Scientists Finally Solved the Mystery of Roman Concrete’s Self-Healing Strength

autumn in rome and vatican city
The Secret Strength of Ancient Roman ConcreteLaszlo Szirtesi - Getty Images
  • Ancient Roman concrete is known as some of the strongest in history, and a new study finally explains why.

  • MIT researchers studied the self-healing properties of the concrete mix.

  • Extreme temperatures while mixing created quicklime, meaning hot mixing leads to self-healing.

Ancient Roman concrete is a global fascination. Remarkably, the Pantheon—Rome’s unreinforced concrete dome, dedicated in 128 A.D.—still stands to this day, and aqueducts from the same period continue to carry water to Rome. The concrete, simply put, holds strong.

For decades, researchers credited the inclusion of volcanic ash from Pozzuoli, a city in Naples, Italy, for the unique strength in the mix; others claimed lime was the key ingredient. A team of researchers from MIT, Harvard, Italy, and Switzerland, led by MIT’s Admir Masic, a civil and environmental engineering professor, knew there was more to the story. Plenty more when you look at the millimeter-scale bright white mineral features dubbed “lime clasts” embedded in the concrete.

The team discovered ancient Roman concrete gets its strength from self-healing properties, which help fill in cracks as they form. Sure, the lime and the ash help, but the true star of the show is “hot mixing,” a process of creating concrete that forms reactive calcium. In other words, these small lime-clast chunks can react with water, post-mixing, to recrystallize as calcium carbonate, and fill cracks while reacting with the ash for further strength. The reactions take place spontaneously, self-healing the cracks before they spread.

The Big Roman Concrete Question

segovia aqueduct, spain
Segovia’s most recognizable symbol is El Acueducto (Roman Aqueduct), an 894-meter-long engineering wonder that looks like an enormous comb plunged into Segovia. First raised here by the Romans in the 1st century A.D., the aqueduct was built with not a drop of mortar to hold more than 20,000 uneven granite blocks together. It’s made up of 163 arches and, at its highest point in Plaza del Azoguejo, rises 28 meters high. The aqueduct was originally part of a complex system of aqueducts and underground canals that brought water from the mountains more than 15 kilometers away.Getty Images

Masic tells Popular Mechanics he moved to MIT with the specific goal of researching ancient Roman concrete. He established a lab, and hired students interested in the topic. As a chemist first and foremost, he says he knew there was something beneath the surface when it came to the strength of Roman concrete.

“The more I was reading about it and studying about it, the lime is not well understood in this fundamental component,” he tells Popular Mechanics. “It was overlooked, and I believe it was overlooked because we focused on the volcanic component.”

Scientists commonly saw ash as the hero ingredient because the Romans were the first to add volcanic ash to concrete, creating a completely new chemistry in the mixture, known as “hydration,” instead of carbonation. A hydration-based concrete—in the past, the aerial mortars required air to harden and capture carbon dioxide—gives the mixture additional strength compared to carbonation, and makes it possible to build in all seasons.

But questions remained. Research showed that Roman concrete formed crystalline phases known to only take shape at high temperatures—at least 80 degrees Celsius (or about 176 degrees Fahrenheit). Masic wondered how that could be, with no reasonable way the concrete mixture, or its final location, ever reached those sorts of temperatures.

Then, a recent study found calcium aluminate hydrate as a binding agent in Roman concrete, creating further intrigue. Even in portions where the Romans added crushed ceramic tiles for a water-resistant mixture, calcium formed. You can credit part of the formula to the volcanic ash, but not all of it. “Where did the calcium come from? This is where the research starts,” Masic explains. “Ask where the calcium comes from.”

The Power of Lime

Calcium comes from lime. But there’s still a difference. Masic notes that modern concrete— when mixed with lime, volcanic components, and water—creates a homogenous mixture, quite like Roman-style concrete. The Romans, though, “would systematically create heterogenous concrete. How is it possible that every construction site would make this heterogeneous material even though they put so much effort [into] making it very standard in terms of ingredients, how to mix it?”

