Roman concrete, the secret of the longevity of ancient structures

More than 2,000 years old, and the Pantheon in Rome still supports the largest dome of antiquity. Such longevity continues to amaze modern architects, well aware that this 40-meter-diameter dome, completed in the 2nd century, saw countless modern structures collapse long before them.

The secret, they know, comes from its material itself: the concrete used by the Romans. This building material which, in fact, has nothing to do with a modern invention. “Concrete is a very old material, used by the Romans, and whose recent archaeological research locates the appearance in the middle of the 2nd century BC, recounts Valérie Nègre, architect and professor of the history of techniques at the University of Paris l Panthéon-Sorbonne, in a program of

Science, CQFD. Archaeologists believe that its expansion dates from around the beginning of our era. In the 1930s to be exact. And it is linked to the fact that the Romans were great builders. They build buildings, infrastructures of considerable size, basilicas, baths, etc. And above all, they use the arch and the vault, for example for the baths of Diocletian or the great bridges of the Appia voice. However, arches and vaults require thick walls to contain the thrusts and therefore, to save time, to save material, the Romans will use concrete.

If we have the impression that it is a modern invention since the industrialization of cement during the 19th century, concrete, in the sense that it is defined as an aggregate of mineral materials (aggregates) mixed with a binder (of cement, clay or bitumen), is actually an old architectural tool. Some ancient structures, 2,000 years after their construction, are also there to attest to the effectiveness of this material, some aqueducts even continuing to supply cities with water.

The longevity of these ancient structures, however, has long questioned architects, who wondered how they could withstand the ravages of time in this way. Until then, experts believed that Roman concrete owed its effectiveness to a particular composition, including pozzolana: “We know that the secret of the solidity of these Roman concretes – or Roman cements – is due to an adjuvant that was added to the mortar: a volcanic powder collected in the region of Pozzuoli, west of Naples”continued Valérie Nègre in La Science CQFD in August 2022. Despite everything, historians and scientists alike struggled to understand one of the specificities of this ancient material: when it cracks, Roman concrete has the strange ability to repair itself. .

Lime in front

It is thanks to the research of a team from MIT, the Massachusetts Institute of Technology, that the secret of this concrete has finally been revealed. Scientists analyzed fragments taken from an ancient city wall in the city of Privernum, Italy, using advanced imaging techniques including energy dispersive X-ray spectroscopy. The researchers were thus able to establish precisely the composition of Roman concrete, and in particular stopped on shards of white minerals called lime clasts. Until then, experts thought that their presence was due to a sloppy or poor quality mortar. A theory to which Admir Masic, professor of civil and environmental engineering at MIT and co-author of

the study published in the journal Science Avances : “The idea that the presence of these lime clasts is simply attributed to poor quality control has always bothered me. If the Romans put so much effort into developing an exceptional building material, following recipes optimized over many centuries, why would they put so little into ensuring a quality production process?”

For two millennia, the recipe for concrete has finally changed little: today, we use hydraulic lime, that is to say quicklime slaked with water, to make concrete. . It is also lime that gives concrete its ability to set and harden, even in humid environments. “Since I started working on Roman concrete, I have always been fascinated by the presence of these piecesadmits Admir Masic. “They are not present in modern concrete, so why were they in the old?”

Spectroscopic analyzes by MIT scientists revealed that the lime clasts were made of calcium carbonate, a compound that only forms at extreme temperatures. This element allowed them to deduce the major difference between modern concrete and Roman concrete: the latter was mixed directly with quicklime, that is to say at very high temperatures.

“The advantages of hot mixing are twofold”,

explains Admir Masic in an MIT press release . “First, when the whole concrete is heated to high temperatures, it allows for chemistries that aren’t possible if you were just using hydrated lime, producing high-temperature associate compounds that wouldn’t otherwise form. Second, this increased temperature dramatically reduces cure and set times, since all reactions are accelerated, allowing for much faster construction.”

Self-repairing concrete

Better yet, it is thanks to these lime clasts that Roman concrete has the ability to self-repair. The hot mix gives it a fragile nanoparticle structure: in other words, the very structure of the concrete will facilitate fractures that will pass through the calcareous lime clasts, thus exposing them to the light of day.

However, once the latter have been exposed, when the water will pass through its cracks, it will become saturated with calcium, which will be able to crystallize again in the form of calcium carbonate. It is this reaction which makes it possible to reinforce the crack, and gives the concrete this astonishing capacity for self-regeneration. Water can also react with pozzolan, volcanic powder, which will help to further strengthen the crack.

To be sure, the researchers tried the experiment again: they created samples of concrete based on quicklime and deliberately cracked them, before adding water. Two weeks later, the cracks had healed. A control sample produced without quicklime remained cracked.

Ancient concrete, concrete of the future?

But how was this ancient knowledge – if the Romans at the time were well aware of the properties of this concrete – lost? The idea that the composition of this concrete was due to luck rather than a well-established recipe remains entirely plausible. But all the same, concrete, if it has been used over the centuries in Rome and Lazio, has gradually been abandoned elsewhere. “In the Middle Ages, we will build much less”, says Valérie Nègre. “It is true that outside Rome, the use of concrete is becoming rarer. Firstly because less monumental buildings are being built there. And then, when you move away from the center of Rome, volcanic powder and brick powder are expensive, even luxurious materials. So for reasons of economy and size of the constructions, we will abandon it, except perhaps in the foundations.”

Concrete will be rediscovered during the Renaissance, precisely when “where we try to revive Antiquity”, completes the professor of history of techniques. From there, it became an indispensable part of architecture, and really took off during the industrial revolution. As the techniques become industrialized, it invades the big cities.

It should be remembered that in many respects, and in particular as regards its chemical composition, modern concrete, better controlled, remains of a quality far superior to Roman concrete. But this astonishing capacity for “self-healing” is already arousing the interest of researchers, at a time when concrete has become a major issue, particularly in terms of the environment. At present, its production represents 8% of global greenhouse gas emissions, and research projects for “green concrete”, which would reduce its environmental impact, are multiplying. According to Admir Masic, understanding how Roman concrete works could help achieve this goal, by making it possible to create a material that is stronger, lighter, and more durable. In short, more ecological.