One of the best kept secrets of the Roman Empire is What was special about your concrete? so that Agrippa’s Pantheon has been there for 2,000 years (and what remains). From time to time science and archeology believe you have discovered the secret and for example, a button: this half-finished work in Pompeii left clues. We have found the umpteenth attempt to explain why concrete is no longer made like it used to be in an emperor’s toilet.
The discovery. To date the explanation for the resistance of Roman concrete is in the reaction between lime and volcanic ash (a pozzolanic reaction), but a new study adds one more ingredient to the recipe: natural carbonation and the progressive formation of calcite were also important for this construction material to withstand the passage of time so well.
In addition to the ingredient, the curious thing is where they took the sample from: from the concrete collector located under one of the seats of one of the common latrines in the western complex of the Canopus, in Hadrian’s Villa in Tivoli. From a toilet.
Why is it important. Because beyond the pozzolanic reaction, what really holds concrete together over the centuries is calcite. So the formation of calcite over centuries also helps the structures of these latrines to remain standing two millennia later. Knowing this mechanism opens the doors to the design of modern cements that are more durable.
Context. Before we thought that all the merit fell on the reaction between the lime and the ash and be careful, that explanation is still valid, but it is not the only one. Furthermore, previous research such as this MIT study from 2023 pointed out that the white fragments of lime present in the concrete were proof that the Romans used a “hot mix” technique, which gave this construction material the ability to self-heal when cracks appeared.
In detail. From 3D images, the team rebuilt in detail the internal composition of the material and what chemical processes it underwent over the centuries. Thus, they verified that calcite was formed through a slow reaction between lime, humidity and carbon dioxide in the air (carbonation) and that mineral progressively filled cracks and pores, making the concrete denser and blocking the passage of potentially harmful substances or even water. The volcanic rock fragments also reacted with the lime in the edge area, which strengthened the bond.
Yes, but. The main limitation of this study is that it is based in a single sample from an indoor environment, so the conclusions obtained cannot be directly extrapolated to other Roman concretes studied outdoors or in marine environments, where Different self-repair mechanisms have been described with salt water. We will have to wait for the results to be replicated in other samples and sites before reaching a general conclusion.
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Cover | Jl FilpoC and Mineralized carbonates contribute to the millennial durability of Roman concrete. Advanced Science
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