Know Your Terms: The word “homogenous” refers to a mixture with a uniform appearance throughout, while a “heterogeneous” mixture has components that you can visually distinguish between. A cup of coffee with creamer mixed into it is homogenous, because every sip is the same; meanwhile, a bowl of chicken noodle soup is heterogenous, because you can see the noodles, vegetables, and hunks of meat suspended in the mixture.

The research team decided to investigate lime clasts, small chunks of lime they found in every sample they analyzed. “The presence of these lime clasts—and mechanical features and structural aspects of lime clasts, and chemical aspects of lime clasts—led to the conclusion,” Masic says, “that lime clasts are not an error in mixing, or sloppy mixing, but they are the root of a very specific way of mixing mortars.

“Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features. These are not found in modern concrete formulations, so why are they present in these ancient materials?”

Roman concrete was mixed in kilns 164 feet tall and 26 feet in diameter. Originally, the belief was that the Romans added water with the lime and powder of volcanic ash and aggregate to make a slurry, called “slaking.” That creates a homogeneously structured mixture. However, the presence of lime clasts, which included calcium carbonate (quicklime), meant this wasn’t the secret after all.

Mixing Matters

“Instead of having this stone reacting with water to create the slurry,” Masic says, “you grind this stone to small pieces that are now calcium oxide, mix them with a volcanic ash and dry aggregate, and then you add water. This is where the reaction of calcium oxide with water happens, and the entire mix gets hot because of the reaction between calcium oxide and water, [which] releases energy, and creates hot spots up to 200–250 degrees Celsius.”

That “hot mixing” introduces a series of chemical consequences not found in cold mixing, only explainable by the reactions produced using quicklime instead of, or in addition to, the slaked lime mixture.

“The benefits of hot mixing are twofold,” Masic explains. “First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times, since all the reactions are accelerated, allowing for much faster construction.”

Self-Healing Concrete

map of ancient concrete that shows a two centimeter fragment of ancient roman concrete with a visible lime clast
A large-area elemental map (Calcium: red, Silicon: blue, Aluminum: green) of a 2-centimeter fragment of ancient Roman concrete (right) collected from the archaeological site of Privernum, Italy (left). A calcium-rich lime clast (in red), which is responsible for the unique self-healing properties in this ancient material, is clearly visible in the lower region of the image.Courtesy Masic et. al

The hot mixing allows lime clasts—characteristically brittle nanoparticulate, an easily fractured and reactive calcium source—to develop. So, when tiny cracks start to form within the concrete, the lime clasts react with water to create a calcium-saturated solution that recrystallizes as calcium carbonate to quickly fill the crack, or react with Pozzolanic materials to strengthen the concrete. The reactions happen automatically, healing the cracks before they spread, all part of the findings Masic’s team published in the journal Science Advances last month.

The key effect in hot-mixing with quicklime, Masic says, is you don’t dissolve all the lime clasts, and they get systematically embedded into the concrete mix, creating a reservoir of calcium. Those reservoirs of calcium, starting out as quicklime, can expand when hydrated, creating cloth-like nanoparticles.

“When water comes in, it just dissolves, and this is where the calcium comes from,” he explains. “These clasts dissolve and recrystallize as a form of calcite in the cracks. Now we are talking about self-healing.”

The team tested the theory, and found the Roman-inspired hot-mixing concrete healed a crack within two weeks. A modern concrete mixture didn’t heal at all, allowing water to flow through the crack, which would eventually destroy the concrete.

The Future of Concrete

Masic says the world already has self-healing concrete, but costs keep the industry from using it. “This technology might be a game changer simply because we can offer the self-healing at an extremely competitive price,” he says.

The latest find—which still shows the importance of both lime and volcanic ash in the mixture—can evolve the design of Roman-inspired solutions in the future, Masic says. The functionality of self-healing concrete allows the mixture to last longer, and requires less maintenance and less new material for new construction, all part of the goal to reduce emissions when creating concrete. Plus, the Roman-inspired self-healing concrete simply performs better when compared to today’s standard mixture.

“It is one step,” Masic says, “in the right direction.”

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