In the quest to build better for the future, some look to the distant past for answers. Ancient builders around the world created structures that still stand today, thousands of years later, from Roman engineers who built thick concrete sea barriers to Mayan masons who crafted plaster sculptures for their gods and Chinese builders who erected walls against invaders.
However, dozens of newer structures are already close to expiration: the concrete that makes up much of our modern world has a lifespan of between 50 and 100 years. A growing number of scientists have been studying materials from distant times—cutting up chunks of buildings, studying historical texts, mixing imitated recipes—in hopes of discovering how they have been preserved over millennia.
This reverse engineering has revealed a surprising list of ingredients that were mixed into ancient buildings: materials such as tree bark, volcanic ash, rice, beer and even urine. These unexpected additions could be key to some pretty impressive properties, like the ability to strengthen 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 support huge skyscrapers and heavy infrastructure, it can't compete with the strength of these ancient materials.
And with the growing threats of climate change, there is 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, with cement production alone accounting for more than 7% of those emissions.
"If you improve the properties of the material using...traditional recipes from the Mayans or the ancient Chinese, you can produce material that can be used in modern construction in a much more sustainable way," said Carlos Rodríguez-Navarro, heritage cultural. Researcher at the University of Granada in Spain.
Is the concrete of ancient Rome better than today's? Many researchers have turned to the Romans for inspiration. Around the year 200 BC. C., the architects of the Roman Empire built 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 ports, where seawater has been pounding structures for years, concrete will be found “basically as 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 very high temperatures and grinding them.
That cement is mixed with water to create a chemically reactive paste. Then, pieces of material such as rock and gravel are added, and the cement paste joins them together, forming a mass of concrete.
According to records of ancient architects such as Vitruvius, the Roman process was similar. Ancient builders mixed materials like burnt limestone and volcanic sand with water and gravel, creating chemical reactions to bind everything together.
Now, scientists believe they have found a key reason why Roman concrete has supported structures for thousands of years: the ancient material has an unusual power to repair itself. It's still unclear exactly how, 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 pieces of lime that are embedded throughout the Roman material rather than mixed evenly.
Researchers used to think that these chunks were a sign that the Romans weren't mixing their materials well enough. Instead, after analyzing concrete samples from Privernum, an ancient city outside Rome, scientists discovered that the chunks could fuel the material's "self-healing" capabilities.
When cracks form, water can seep into the concrete, Masic explained. That water activates excess lime pockets, causing new chemical reactions that can fill in damaged sections. Marie Jackson, a geologist at the University of Utah, has a different opinion.
Their research has discovered that the key could lie in the specific volcanic materials used by the Romans. Builders would collect volcanic rocks left after eruptions to mix with 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 continue to adapt over time "is truly the genius of the material," Jackson said. "The concrete was so well designed that it stands on its own."
At Copán, a Mayan site in Honduras, intricate lime sculptures and temples remain intact even after more than 1,000 years exposed to a hot, humid environment.
Researchers here had a living link to the structures' creators: They met with local masons in Honduras who traced their lineage to the Mayan builders, Rodríguez-Navarro explained.
The masons suggested using extracts from local chukum and jiote trees in the lime mixture. When the researchers tested the recipe (they collected bark, put the pieces in water, and added the "juice" of the resulting tree to the material), they found that the resulting plaster was especially durable against physical and chemical damage.
As the scientists got closer, they saw bits of organic material from the tree juice being incorporated into the molecular structure of the gypsum. In this way, Mayan plaster was able to imitate tough natural structures, such as seashells and sea urchin spines, and borrow some of their hardness, Rodríguez-Navarro said.
Studies have found all kinds of natural materials mixed into long-ago structures: fruit extracts, milk, cheese curds, beer, and even manure 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 sticky rice starch.
Some of these ancient builders might have been lucky, said Cecilia Pesce, a materials scientist at the University of Sheffield in England. They threw almost anything into their mixes as long as it was cheap and available, and the ones that didn't work have long since collapsed.
"They would put all kinds of things into the construction," Pesce said. "And now we only have the buildings that survived. So it's like a process of natural selection."
But some materials appear to show more intent, such as in India, where builders created blends of local materials to produce different properties, said Thirumalini Selvaraj, a civil engineer and professor at the Vellore Institute of Technology in India.
According to Selvaraj's research, in humid areas of India, builders used local herbs that help structures deal with humidity. Along the coast, they added brown sugar, an unrefined sugar, which can help protect against salt damage. And in areas with a higher risk of earthquakes, they used superlight "floating bricks" made from rice husks.
"They know the region, they know the condition of the soil, they know the climate," Selvaraj said. "So they design a material accordingly." Today's builders cannot simply copy old recipes. Although Roman concrete lasted a long time, it couldn't withstand heavy loads: "You couldn't build a modern skyscraper with Roman concrete," Oleson said. "It would collapse when you got to the third floor."
Instead, researchers are trying to take some of the specialties of the ancient material and add them to modern mixes. Masic is part of a startup trying to build new projects using Roman-inspired "self-healing" concrete. And Jackson is working with the Army Corps of Engineers to design concrete structures that can withstand seawater well (like those in Roman ports) to help protect coastlines from rising sea levels.
We don't need to make things last as long as the Romans did to have an impact, Masic said. If we add 50 or 100 years to the useful life of concrete, "we will need less demolition, less maintenance and less material in the long term."
