Ancient Roman Manufacturing Technique Found by Researchers Could Help Create Self-Healing Concrete: Study

Actors dressed as maids of ancient Rome march in front of the Coliseum in a commemorative parade during festivities marking the 2,766th anniversary of the founding of Rome on April 21, 2013, in Rome, Italy. Giorgio Cosulich/Getty Images

A group of researchers from MIT, Harvard University, and facilities in Italy and Switzerland has made significant progress in uncovering ancient concrete-manufacturing processes that included numerous crucial self-healing features.

The findings were reported in Science Advances in an article co-authored by MIT professor of civil and environmental engineering Admir Masic, former doctoral student Linda Seymour, and four others.

For years, scholars have concluded that the key to the ancient concrete’s longevity was pozzolanic material, such as volcanic ash from the Pozzuoli area on the Bay of Naples. This particular type of ash was even brought all the way across the vast Roman empire to be utilized in construction, and was identified as a major element for concrete in accounts written at the time by architects and historians.

These ancient samples also include microscopic, unmistakable millimeter-scale bright white mineral structures, which have long been recognized as a common component of Roman concretes. These white pieces, known as “lime clasts,” are made of lime, another important component of the ancient concrete mix. The lime clasts have been dismissed as indications of sloppy mixing or low-quality raw materials. However, the new study reveals that these small, previously disregarded ingredients, give the concrete a previously unknown self-healing power.

Questions by Researchers

“Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” Masic tells David Chandler in MIT News. “These are not found in modern concrete formulations, so why are they present in these ancient materials? The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” he says.

“If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”

The researchers gained fresh insights into the potential usefulness of these lime clasts after additional analysis of these lime clasts, utilizing high-resolution multiscale imaging and chemical mapping techniques pioneered at Masic’s research group. It was previously considered that when lime was used in Roman concrete, it was first mixed with water to generate a highly reactive paste-like material, a process known as slaking. However, the presence of the lime clasts could not be explained only by this procedure.

“Was it possible that the Romans might have actually directly used lime in its more reactive form, known as quicklime?” Masic posits.

After studying samples, he and his colleagues established that the white inclusions in this old concrete were formed of various kinds of calcium carbonate. And spectroscopic investigation revealed that they were created at high temperatures, as would be predicted given the exothermic reaction caused by employing quicklime instead of, or in addition to, slaked lime in the combination. The team has recently established that hot mixing was the key to its super-durable nature.

“The benefits of hot mixing are twofold,” Masic said, according to the publication. “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.”

The lime clasts generate a particularly brittle nanoparticulate architecture during the hot mixing process, resulting in a readily broken and reactive calcium source, which, as the team hypothesized, could provide important self-healing activity. When microscopic cracks emerge within the concrete, they preferentially travel through the high-surface-area lime clasts. This substance can then react with water to form a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the fracture, or it can combine with pozzolanic materials to reinforce the composite material even further. Because these processes occur spontaneously, they automatically mend the gaps before they propagate. Previous evidence for this notion came from the investigation of additional Roman concrete samples with calcite-filled fissures.

To demonstrate that this was the process responsible for the endurance of Roman concrete, the researchers created samples of hot-mixed concrete that included both ancient and modern formulations, broke them purposefully, and then flowed water through the cracks. Within two weeks, the cracks had healed completely and the water could no longer flow. A comparable piece of quicklime-free concrete never healed, and the water just continued to flow through the sample. Following the success of these experiments, the team is trying to commercialize this improved cement material.

“It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete formulations,” says Masic.

How Changes in Concrete Could Impact Building Today

Contractor Robert Blume, owner of Blume and Son Construction and a 43-year veteran of the construction business, told The Epoch Times that many of the decisions made about concrete in construction today are largely based on cost. The contractor said that self-healing concrete is something people in his industry are already actively testing but said the research being done into how to make concrete that stands the test of time could have short-term implications on his business if it’s cheap enough to fit into home and business owners’ already strained construction budgets.

“The basic components of what [the Romans] used are still the basic components of what we use,” Blume said. “The biggest deal is we do this commercially… so we have to ask ourselves if making a stronger concrete is worth the price. If we are putting in a slab for something that will have extremely heavy stuff on it, lots of vibration, we used a better cement and more of it. So even now we have all kinds of components we can put in, like fiberglass, and really make it strong. It’s just that nobody wants to spend $100,000 on a $10,000 slab. Economics affects this greatly.”

Blume, who lived in Eastern Europe for 10 years, said many of the buildings there have been around for hundreds of years and are still standing: “The buildings in Romania, they’ve held up through 20 earthquakes a year and a thousand years. But the walls may be four feet thick. If I built that way now,  a house would cost you $2 million dollars.

“Right now we could build concrete homes that would last 1,000 years, but nobody is going to want to spend that on it. Plus in another 10-20 years the technology is going to be so different it will have been a waste.”

Blume said there are dozens of types of concrete that can be used, but medium-grade commercial concrete is used because it’s cheap, and the discoveries being tested right now will be used as standard building materials if they can be sourced easily and economically.

Janille Maragh of MIT, Paolo Sabatini of DMAT in Italy, Michel Di Tommaso of the Instituto Meccanica dei Materiali in Switzerland, and James Weaver of Harvard University’s Wyss Institute for Biologically Inspired Engineering were also part of the study team. The Archeological Museum of Priverno in Italy assisted with the work.