Roman Concrete Healed Itself

Why millenia-old concrete outlasts its modern version.

MIT News (“Riddle solved: Why was Roman concrete so durable?“):

The ancient Romans were masters of engineering, constructing vast networks of roads, aqueducts, ports, and massive buildings, whose remains have survived for two millennia. Many of these structures were built with concrete: Rome’s famed Pantheon, which has the world’s largest unreinforced concrete dome and was dedicated in A.D. 128, is still intact, and some ancient Roman aqueducts still deliver water to Rome today. Meanwhile, many modern concrete structures have crumbled after a few decades.

Researchers have spent decades trying to figure out the secret of this ultradurable ancient construction material, particularly in structures that endured especially harsh conditions, such as docks, sewers, and seawalls, or those constructed in seismically active locations.

Now, a team of investigators from MIT, Harvard University, and laboratories in Italy and Switzerland, has made progress in this field, discovering ancient concrete-manufacturing strategies that incorporated several key self-healing functionalities. The findings are published today in the journal Science Advances, in a paper by MIT professor of civil and environmental engineering Admir Masic, former doctoral student Linda Seymour ’14, PhD ’21, and four others.

For many years, researchers have assumed that the key to the ancient concrete’s durability was based on one ingredient: pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples. This specific kind of ash was even shipped all across the vast Roman empire to be used in construction, and was described as a key ingredient for concrete in accounts by architects and historians at the time.

Under closer examination, these ancient samples also contain small, distinctive, millimeter-scale bright white mineral features, which have been long recognized as a ubiquitous component of Roman concretes. These white chunks, often referred to as “lime clasts,” originate from lime, another key component of the ancient concrete mix. “Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” says Masic. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”

Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognized self-healing capability. “The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” says Masic. “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.”

Upon further characterization of these lime clasts, using high-resolution multiscale imaging and chemical mapping techniques pioneered in Masic’s research lab, the researchers gained new insights into the potential functionality of these lime clasts.

Historically, it had been assumed that when lime was incorporated into Roman concrete, it was first combined with water to form a highly reactive paste-like material, in a process known as slaking. But this process alone could not account for the presence of the lime clasts. Masic wondered: “Was it possible that the Romans might have actually directly used lime in its more reactive form, known as quicklime?”

Studying samples of this ancient concrete, he and his team determined that the white inclusions were, indeed, made out of various forms of calcium carbonate. And spectroscopic examination provided clues that these had been formed at extreme temperatures, as would be expected from the exothermic reaction produced by using quicklime instead of, or in addition to, the slaked lime in the mixture. Hot mixing, the team has now concluded, was actually the key to the super-durable nature.

“The benefits of hot mixing are twofold,” Masic says. “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.”

During the hot mixing process, the lime clasts develop a characteristically brittle nanoparticulate architecture, creating an easily fractured and reactive calcium source, which, as the team proposed, could provide a critical self-healing functionality. As soon as tiny cracks start to form within the concrete, they can preferentially travel through the high-surface-area lime clasts. This material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material. These reactions take place spontaneously and therefore automatically heal the cracks before they spread. Previous support for this hypothesis was found through the examination of other Roman concrete samples that exhibited calcite-filled cracks.

The full Science article is available, at least for now, without a paywall. While I’ve only skimmed it, it’s noteworthy that the researchers were not only able to ascertain why the Roman variant was more durable but to replicate it with currently-available materials. Whether they’re sufficiently cost-effective and doable at major construction project scale isn’t clear.

What’s also not clear—and is likely unknowable—is whether the Roman process was a happy accident or they understood at the time what they had achieved. I’m puzzled and fascinated by those rare occasions when ancient civilizations were able to undertake projects that we can’t replicate today. Science, and all learning, tends to be cumulative so I find it strange. The Romans had a written language, so it’s not as if their work was pre-historic.

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James Joyner
About James Joyner
James Joyner is Professor and Department Head of Security Studies at Marine Corps University's Command and Staff College. He's a former Army officer and Desert Storm veteran. Views expressed here are his own. Follow James on Twitter @DrJJoyner.

