Roman concrete’s longevity: carbonation and calcite help it self-seal

Time only moves in one direction, and everything built within it is subject to wear.

We can design buildings and infrastructure to endure, yet even the strongest substances will, sooner or later, fracture, lose strength, and break down.

Against that backdrop, ancient Roman concrete stands out.

Researchers have understood for years that concrete from the Roman Empire appears to become more robust as it ages. Earlier studies attributed much of this exceptional staying power to a chemical reaction between volcanic ash (known as pozzolan) and quicklime, which formed unusually resilient minerals inside the mix.

Scientists have now added an important piece to that picture: long-term, gradual reactions with carbon dioxide from the surrounding air.

"While the pozzolanic reaction is of fundamental importance," says engineer Paulo Monteiro of UC Berkeley, "our findings suggest that carbonation over a long period of time also enhances the durability of concrete and can help it seal cracks as it ages."

The results, reported in Science Advances, highlight the sophistication embedded even in ordinary Roman construction.

Roman engineering and the durability of Roman concrete

A striking feature of Roman engineering is the sheer number of surviving structures that remain in remarkably good condition, while many buildings from the same era have collapsed into debris.

The best-known example is the Pantheon in Rome: a 2,000-year-old temple topped by a vast dome made from unreinforced concrete, still the largest structure of its type anywhere.

To dig into what makes Roman concrete so enduring, Monteiro, his co-lead Xiaohong Zhu of Beijing University of Technology, and other colleagues chose a much less celebrated source material.

A Hadrian-era sample from Tivoli

In the 2nd century CE, Emperor Hadrian built a villa at Tivoli, Italy-large parts of which, unsurprisingly, are still standing today.

From this site, the team removed a small fragment of concrete from a communal toilet that once bore the weight of imperial visitors.

Using a set of high-resolution imaging methods, the researchers analysed the material down to the nanoscale.

They saw what prior work would predict: signs of the pozzolanic reaction, where volcanic ash and lime combine to create minerals that are exceptionally durable within the concrete.

But the sample also contained another clear signal.

Carbonation, calcite, and self-sealing cracks

Over many centuries, carbon dioxide in the atmosphere had reacted with remaining lime in the concrete, producing calcite-the same mineral found in limestone.

The researchers concluded this was not simply an incidental product of ageing.

Instead, the calcite seems to have reinforced the material. By crystallising inside minute pores and hairline cracks, it increased density and slowly closed off weak points that might otherwise have widened and spread over time.

Calcite in Roman concrete had been spotted before, but earlier research had not examined it in three dimensions or charted how it is arranged through the material.

According to the team, the new evidence indicates calcite may have been an underappreciated contributor to Roman concrete’s extraordinary lifespan-not supplanting the established importance of the pozzolanic reaction, but complementing it.

Scientists had already been attempting to recreate Roman concrete.

Carbonation occurs naturally in lime-based concrete whether or not it is recognised, but pinpointing what it does could provide an additional lever for engineers aiming to make concrete that lasts longer while generating less carbon.

"Understanding how calcium carbonate crystallization dynamics bind concrete together and contribute to its long-term durability could provide new insights into the long-term mineralogical evolution and natural carbonation of lime-based binders," Monteiro says.

Ancient Roman structures-from the magnificent Pantheon to Hadrian’s modest toilet-offer astonishing examples of concrete that has remained structurally sound for millennia.

That is not to say we can straightforwardly copy Roman methods today.

Modern construction places much heavier demands on materials, and reinforced concrete must deal with a problem Roman engineers did not face: corrosion of the steel rebar within.

What the findings could mean for modern concrete

What this new work can do, however, is support efforts to create longer-lasting, more sustainable concrete in the future.

"This study shows how exploring ancient engineering techniques can lead to important revelations," Monteiro says.

"We hope that by unlocking Roman secrets for enhancing concrete durability, we can someday attain sustainable modern infrastructure development."

The research has been published in Science Advances.

This article was fact-checked by Rachel Garner and edited by Clare Watson. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.

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