If we were to hit the concrete joints of the Roman Colosseum with a pick today, the tip would bounce off and not a bit of gravel would come off.
If we insisted a lot, and after endless attacks a crack was created, the most surprising thing would come: that gap would take less than two weeks to close again.
She alone.
The cement, upon coming into contact with the air, would restore its own pores.
Although Roman concrete had been studied since the sixties, this last curious quality has been explained just a few days ago.
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In early January, universities in Italy, Switzerland and the United States published a latest scientific proposal on the secrets of Roman cement.
Until now, it was suspected that it owed its strength to the pozzolan rocks, volcanic in nature and found next to the small town of Pozzuoli, on the slopes of Vesuvius.
But today we know more.
The great Roman buildings have survived millennia of history not only because of the purity of their materials, but also thanks to a specific element: quicklime.
Quite a paradox: the Romans created architecture with the same substance that we know to the contrary, by corroding matter.
Traditionally, concrete results from mixing two different ingredients.
One is the framework, that is: the solid, the gravel that will make up the bulk of the compound.
This is part of the success of Roman cement because they found this material, always pure, in Pozzuoli, and from there it traveled to the rest of Rome.
The twist comes in the second component: the binder or glue, a substance that helps hold the gravel together.
Until now, it was known that the Romans chose lime, but always cold, quenched with water.
"Vitruvio himself, known for his architectural treatises, recommends hydrating the lime for at least six months," recalls Pablo Guerra, an archaeologist and professor at the University of Castilla-La Mancha.
His doctoral thesis dealt with Roman construction materials.
As he explains, the mixture of gravel and slaked lime causes that chemical reaction that manages to bind the concrete that we all know.
So, what have the universities that have studied together, from different corners of the world, discovered the concrete of Rome?
Well, to the traditional preparation, made up of gravel and slaked lime, the Romans also added quicklime, fresh from the oven and still burning, during the mixture.
Thanks to this, the great constructions of Rome have not only reached our days.
They took much less time to get up.
“Quicklime accelerates the hardening of concrete and makes it more resistant, as long as it is added during the mixing of the materials.
In addition, thanks to it, the material reacts very well to contact with air.
The concrete pores close on their own when mixed with the carbon in the environment”, explains Guerra.
For this reason, centuries later, any crack in a Roman construction will only require a few days to close on its own.
It is also the reason why the concrete of the Colosseum, or Agrippa's Pantheon, has never been reinforced with other materials.
This cement could be used not only to support huge constructions, but also very fine mosaics.
Guerra mentions those present in the Roman villa of Noheda, in Cuenca.
When the gravel of the compound is especially small, and the mixture can be poured between narrower holes,
better than now
“Roman cement is the best.
No historical civilization, including the current one, has created such a durable concrete.
Until now, this has always been attributed to pozzolan sand.
But this research also promises to shake up the construction of our time”, says Carmen Martínez.
This doctoral student is investigating Roman archeology in Cartagena, where she has been working for 12 years.
And she continues: “I still remember an excavation in which we had to remove a piece of Roman concrete.
It was tremendous.
There was no way, no matter how hard we stung, and I don't remember any other material with which something like this happened”.
There will be those who wonder how it is possible that today, despite all the technological advances that accompany us, concrete is weaker than in Roman times.
“Although it may surprise us, the Romans did not know the chemistry of materials.
They worked by trial and error.
After trying some ingredients and others, they were very skilful when it came to choosing and using them,” says Guerra.
Like Martínez, he maintains that Roman cements are the most resistant, and that the latest discoveries about them can improve current architecture.
Subterranean passages of the Flavio amphitheatre, in Pozzuoli (Naples), the town where the Romans extracted gravel for cement.A.
VERGANI (Getty)
Now, Roman concrete was also exceptional for political reasons, for a sense of the State and production.
“Unlike what we do, the Romans never built 15,000 houses at once.
What would they do it for?
They created a lot, but in a balanced way”, says Guerra.
When he does some restoration, he prepares his own compound, instead of buying it in bags, and follows Vitruvio's recipe: he leaves the lime soaking for at least half a year.
He also sifts the gravel himself to choose the best, as the Romans did.
According to him, this is how concrete is not only more resistant, but also respectful of the planet.
In fact, the Wikipedia entry defines pozzolan as an ecomaterial.
Because not only historians praise Roman cement and understand that it surpasses the current one.
Alfonso Barrón, contemporary architect and materials expert, shares this idea: “In addition to repairing its own cracks, the lime in Roman concrete becomes hard over the years.
As time goes by, it recovers the properties of limestone, the original rock from which it was extracted.
He is younger every day.
Meanwhile, Portland cement, the one we've used since the 19th century, goes the opposite way: it ages and deteriorates."
As the archaeologists consulted intuit, none of the Chamartín towers will survive the Coliseum.
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