Comments

  1. OzarkHillbilly says:

    What’s also not clear—and is likely unknowable—is whether the Roman process was a happy accident or they understood at the time what they had achieved.

    The article I posted yesterday said that it is thought to have been just a happy accident(we will never know for sure). Regardless, it’s cool stuff to read about.

  2. Kathy says:

    What’s also not clear—and is likely unknowable—is whether the Roman process was a happy accident or they understood at the time what they had achieved.

    The answer is probably: yes.

    I’m puzzled and fascinated by those rare occasions when ancient civilizations were able to undertake projects that we can’t replicate today.

    How do you skin a bear, flake a spear point, produce fire without matches or a lighter, render fat from a whale, build a pyramid with bronze tools, etc, etc?

    Or how do you increase the efficiency of a steam engine*?

    We stop using old methods when they’re no longer useful or when better, easier, more efficient, or cheaper methods supplant them. No one builds computers with vacuum tubes anymore, because first transistors and then microchips proved cheaper and more capable. No one uses bone needles, either. And there’s no engineer with a wood bow and kindling to light the engines of a commercial jet.

    That’s why we don’t know how the ancients did it, because we don’t have the problems they had. We have different problems.

    This, BTW, produces many idiot conspiracy theories. Because we can’t figure out how the Egyptians or the Maya built monumental stone pyramids without even iron tools, never mind modern construction tools, then it must have been impossible for them either, and aliens did it.

    Here’s another question, as I abhor those who worship the ancients as much as those who discount them: is Roman concrete stronger than modern concrete?

    The Romans were first rate engineers in their time. But a Roman bridge that has stood millennia is not as good as a modern steel bridge across the same gap, which makes more efficient use of materials and can support a vastly larger tonnage.

    Trivia: while the materials used are different, essentially today’s roads employ the same design as Roman roads did. We also use the same calendar, plus one correction.

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  3. Joe says:

    @Kathy: I had the opportunity to visit Peru and Machu Picchu about 3 years ago. I was invited to admire the stunningly detailed Incan stone work, especially the huge stones on the mountain top at Machu Picchu, but I never hears any explanation about how they did it.

  4. Chip Daniels says:

    @Kathy:
    We in the modern times often overlook just how clever and intelligent pre-modern people were.
    Which is ironic since trying to do even simple things, like constructing a building which is level, square and plumb, or trying to determine when the crops are ripe, or even just refining copper ore into a useful material, all done without any of the modern technology and knowledge.

    Also, the whole “Aliens musta done it” theory is incredibly culturally chauvinistic.
    Like, the Meso-American structures like Machu Pichu were constructed around the same time as the Gothic cathedrals, but you don’t hear anyone saying, “By golly, how could those primitive Frenchmen have built Notre Dame- They musta had help from space aliens!”

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  5. Chip Daniels says:

    @Chip Daniels:
    First paragraph missing a concusion-
    …all done without any of the modern technology and knowledge requires a tremendous amount of inventive skill and cleverness.

  6. Mister Bluster says:

    @Kathy:..aliens did it.

    No doubt the outer space visitors who built the ancient structures used anti gravity machines that never came to rest on the projects that they were assembling leaving no evidence of their presence.

  7. OzarkHillbilly says:

    @Chip Daniels: Which is ironic since trying to do even simple things, like constructing a building which is level, square and plumb,

    What might be surprising to people not in the construction business is the fact that we largely still use the same techniques/tools for plumb, level, and square.

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  8. Just nutha ignint cracker says:

    @Chip Daniels: I don’t have any cognitive dissonance about that at all. The Frenchmen who built the cathedral at Notre Dame were at the apex of civilization as they understood it at the time. The Incas were ignorant savages worshiping false gods and barely able to dress themselves. Completely different! 😉

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  9. dazedandconfused says:

    I think it important to note they are really talking about mortar to hold bricks, which is a different mix from what today we call concrete, which is cement mixed with aggregate (gravel) and reinforced with steel. They also may have been unaware their crude mixing techniques resulted in very long lasting mortar. They may have preferred it because fast cure allows for faster construction.

