How are ancient Roman and Mayan buildings still standing? Scientists are unlocking their secrets

NEW YORK (AP) — In the quest to build better for the future, some are looking for answers in the long-ago past.

Ancient builders across the world created structures that are still standing today, thousands of years later — from Roman engineers who poured thick concrete sea barriers, to Maya masons who crafted plaster sculptures to their gods, to Chinese builders who raised walls against invaders.

Yet scores of more recent structures are already staring down their expiration dates: The concrete that makes up much of our modern world has a lifespan of around 50 to 100 years.

A growing number of scientists have been studying materials from long-ago eras — chipping off chunks of buildings, poring over historical texts, mixing up copycat recipes — hoping to uncover how they’ve held up for millennia.

This reverse engineering has turned up a surprising list of ingredients that were mixed into old buildings — materials such as tree bark, volcanic ash, rice, beer and even urine. These unexpected add-ins could be key some pretty impressive properties, like the ability to get stronger over time and “heal” cracks when they form.

Figuring out how to copy those features could have real impacts today: While our modern concrete has the strength to hold up massive skyscrapers and heavy infrastructure, it can't compete with the endurance of these ancient materials.

And with the rising threats of climate change, there's a growing call to make construction more sustainable. A recent UN report estimates that the built environment is responsible for more than a third of global CO2 emissions — and cement production alone makes up more than 7% of those emissions.

“If you improve the properties of the material by using … traditional recipes from Maya people or the ancient Chinese, you can produce material that can be used in modern construction in a much more sustainable way,” said Carlos Rodriguez-Navarro, a cultural heritage researcher at Spain’s University of Granada.

Many researchers have turned to the Romans for inspiration. Starting around 200 BCE, the architects of the Roman Empire were building impressive concrete structures that have stood the test of time — from the soaring dome of the Pantheon to the sturdy aqueducts that still carry water today.

Even in harbors, where seawater has been battering structures for ages, you’ll find concrete “basically the way it was when it was poured 2,000 years ago,” said John Oleson, an archaeologist at the University of Victoria in Canada.

Most modern concrete starts with Portland cement, a powder made by heating limestone and clay to super-high temperatures and grinding them up. That cement is mixed with water to create a chemically reactive paste. Then, chunks of material like rock and gravel are added, and the cement paste binds them into a concrete mass.

According to records from ancient architects like Vitruvius, the Roman process was similar. The ancient builders mixed materials like burnt limestone and volcanic sand with water and gravel, creating chemical reactions to bind everything together.

Now, scientists think they’ve found a key reason why some Roman concrete has held up structures for thousands of years: The ancient material has an unusual power to repair itself. Exactly how is not yet clear, but scientists are starting to find clues.

In a study published earlier this year, Admir Masic, a civil and environmental engineer at the Massachusetts Institute of Technology, proposed that this power comes from chunks of lime that are studded throughout the Roman material instead of being mixed in evenly. Researchers used to think these chunks were a sign that the Romans weren’t mixing up their materials well enough.

Instead, after analyzing concrete samples from Privernum — an ancient city outside of Rome — the scientists found that the chunks could fuel the material’s “self-healing” abilities. When cracks form, water is able to seep into the concrete, Masic explained. That water activates the leftover pockets of lime, sparking up new chemical reactions that can fill in the damaged sections.

Marie Jackson, a geologist at the University of Utah, has a different take. Her research has found that the key could be in the specific volcanic materials used by the Romans.

The builders would gather volcanic rocks left behind after eruptions to mix into their concrete. This naturally reactive material changes over time as it interacts with the elements, Jackson said, allowing it to seal cracks that develop.

The ability to keep adapting over time “is truly the genius of the material,” Jackson said. “The concrete was so well designed that it sustains itself.”

At Copan, a Maya site in Honduras, intricate lime sculptures and temples remain intact even after more than 1,000 years exposed to a hot, humid environment. And according to a study published earlier this year, the secret to these structures' longevity might lie in the trees that sprout among them.