MIT, Harvard, Italian, and Swiss scientists re-discover why Roman concrete structures last millennia while modern concrete doesn’t

Visiting the Pantheon, one of Rome’s premier tourist sites, it’s hard not be humbled by the knowledge that this building has lasted 1900 years and still stands as the world’s largest unreinforced concrete dome, while modern concrete structures deteriorate. Take a look at the magnificence of the Pantheon.

Photo credit:  CC BY-SA 4.0 license

Photo credit: Macrons CC BY-SA 4.0 license

Photo credit: Anthony Majanlahti CC BY 2.0 license

The Romans’ concrete technology was lost for almost a millennium as the Dark Ages unfolded, and Europe regressed technologically and economically.  The discovery of Roman manuscripts on making concrete in 1414 sparked gradual reintroduction of concrete as a building material, eventually including the use of reinforcing materials like iron bars that enabled feats that the Romans could not accomplish.

Yet for all our scientific and technical sophistication, Roman concrete resisted cracking and eventually crumbling far better than ours.

Now, as MIT News reports, scientists from MIT, Harvard, and 2 European research institutes have discovered the secret to Rome’s concrete recipe. We may finally be ending the Dark Ages for concrete. David L. Chandler writes:

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.

The Romans used:

…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.

But moderns disregarded the presence in Roman concrete of:

…white chunks, often referred to as “lime clasts,” [which] 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?”

If anything, the lime clasts were attributed to low quality, perhaps sloppy mixing practices, but they turned out to be the key to durability. Using quicklime a more reactive form of lime, the Romans reached high temperatures while mixing the concrete:

“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. 

With this new high technology that is actually lost ancient technology, will we moderns start building structures that will stand the test of the centuries? Will future civilizations marvel at our engineering achievements?

Maybe. But consider the fact that with discounted cash flow analysis, the methodology used for evaluating major investment projects, the long term durability of a structure matters not at all. A dollar invested today is worth 95 cents in a year, if a discount rate of 5% is chosen as the cost of capital. Follow that logic out for 50 years, and values at that point are close to zero.

The new-old technology may still be adopted, though at least at first the cost of re-configuring the production facilities to allow hot mix techniques and the costs of different raw materials may raise costs and prices. But the ability of the new formulation to resist cracking may mean less concrete is needed for certain structural elements. Engineering studies and building code revision may be needed, but the new formulation may do more with less concrete.

The song “everything old is new again” debuted on Broadway in 2003. Twenty years later, it has a new embodoment.

Edit: cut reference to and picture of Nazi structure.

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