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Researchers Look at Roman Concrete Durability
A team of researchers from the Massachusetts Institute of Technology, Harvard University, and laboratories in Italy and Switzerland have recently taken to studying ancient Roman concrete to crack the code of what makes the material ultradurable.
For their study, the team looked at how concrete structures like the Pantheon and ancient Roman aqueducts, as well as structures that have undergone harsh conditions such as docks, sewers, and seawalls or those constructed in seismically active locations, managed to stay intact after all this time.
Findings from the team’s research have since been published in the journal Science Advances, in a paper by MIT professor of civil and environmental engineering Admir Masic, former doctoral student Linda Seymour ’14, Ph.D. ’21. The team also included Janille Maragh at MIT, Paolo Sabatini at DMAT in Italy, Michel Di Tommaso at the Instituto Meccanica dei Materiali in Switzerland and James Weaver at the Wyss Institute for Biologically Inspired Engineering at Harvard University.
The work was carried out with the assistance of the Archeological Museum of Priverno in Italy.
Cracking the Concrete Code
In an MIT-issued news release, the Institute shared that a team of researchers recently discovered ancient concrete-manufacturing strategies that incorporated several key self-healing functionalities.
Previously, other researchers observing the same ancient concrete were often noted to attribute the material’s durability on a mixture of pozzolanic material, such as volcanic ash, and millimeter-scale bright white mineral known as “lime clasts.”
“Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” said Masic. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”
While the lime clasts were chalked up as “sloppy mixing practices” in older studies of the material, the new study suggests that the mineral material is what gave the ancient concrete its previously unrecognized self-healing capability.
“The idea that the presence of these lime clasts was simply attributed to low-quality control always bothered me,” Masic continued. “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.”
Using high-resolution multiscale imaging and chemical mapping techniques pioneered in Masic’s research lab, the team went on to further characterize the lime clasts. As a result, they were able to gain new insights into the functionality of the mineral.
Of those insights was realizing that the Romans may not have mixed their concrete materials in a process known as slaking (where lime is incorporated first with water). Instead, researchers believe that the Romans may have used lime in a more reactive form.
This conclusion was made after the team studied additional samples of the ancient concrete, finding that the white inclusions also featured various forms of calcium carbonate—which could only be achieved through “hot mixing.”
“The benefits of hot mixing are twofold,” Masic explained. “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.”
As for the self-healing properties, the team explained in the study that because lime clasts develop a characteristically brittle nanoparticulate architecture through hot mixing, the material becomes an easily fractured and reactive calcium source.
As described by the team, when a tiny crack forms within the concrete, the material preferentially travels through the high-surface-area lime clasts, reacts with water to make a calcium-saturated solution, recrystallizes as calcium carbonate quickly fill the crack, or reacts with pozzolanic materials to further strengthen the composite material.
And, because the reactions take place spontaneously, the cracks can heal automatically before they spread throughout the structure.
To prove the mechanism was behind the durability and self-healing properties of the ancient concrete, researchers produced samples of hot-mixed concrete that incorporated both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks.
Within two weeks, the cracks were observed to have healed completely in the ancient formulation.
“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,” said Masic.
Since making the discovery, the team is reportedly working to commercialize the modified cement material with the hopes that the concrete could help reduce the environmental impact of cement production.
Previous Observations
Back in 2017, researchers at Rice University launched an investigation into the Romans’ concrete formula to better understand how modern cement could be made stronger and more resistant to cracking. Specifically, the team looked at tobermorite, a crystalline mineral that’s analogous to the calcium-silicate-hydrate of cement and was once used in the concrete made by the Romans.
As a result of research efforts, materials scientist Rouzbeh Shahsavari, at Rice University’s Multiscale Materials Laboratory, published a study looking at the atomic structure of tobermorite in hopes of better understanding how it helped to make ancient Roman concrete an “extraordinarily durable, high-performance composite,” as some researchers have called it.
The study also describes how tobermorite particles often exhibit what is called screw dislocations, defects in layers at the molecular level.
Later that same year, Marie Jackson, an expert in Roman concrete from the University of Utah, along with colleagues, looked at the structure of an ancient Roman concrete sample at a microscopic level.
These samples, which had been extracted from seawalls and piers as part of the Roman Maritime Concrete Study project, revealed that the concrete behaves much like volcanic deposits in submarine environments, according to Jackson.
Furthermore, Jackson noted that the crystals within the concrete act like small armor plates, and prevent the concrete from fracturing.
After a series of spectroscopic tests and observation with advanced imaging techniques, what became evident in the study was that the concrete’s strength stems from a rare chemical reaction: aluminous tobermorite crystals growing out of another mineral called phillipsite.
According to Jackson, as seawater found its way into cracks in the concrete, it reacted with the phillipsite, which is naturally occurring in volcanic rock, and formed tobermorite crystals.
With all of these factors combined, it becomes evident that Roman cement itself is hydraulic—it can set underwater, or in wet conditions. According to Popular Mechanics, the Romans mixed this cement with volcanic ash from the Naples region. Between 22 and 10 BCE, the Romans constructed an underwater concrete foundation for Caesarea, an ancient city in what is now Israel.
At the time of the research, the marine structures were noted to be still standing.
More recently, in October 2021, researchers from MIT and the University of Utah conducted research on a 2,050-year-old Roman tomb for insight into concrete resilience. For their study, researchers looked at the tomb of first-century noblewoman Caecilia Metella.
Built using volcanic aggregate, the large cylindrical structure was of interest to lead co-authors of the study, Admir Masic, associate professor of civil and environmental engineering at MIT, and Marie Jackson, research associate professor of geology and geophysics at the University of Utah, for its unusual chemical interactions with rain and groundwater over the course of two millennia.
Due to these design choices, the concrete quality of the tomb may actually exceed its male contemporaries’ monuments.
A landmark on the Via Appia Antica, the tomb of Caecilia Metella consists of a rotunda-shaped tower on a square base, measuring roughly 70 feet tall and 100 feet in diameter. The tomb was constructed in about 30 BCE, when the Roman Republic was transforming into the Roman Empire, using a mixture of coarse brick or volcanic rock aggregate bound with mortar made with lime and volcanic tephra (porous fragments of glass and crystals from explosive eruptions).
The crystals, formed from the potassium-rich mineral leucite, dissolved over time, causing the structure to remodel and reorganize the interface between volcanic aggregates and the cementitious binding matrix, improving the cohesion of the concrete.
Looking at the microstructure of the concrete, researchers discovered that the concrete mixture used for the tomb was similar to the mortar used in the Markets of Trajan 120 years later. The glue of the Markets of Trajan mortar consists of a building block called the C-A-S-H binding phase (calcium-aluminum-silicate-hydrate), along with crystals of a mineral called strätlingite.
What made the tomb’s concrete stronger, however, was the leucite that managed to strengthen the structure over centuries of rainwater and groundwater percolating through the tomb’s walls, reconfiguring the C-A-S-H binding phase.
Stefano Roascio, the archaeologist in charge of the tomb, reported that the study has a great deal of relevance to understanding other ancient and historic concrete structures that use Pozzolane Rosse aggregate.
The research has since been published in the Journal of the American Ceramic Society.
