Scientists have uncovered the secret behind Roman concrete, which has remained intact for 2,000 years

Scientists from the US and Europe have found another piece of the answer. They discovered that Roman concrete was not only very strong from the outset — it continued to strengthen over the centuries thanks to a slow chemical process which also helped it to ‘heal’ minor damage.

The findings have been published in the journal *Science Advances*.

Not just volcanic ash

For a long time, it was thought that the main secret of Roman concrete lay in a mixture of lime and volcanic ash. This combination does indeed produce a particularly strong material.

However, new research has shown that this explanation is insufficient.

It turned out that, after construction, the concrete continued to change for hundreds of years. The lime gradually reacted with moisture and carbon dioxide from the air. As a result, the mineral calcite formed within the material, filling microcracks and voids.

An unusual sample

For their research, the scientists used a piece of concrete around 1,900 years old, found beneath a public latrine at the villa of the Roman Emperor Hadrian, not far from Rome.

At first glance, this choice may seem strange, but it is precisely this concrete that has been exposed to constant humidity for centuries, making it particularly valuable for studying the material’s durability.

The researchers examined the sample using 3D X-ray tomography, electron microscopes and chemical analysis.

How concrete ‘heals’ itself

The scans revealed that calcite had gradually formed a dense mineral network within the concrete.

This network performed several important functions at once:

• filling microscopic cracks;
• it made the concrete denser;
• prevented water penetration;
• it protected the material from further deterioration.

Furthermore, the volcanic particles continued to interact slowly with the lime, forming additional cementitious compounds.

It was precisely this combination of processes that helped Roman concrete retain its strength for almost two millennia.

This will benefit modern construction

The authors believe that understanding the mechanisms behind ancient Roman concrete could help in the development of new building materials.

Modern concrete is one of the world’s largest sources of carbon dioxide emissions. If more durable mixtures can be developed that are capable of sealing small cracks on their own, this will reduce the need for repairs to structures and cut down on the use of building materials.

Why this is important

The research shows that the durability of Roman concrete is not down to a single ‘secret ingredient’, but rather to several slow-acting processes that continue for decades and even centuries after construction.

Understanding these mechanisms could help engineers develop stronger and more environmentally friendly building materials for the future.

Background

Roman concrete (opus caementicium) was widely used in the construction of aqueducts, harbours, baths, temples and other structures across the Roman Empire. Many of these have survived to this day.

In recent years, scientists have discovered that the material is capable of partially ‘self-healing’ cracks thanks to its limestone inclusions. This new study adds to this picture, showing that the formation of calcite within the concrete over the course of centuries also plays a crucial role.

Source

Xiaohong Zhu et al. Mineralised carbonates contribute to the millennial durability of Roman concrete. Science Advances (2026).