Still standing: Researchers crack secret of ancient Rome’s self-healing concrete

The Start-up Nation’s crumbling concrete buildings show modern Israel has a thing or two to learn from 2,000-year-old Roman tech. Through earthquakes, severe weather, and even after being submerged underwater, ancient Roman concrete structures have survived millennia without the spiderwebs of cracks that plague Israel’s brutalist architecture after just a few decades.

That’s because Roman concrete has the unique ability to “heal” its own cracks after being exposed to water, according to a group of Italian and American scientists who were able to shed more light on the chemical reaction that helps concrete make cracks disappear.

Using new information gathered by infrared cameras and other microarchaeology methods on samples taken from ancient concrete in central Italy, the group was able to recreate their own “Roman concrete” and witnessed their samples make inflicted cracks disappear in a matter of weeks.

Some of the Roman construction in Israel, including the Caesarea port, parts of Herod’s palace in Jericho, and Herod’s family tomb in Jerusalem, contain concrete similar to the Italian concrete studied. Concrete in these sites used imported Italian ash, called pozzolana, which was extracted from an area near Naples and brought to Israel by ship.

In addition to solving the riddle of these enduring Roman monuments, the research could help the modern concrete industry, one of the most polluting industries in the world, learn how to create more durable concrete that requires fewer materials and less maintenance.

“Romans were very savvy in how to formulate concrete for different applications,” Dr. Linda Seymour, the lead author of the study published Friday in the prestigious journal Science Advances, told The Times of Israel this week.

The Romans came across these concrete varieties after much trial and error, Seymour explained.

“As early as Phoenician times [1500 to 300 BCE], they saw that when they added crushed pottery, [the concrete] was able to set in different conditions, like with more water, or even underwater,” Seymour said. “There was kind of this evolution of them trying out different aggregates, and they saw when they used volcanic aggregate [based on volcanic ash] they could build even larger structures.”

Seymour, who is currently an independent consultant in the concrete industry, studied ancient Roman concrete at MIT during her PhD research, delving into how Romans were experts at creating concrete that was uniquely adapted to each specific location and purpose. Some concrete set quickly, which allowed the Romans to build large and impressive structures such as the giant dome of the Pantheon. Others set slowly, ideal for creating smooth water channels on aqueducts, such as the one found near Caesarea.

But one of the most impressive aspects across many types of Roman concrete — and one that has stumped scientists until recently — is that the concrete continues to react with water after it has set, allowing it to heal cracks that develop over time.

Seymour and her team were able to recreate a type of concrete similar to one they found at Roman ruins they studied at Privernum, about 100 kilometers (60 miles) southeast of the Italian capital.

The researchers created cylinders of their own Roman concrete, then inflicted tiny cracks half a millimeter wide in the samples. When the cylinders were immersed in running water for one to three weeks, the cracks shrunk and sealed. After a year, the cracks were completely gone.

Concrete with designer Italian ash

Modern concrete consists of gravel, sand and water, held together by the binding element of cement. Most modern concrete uses Portland cement, invented in the 1800s in England, which is a combination of materials such as calcium, silicon, aluminum, iron, limestone, shells, and chalk, heated at exceptionally high temperatures and then ground into a fine powder.

Modern cement requires the materials to be heated to a temperature of more than 1,500 degrees Celsius (2,700 degrees Fahrenheit), leading to enormous greenhouse gas emissions. Each ton of concrete produced creates a metric ton of carbon dioxide emissions. If the cement industry were a country, it would have the third-highest emissions in the world, after only the United States and China.

Modern concrete is also notoriously fragile and will crack and crumble after just a few decades, especially when wet or near salt water, as evidenced by the crumbling brutalist concrete architecture popular in many Israeli cities.

Roman concrete, especially in marine environments such as the Caesarea port, has long captured the fascination of researchers for its durability. The Romans themselves were exceptionally proud of their concrete and left extensive written records about their concrete mixing. Pliny the Elder’s first century CE opus “Naturalis Historia,” described a process whereby volcanic ash mixed with water formed an “impenetrable stone.”

University of Utah geologist Prof. Marie Jackson led a team in 2017 that used Pliny the Elder’s description to guide their groundbreaking research about Roman cement that was used underwater. Their research found that seawater creates a reaction with the concrete — specifically the volcanic ash contained in it — to seal cracks.

The construction of Caesarea’s breakwaters was an ingenious use of dense “hydraulic concrete,” a mixture of mortar, Italian pozzolana, sandstone and slaked lime.

There are approximately 35,000 cubic meters (1.2 million cubic feet) of concrete in the Caesarea port, and 24,000 cubic meters (850,000 cubic feet) of this concrete are made up of the pozzolana ash, said Prof. Boaz Zissu, an expert in the Roman, Hellenistic and Byzantine periods at the Martin (Szusz) Department of Land of Israel Studies and Archaeology at Bar Ilan University. This represented at least 52,000 tons of imported volcanic ash, the equivalent of 100 to 150 large boatloads, archaeologists determined in an earlier study focusing on the port’s construction.

“There were boats bringing wheat from Alexandria to Italy, because the Italians were hungry for Egyptian wheat, and we believe that on the way back to Egypt, the ships were likely bringing the pozzolana,” Zissu said.

Caesarea is an amazing story. It was the idea of a stubborn king with an agenda that goes against the laws of nature

Because the pozzolana was such a luxury item, it was only used for the most important projects, ones that were near and dear to Herod’s heart — like his family tomb — or that required extraordinary engineering feats, like Caesarea.

“Caesarea is an amazing story,” Zissu said, because the geographic location is not necessarily suited to a port, as there is no protection from the open sea. “It was the idea of a stubborn king with an agenda that goes against the laws of nature.”

