‘Self-Healing’ Concrete May Have Preserved Ancient Roman Structures

The durable material could fill its own cracks, new research suggests

A view of the Pantheon's dome in Rome from the inside of the building.
The Pantheon's dome, the largest unreinforced concrete dome in the world, is still standing despite being nearly 2,000 years old. Stephen Knowles Photography via Getty Images

Ancient Roman infrastructure can put modern buildings to shame. While today’s concrete structures might only last a few decades, some long-lived concrete in Rome has survived for 2,000 years. The Pantheon’s unreinforced concrete dome, completed around 125 C.E., remains intact, according to a statement from MIT.

“The Pantheon would not exist without the concrete as it was in the Roman time,” Admir Masic, a chemist at MIT, tells the Guardian’s Nicola Davis.

Scientists have long wondered how the Romans achieved such an immense feat of engineering. In a new paper, published Friday in the journal Science Advances, Masic and other researchers propose that the concrete mixed by Romans could repair cracks on its own. While the finding reveals new insights on ancient Rome, it also provides a blueprint for improving modern concrete, scientists say.

To study the durable building material, researchers took mortar samples from walls in the ancient city of Privernum, near Rome. The samples had similar compositions to other Roman concrete samples from the same period, the researchers write in the paper.

The mortar contained small white chunks of calcium deposits, called lime clasts. Previously, researchers thought the chunks meant that Romans weren’t mixing the concrete well enough. But Masic wasn’t convinced.

“For me, it was really difficult to believe that ancient Roman [engineers] would not do a good job, because they really made careful effort when choosing and processing materials,” he tells CNN’s Katie Hunt.

The analysis revealed the lime clasts’ origin: Ancient engineers used the dry, most reactive form of limestone called quicklime, instead of or in addition to slaked lime, which is combined with water first. Mixing with quicklime would have set off chemical reactions, causing extreme temperatures—known as “hot mixing”—and creating the calcium deposits.

The deposits also served a purpose, the researchers found. They theorized that as water entered cracks in the concrete, it could dissolve the chunks of calcium. Then, the dissolved chemicals could recrystallize or react with other materials, filling the cracks and strengthening the structure.

To test this, the team made concrete using a Roman recipe and a modern recipe. They then broke the concrete and let water pass through it for 30 days. Afterward, the modern concrete still let water pass through, but the Roman concrete did not, suggesting the cracks had been filled.

Samples of ancient Roman concrete appeared to have cracks filled in the same manner, writes Ars Technica’s Jennifer Ouellette.

The new findings could lead to more durable modern concrete, Marie Jackson, a geologist who studies ancient Roman concrete at the University of Utah and did not contribute to the new paper, tells Science’s Jacklin Kwan.

Taking inspiration from Roman concrete “might be a cost-effective way to make our infrastructure last longer through the self-healing mechanisms we illustrate in this study,” Masic tells the Guardian.

Since around 8 percent of global greenhouse gas emissions come from making cement, longer-lasting concrete could also reduce the industry’s contribution to climate change, Masic says in the statement.