Roman concrete

The story

A liquid stone

Imagine being able to build a wall, a vault, an arch, by pouring a sludge into a mould and waiting for it to harden until it is as hard as stone. Today we call it concrete, or cement, and it is the most widely used building material. The Romans used a version all of their own, which they called opus caementicium, that is, “work of rubble”. It was one of the secrets of their success.

It was not a sudden invention. The technique seems to have developed gradually between the 3rd and 2nd centuries BC. At first it was a poor material, used for fillings and private buildings, with a mortar that was still imperfect. There are examples at Pompeii that are said to go back to the middle of the 3rd century. Then the builders discovered how to make it much stronger.

The Romans' recipe

The base was lime. You get it by heating limestone in a kiln at a temperature of about 900 degrees or less: the product is called quicklime. When it is mixed with water, it is “slaked” and becomes a paste, which serves as a binder, that is, as glue. Making lime takes much less energy than producing modern Portland cement.

The real secret, however, was one ingredient: pozzolana, a volcanic ash. The name comes from Pozzuoli, the city near Naples, in the Phlegraean Fields, where it was dug in large quantities. The strongest pozzolanas came precisely from the volcanic areas of the Gulf of Naples. The Roman writer Vitruvius described quarry sand in several colours and recommended the “dust of Pozzuoli” for work in water. Mixed with lime and water, pozzolana reacts and gives a mortar much stronger and longer-lasting than lime alone.

Stones and fragments, the caementa, often of tuff, a soft volcanic rock, were added to the mortar. The mixture was poured into wooden formwork, that is, moulds. The outer faces of the walls, the facings, were finished in different ways, which archaeologists call by the name of “opus”: opus incertum with irregular stones, opus reticulatum with small stone pyramids laid out like a net, and opus testaceum with tiles and bricks. Even today, looking at a Roman wall, you can recognise these patterns.

Setting even under water

A special feature of the mixture with pozzolana is that it sets even under water. Instead of ruining the mixture, the water helps it to harden. This is why the Romans were able to build piers, harbours, bridge piers and foundations in the sea. If the material had not had this property, many harbour works would have been impossible.

Vaults, arches and domes

Concrete was not used only for walls. It could be poured over wooden structures, the centering, to form arches, vaults and domes. The first known vaults are those of the Porticus Aemilia, around 174 BC. With this material Roman architects could cover large interior spaces without filling the hall with columns. The Pantheon is the grandest example: its dome, with a diameter of more than 43 metres, was made with a concrete that was heavy at the base, with chips of travertine and tuff, and lighter and lighter towards the top, where pumice and volcanic lapilli were used. The thickness of the dome goes from over 6 metres at the base to a little over a metre near the opening at the top.

In later centuries, to lighten the vaults, empty amphorae were also built in, as in the mausoleum of Helena in Rome. It is one of the many tricks of the Roman builders, who were constantly experimenting.

Why does it last so long?

Roman concrete structures have stood for almost two thousand years, even in places exposed to the sea and to earthquakes. To understand why, in 2023 a group of researchers led by Admir Masic, of the Massachusetts Institute of Technology, published a study on walls of the ancient city of Privernum, near Rome. They noticed small white fragments, until then thought to be mixing defects. According to the researchers they come instead from the use of quicklime, mixed directly with the other materials and slaked with water. These fragments might help the material to repair itself: when a crack forms, water dissolves a little lime which then recrystallises and closes the gap. It is a very interesting hypothesis, still being researched.

One last curiosity: archaeologists can date some of these buildings by studying the mortar too, thanks to radiocarbon, the same method used to date ancient bones and wood.