Roman concrete: buildings that have stood for 2,000 years

Almost every building today uses concrete, and it usually needs repair after a few decades. Yet in Rome there is a dome that has stood for nearly 1,900 years without steel reinforcement. On the shores of the Mediterranean, the walls of Roman-era harbours have withstood the sea for 2,000 years. This is the achievement of Roman concrete, the secret of which scientists are only now beginning to understand.

How concrete began

According to Smithsonian Magazine, the Romans began making concrete more than 2,000 years ago. By the start of the second century BC, they were using it in large building projects. The Greeks probably also made lime mortar. However, Renato Perucchio, an engineer at the University of Rochester in the United States, says that mixing mortar with pieces of brick or stone to make concrete was probably a Roman method.

The recipe was simple. Limestone was burned in a kiln to make quicklime. Water was added to make a paste, and then volcanic ash was mixed in. According to Smithsonian, the writings of Vitruvius, a Roman architect of the first century BC, generally give three parts ash to one part lime. Fist-sized pieces of brick or a volcanic stone called tuff were mixed into this mortar, which was laid in layers.

Volcanic ash

Volcanic ash was the key ingredient. In his book on architecture, Vitruvius writes that a special sand is found near Baiae, close to the Bay of Naples, and around Mount Vesuvius. Mixed with lime and stone, it hardens under water as well as in ordinary buildings, so that even the waves cannot break it apart. The ash became known as "pozzolana", after the nearby city of Pozzuoli.

According to the Museum of Classical Antiquities at the University of Ottawa in Canada, the soluble silica in the ash combines with lime in the presence of water to form a thick, strong, cement-like substance.

Concrete in the sea

The most astonishing use of Roman concrete was in the sea. Vitruvius describes how a harbour was built. Two parts of ash from the Cumae region were mixed with one part of lime. A enclosure was formed under water with oak piles, and it was then filled with stone and the same mortar.

According to Smithsonian, the geologist Marie Jackson says the Romans loaded thousands of tonnes of this ash onto ships and carried it from the Italian coast to build harbours as far away as Alexandria and Turkey. According to a 2017 report by the University of Utah in the United States, the Roman writer Pliny wrote around AD 79 that harbour concrete becomes a single solid mass of stone that grows stronger every day.

According to the University of Utah, Jackson's team found the reason in samples taken from Roman harbours. Seawater seeps into the concrete and dissolves parts of the volcanic ash. In their place grow layered crystals of minerals called Al-tobermorite and phillipsite. These interlock and make the mortar stronger still.

The dome of the Pantheon

The Pantheon in Rome is the most famous example of Roman concrete. According to the World History Encyclopedia, the present building was completed around AD 125 under Emperor Hadrian, while Smithsonian and the Massachusetts Institute of Technology (MIT) give the date as AD 128. Its dome is 43.2 metres wide, and its height from floor to top is the same. A round opening, or oculus, 8.8 metres across, is open at the top.

To keep the weight of the dome down, light tuff and a light volcanic rock similar to scoria were mixed into its concrete. Square recesses, or coffers, were cut into the inside to reduce the weight further. According to Smithsonian, it remains the largest dome of its kind in the world without iron reinforcement.

Cracks that heal themselves

Millimetre-sized white particles are commonly seen in Roman concrete. Experts earlier took them to be signs of careless mixing. According to MIT News, in January 2023 a team led by Professor Admir Masic reported in the journal Science Advances that these particles are in fact a strength. The team had examined mortar from the wall of the ancient Italian city of Privernum.

According to them, the Romans probably used quicklime directly, either with or instead of slaked lime. Quicklime gives off intense heat when water is added. This "hot mixing" produced the lime particles. When a crack reaches them, water dissolves their calcium, which turns back into calcium carbonate and fills the crack. In the experiment, water flow through broken samples stopped within a few weeks. A sample made without quicklime did not heal.

The lesson for today

According to the Science Advances paper, the cement industry produces about 8 per cent of the world's greenhouse gas emissions. More durable concrete means less repair and lower emissions.

The houses, bridges and roads built every day in Pakistan's cities are a new chapter in this old story. Roman builders had no microscope, but through observation and experience they made a material from which engineers are still learning today.