The Romans built the Colosseum over two thousand years ago. It is a giant, oval amphitheatre. Even though a 14th-century earthquake knocked down the south side of the Colosseum, most of the 150-foot building remains intact.

Romans built the Colosseum and many other structures using a specific kind of concrete. As a result, many scientists and engineers suspected the secret behind the structure’s durability was Roman concrete.

However, how this concrete has contributed to the architecture’s strength remains a mystery.

Recently, a team of interdisciplinary researchers discovered an answer to the question. The answer is crucial to modern real estate because the ancient Colosseum has weathered the test. Modern concrete structures crumble after a few decades.

The answer is simple yet mind-boggling, self-healing concrete.

The science behind self-healing concrete

Self-healing concrete comprises three components: limestone, volcanic material, and water. For years, architects and historians speculated that cement concrete contains volcanic material that makes it solid and sturdy, which is true but not the whole story. It still does not explain the material’s self-healing ability.

Standing tall. The Colosseum as seen in 1757 in an engraving by Giovanni Battista Piranesi

Researchers have reason to believe that the self-healing properties might be chemical manipulation. However, the secret lies in the use of limestone.

When ancient Romans made mortar, they first heated the lime to turn it into a substance called ‘quicklime’ .Quicklime is a very reactive substance, a chemical sibling to limestone.

While mixing quicklime, Romans introduced water into the process. The heat it produced created a chemical foundation that could strengthen the concrete later.

Hence, when tiny cracks form in the structure, the quicklime stops them from spreading. The rain, when it pours, reacts with the lime to recrystallize as various forms of calcium carbonate, quickly filling the crack. Or respond with the volcanic ash to “heal” the material.

The Colosseum showing the exterior and interior walls. It stands proud even after a powerful earthquake.

Ainissa Ramirez, a material scientist, believes the new understanding of ancient Roman concrete is a welcoming discovery.

“This is one way that the material can be greener. It’s sort of like a message in a bottle. The Romans made the material. So we had to figure out how they did it so that we can make better materials — and then, you know, be better stewards of our environment.” said Ramirez.

The Roman Colosseum

The Colosseum is an elliptical amphitheatre located in Rome, Italy. To this date, it is the largest ancient amphitheatre ever built. Its construction started under emperor Vespasian (r. 69–79 AD).

The Colosseum comprises travertine limestone, tuff (volcanic rock), and brick-faced cement. It could hold approximately 50,000 to 80,000 spectators at various historical points. It had an average audience of 65,000.

The amphitheatre was known for gladiatorial contests and public spectacles, including animal hunts, executions, re-enactments of famous battles, and plays based on Roman mythology.

The building stopped being used for entertainment in the early medieval era. However, it was later reused for housing, workshops, quarters for a religious order, a fortress, and a Christian shrine.

Self-healing concrete for the future

Concrete is a material that is widely used in construction. It is mainly due to its accessibility and cost, even though it is prone to fracture formation. Hence, there has been a surge in interest in self-healing materials.

Material scientists are particularly interested in self-healing capabilities in green and sustainable concrete materials. Different techniques have been offered by dozens of researchers worldwide in the last two decades.

In a nutshell, the material works by mending fissures due to the ongoing hydration of clinker minerals or carbonation of calcium hydroxide. If the ‘self-healing’ concrete is made public, would you use it to construct your house?

If you did, it may stand as long as those from the Roman Empire.