Scientists unlock secrets of ancient Roman and Mayan buildings

NEW YORK

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.

Many researchers have turned to the Romans for inspiration. Starting around 200 B.C., 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.

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.”

Using tree juice

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.

Researchers here had a living link to the structures' creators: They met with local masons in Honduras who traced their lineage all the way back to the Mayan builders, explained Rodriguez-Navarro, who worked on the study.

The masons suggested using extracts from local chukum and jiote trees in the lime mix. When researchers tested out the recipe - collecting bark, putting the chunks in water and adding the resulting tree “juice” into the material - they found the resulting plaster was especially durable against physical and chemical damage.

When scientists zoomed in, they saw that bits of organic material from the tree juice got incorporated into the plaster’s molecular structure. In this way, the Mayan plaster was able to mimic sturdy natural structures like seashells and sea urchin spines and borrow some of their toughness, Rodriguez-Navarro said.

Studies have found all kinds of natural materials mixed into structures from long ago: Fruit extracts, milk, cheese curd, beer, even dung and urine. The mortar that holds together some of China’s most famous structures, including the Great Wall and the Forbidden City, includes traces of starch from sticky rice.