According to Zissu, the quality of work at the Caesarea port is not as refined as other Roman construction. But that could be because the local workers, unfamiliar with the imported materials, were trying to pour concrete into blocks while bobbing in a boat in the middle of the open sea, he said.

“They were required to build a port in difficult conditions, where it’s totally not suitable,” Zissu said. “So part of the support for this project [from Rome] came in the form of this pozzolana.”

Only the concrete made with the pozzolana ash has the self-healing qualities that Seymour’s team observed in their experiments.

Other Roman ruins in Israel such as Apollonia, Beit Shean, and Masada utilized similar building methods to Roman construction in Italy, but used local labor and local materials. When making concrete, workers used things like crushed pottery or cement as the binding agent.

Why does this concrete have white specks?

Seymour and her team started questioning why the concrete they found had lime clasts, or white granules rich in calcium, that are visible to the naked eye, especially because it looked like these lime clasts had reacted in strange ways within the concrete.

There is lots of historical writing about what went into [Roman concrete], but it is hard to recreate in the lab

“One of the elusive features of Roman concrete is understanding the exact order of operations, how it was produced, and how it was produced at scale,” said Seymour. “There is lots of historical writing about what went into it, but it is hard to recreate in the lab.”

“One of the things we noticed from previous studies, particularly in Roman marine concrete, is that these lime clasts were pretty prevalent and looked like they had partially reacted or fully reacted over time to become different,” said Seymour.

They were deep into the research when Seymour took part in an exchange between MIT and Israeli scientists through MISTI, the MIT International Science and Technology Initiative, in 2018. Seymour visited an archaeological dig outside of Rehovot managed by the Weizmann Institute’s Prof. Stephen Weiner, the founder and director of the Kimmel Center for Archaeological Science.

Seymour was struck by the way Israeli researchers on-site were using Fourier Transform InfraRed (FTIR) spectroscopy, an advanced camera that determines how infrared rays are absorbed in the material and can provide a unique “thumbprint” of that material’s chemical composition.

It’s a popular process in microarchaeology, but Seymour’s team at MIT hadn’t yet utilized it for their Roman concrete. After this “aha” moment, when she returned to the lab in Boston, they sent a few samples through a neighboring lab’s FTIR machine and were able to further characterize the calcium carbonate in the concrete. This helped them understand more about its chemical composition.

The team surmised that the Romans were likely using “hot mixing” for this type of concrete, which occurs when sand, volcanic ash and burned limestone are mixed. When water is added, a chemical reaction occurs that heats the mixture up to 200 degrees Celcius (392 Fahrenheit). This process also leaves tiny but visible white granules rich in calcium.

As time goes on, if the concrete cracks and water enters it, the water dissolves these bits of calcium and they recrystallize along the fissure, sealing it over time. This reaction between the water and the bits of calcium is what gives the concrete a self-healing property.

Archaeology the eye can’t see

Weizmann’s Weiner is one of the pioneers in the field of microarchaeology — and one of the first scientists to bring advanced microscopes and lab equipment to the site of the dig for real-time analysis.

Weiner first started bringing infrared spectroscopy machines to the field in the 1990s, encouraged by the possibilities of understanding the chemical makeup of different materials and the stories these materials can tell about daily life. Having the lab equipment on site is important because the material that can provide the most information on a microscopic level, such as bits of mortar or ash, often gets cleared out during the excavations, Weiner said. When the lab equipment is on hand, they can determine within a few minutes if a certain material is useful and excavation should proceed in a different manner.

Weiner’s lab runs a field school each summer for PhD and master’s students from around the world, which was how Seymour first saw the onsite testing in 2018.

Mortar was a huge revolution and one of the first materials synthetically produced by humans. Once they did that, they could do all sorts of things

Weiner is also fascinated by the stories found in the material that holds the stones together. “Mortar was a huge revolution and one of the first materials synthetically produced by humans,” he said. “Once they did that, they could do all sorts of things.”

All roads to Rome are paved with Portland cement… for now

Seymour is now trying to apply her ancient Roman knowledge to modern concrete in hopes that the cement industry can replicate some of the self-healing mechanisms the Romans perfected two millennia ago.

“One of the things we are looking at is understanding the durability mechanisms in Roman concrete in order to make better materials for the future,” she said. “This is about sustainability because more durable structures require less work and less materials to maintain.”

MIT’s Prof. Admir Masic, Seymour’s advisor and a co-author of the paper, has a patent exploring how modern concrete can maintain its structural integrity while incorporating self-healing properties. Masic is also the co-founder of a company, DMAT, which is trying to implement these practices.

Seymour is now a consultant in the private sector of the concrete industry. She focuses on supplementary cementitious materials or materials that can be added to concrete to replace some of the Portland cement. Her goal is to make concrete more environmentally friendly, based on Roman practices.

“It’s important to note this study is one small piece of the puzzle of how Roman concrete remains durable,” Seymour added. “There were so many different formulations the Romans used based on location and application, we may never know every single process going on with this material. We have so much to learn about how these processes work.”

Seymour spoke to The Times of Israel on the first day of a personal trip to Israel, and the day before her first visit to Caesarea, the ancient port city built by Herod the Great in around 25-13 BCE.

“Aqueducts are my favorite type of Roman construction,” she said. “But I’ve only seen aqueducts in Rome and Naples, so this is really an opportunity to see how material was being made and used in a totally different context.”

That’s one of the things that fascinates Seymour about Roman concrete: In each place it’s used, the composition is a little bit different, uniquely adapted to the specific function and location while still maintaining the same properties that allowed it to endure for millennia.

“I could study this for the rest of my life, and I’d still be coming across new types,” she said. “I’m still in awe of how they were able to manipulate the material, and what control they had over the material.”