  10. OzarkHillbilly says:

    @dazedandconfused: I think it important to note they are really talking about mortar to hold bricks, which is a different mix from what today we call concrete, which is cement mixed with aggregate (gravel)

    I think you misread something. They are in fact talking about concrete with (volcanic) aggregate sans the reinforcement.

  11. OzarkHillbilly says:

    @dazedandconfused: From the article I posted up yesterday:

    Roman concrete was produced using lumps of volcanic rock and other aggregates held together with a mortar made with ingredients including a pozzolan (such as volcanic ash), a lime source (calcium oxide) and water. Among previous explanations for the strength of the material, researchers have revealed that concrete from Roman breakwaters and piers contain the minerals aluminous tobermorite and phillipsite that helped to reinforce to concrete.

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  12. Kathy says:

    @Chip Daniels:

    It goes farther than that. No one proposes aliens built the Pantheon, the Acropolis, the Pharos, or any other Greek or Roman monumental buildings.

    I think the reason is obvious.

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  13. dazedandconfused says:

    @OzarkHillbilly:
    All we need is an example of Roman cast concrete that held up. Still looking for one. All the cases they mention are masonry in which the mortar has held up incredibly well.

  14. Kathy says:

    @dazedandconfused:

    I think the Pantheon qualifies. See the link above.

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  15. Lounsbury says:

    Science, and all learning, tends to be cumulative so I find it strange. The Romans had a written language, so it’s not as if their work was pre-historic.

    Not strange if you understand language change, as well as the incompleteness of the historical record. The fact one has a written recipe contemporaneous to usage does not automatically mean (i) the recipe written before standardised terminologies and naming – notably on a scientific basis is complete; (ii) that the understanding of specific terms and names meaning has not been clouded by terminology shift even within Latin during either the actual classical period but more importantly after collapse, obscuring terms, meanings…

    Am quite familiar with this in the context of Arabic as the pretence that the Standard Arabic is the same as the Quranic Arabic leads to all kinds of peculiarities (notably Islamist political ones) as the accretion and shifting of meanings over a millenia & half rather obscures more than the Salafistes would ever acknowledge even what the original textual meanings of certain terms and words in the direct text were, even as finally written down…

    @Kathy: there is the aspect in addition to your obs of Selection Biais. The rubbish Roman construction did not survive to same extent as the excellent.

    @Chip Daniels: Such observations must be qualified that there is the partial excuse of documentation (insofar as the pre-historic Stonehendge etc gets similar woo treatment). But that is the breach into which the racialist-chauvinists rush of course to exploit as a pretext for non-Euro, nevertheless one should not overdraw.

  16. grumpy realist says:

    @Lounsbury: It has been pointed out that the major reason why Sappho’s works haven’t made it down to us is because she wrote in a Greek dialect which wasn’t one of the major ones used.

    Translation of something in order to have it survive is a big reason why literature and poetry vanishes. Other stuff ends up surviving only because it has the name of a famous author associated with it. (Go read up on the Art of Memory and memory palaces to discover how the seminal treatise on such only survived because people thought it was written by Cicero.)

  17. Jay L Gischer says:

    Yes, I would be surprised to find that the Romans did not make written records of how to do things. But now that I’ve written that I wonder, maybe they didn’t think that’s what writing was for? Are there surviving engineering texts in Latin from those days? Is there evidence that there were some but they did not get copied over by medieval monks because they didn’t seem important?

    I’m very curious about this process.

  18. dazedandconfused says:

    @Kathy:
    Yeah, I see it now, the Pantheon should’ve crumbled from water some time ago..but didn’t.

    Smithsonian article from 2014

  19. dazedandconfused says:

    @Jay L Gischer:

    Pretty common for artisans in history to keep their trade secrets.