large buildings. Herod's harbour at Caesarea, built from about 22 BCE, was made with ash shipped from Italy. The Pantheon, rebuilt under Hadrian between about 117 and 125 CE, has a dome 43.3 m across with a round opening about 9 m wide at the top. Its walls are about 6 m thick, and the aggregate changes from heavy travertine in the foundations to tuff, brick, and light pumice and tuff in the upper dome. It remains the largest unreinforced concrete dome in the world.

After the western empire fell, large concrete construction faded and lime mortar went back to binding masonry. Modern concrete, made with Portland cement and often reinforced with steel, sets faster and is stronger, but in seawater it can crack, and its steel can corrode, within decades if it is poorly made or poorly protected. Researchers now study Roman harbour cores and lime clasts to make concrete that lasts longer and emits less carbon dioxide.

## Key figures

- Binder: Lime, often with volcanic ash (pozzolana)
- Mix for work under water: About 1 part lime to 2 of ash (Vitruvius)
- Pantheon dome: 43.3 m across, no reinforcement, completed c. 125 CE
- Density of the Pantheon's concrete: About 2,200 kg/m³ in the foundations, 1,350 kg/m³ near the top

## Requires

- [Bricks](https://lastcodex.org/en/inventions/bricks/): Standard blocks of earth (c. 7,500 BCE)
- [Control of Fire](https://lastcodex.org/en/inventions/control-of-fire/): Light, heat and cooked food (1 million years ago)
- [Lime and lime mortar](https://lastcodex.org/en/inventions/lime-mortar/): Burnt stone that turns back to stone (10,000 years ago)

## Leads to

- [Portland Cement](https://lastcodex.org/en/inventions/portland-cement/): The binder that hardens under water (1824)

## Sources

- [Vitruvius, The Ten Books on Architecture, Book II (lime and pozzolana)](https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Vitruvius/2*.html), LacusCurtius, University of Chicago
- [Vitruvius, The Ten Books on Architecture, Book V, chapter 12 (harbours)](https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Vitruvius/5*.html), LacusCurtius, University of Chicago
- [Hot mixing and the durability of ancient Roman concrete (news release, 6 January 2023)](https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106), MIT News
- [How seawater strengthens ancient Roman concrete (news release, 3 July 2017)](https://www.eurekalert.org/news-releases/778694), University of Utah, via EurekAlert!
- [The Pantheon: crown jewel of Roman concrete (David Moore, Constructor, 2002)](https://www.ancientportsantiques.com/wp-content/uploads/Documents/ETUDESarchivees/Ma%c3%a7onnerie/B%c3%a9tonRomain/RomanConcretePantheon-Moore2002.pdf), Associated General Contractors of America (Constructor magazine)
- [Roman concrete](https://en.wikipedia.org/wiki/Roman_concrete), Wikipedia
- [A comparison of the chemical and engineering characteristics of ancient Roman hydraulic concrete with a modern reproduction of Vitruvian hydraulic concrete (Gotti, Oleson et al., Archaeometry 50, 2008)](https://www.ancientportsantiques.com/wp-content/uploads/Documents/ETUDESarchivees/Ma%C3%A7onnerie/B%C3%A9tonRomain/RomanConcrete-Gotti2008.pdf), University of Oxford / Wiley (Archaeometry)
- [Building for Eternity: the History and Technology of Roman Concrete Engineering in the Sea (J. P. Oleson ed., 2014)](https://www.ancientportsantiques.com/wp-content/uploads/Documents/AUTHORS/Oleson2014-BuildingforEternity.pdf), Oxbow Books
- [Building a Roman pila: the ROMACONS experiment at Brindisi, September 2004](https://web.uvic.ca/~jpoleson/Harbour%20Concrete/ROMACONS04Pila.html), J. P. Oleson, University of Victoria
- [FEM static and dynamic modeling of Roman vaulted monuments in opus caementicium (R. Perucchio, 2012)](https://msc-sahc.org/wp-content/uploads/2020/07/5-fem-roman-vaults-guimaraes-1oct2012.pdf), University of Rochester, via SAHC Masters

Fact-checked on 2026-10-12.

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