Almost 2,000 years have passed since Mount Vesuvius unleashed a pyroclastic hell that devastated everything that was around it. That was what ended Pompeii, but it was also what gave it eternity.
The Roman city began to be excavated in 1739 and, we believe, a third of the city is still underground. That’s why it still continues to surprise us.
A work half done. That week in the summer of the year 79 AD, the first domes of the X insula of the IX regio was under construction. This is not surprising, of course. All of Pompeii had been under construction for almost two decades (since the earthquake of 62). However, the curious thing is that on the night of August 24, the workers were caught with everything bogged down.
Plumb lines, chisels and weights; stacked tiles, tufa bricks; amphorae filled with lime, reused demolition materials and piles of pozzolans scattered on the ground. Everything has remained there, untouched, until a team from the MIT Department of Civil Engineering found and cataloged them.
“The weapon of crime.” By reconstructing the scene and studying the processes, researchers concluded that these masons left incontrovertible evidence of how they mixed “hot” quicklime with volcanic ash to create concrete capable of repairing itself.
In fact, as Miguel Ángel Criado collectsthings go further: the chemical and crystallographic analysis of the materials reveals quicklime (calcium oxide) in the structural concrete and slaked lime (calcium hydroxide) in the finishing mortars, thus confirming a double differentiated use.
Have we finally found the key to Roman concrete?
A recurring question. In 2023, I already said that “Every so often, the world rediscovers Roman concrete and is amazed by the durability of a material that allows Agrippa’s Pantheon to stand for 2,000 years.” “While modern concrete cracks after a few decades,” they usually add.
By the way, “almost with the same regularity, there is some scientist or engineer who claims to have found the key secret to making this so.” But the truth, the true truth, is that despite its undoubted historiographical interest (and its potential for illuminate our knowledge of the masonry practices of 1st century Rome), the hype is always unjustified.
The two mistakes of Roman concrete. When talking about Roman concrete, a lot of mistakes are usually made, but there are two recurring ones: the first is “the survivor’s bias.”
As Manuel F. Herrador reminded usprofessor of Structural Concrete at the School of Civil Engineering of the Universidade da Coruña, the idea of the extraordinary quality of Roman concrete comes from studying, precisely, the best structures they made, the ones that have been best preserved. Instead, most of what the Romans built has already completely disappeared and cannot be studied.
The second error is even more basic and is based on ignoring that with Roman concrete we couldn’t do even a tenth of the things we do today with modern concrete. For example, today we can make long and relatively narrow “pieces” thanks to reinforced concrete. That was impossible with Roman construction techniques and is what makes our structures corrode faster.
Who wouldn’t like a Roman concrete…? We already knew that Roman concrete is not a single miraculous material, but a family of recipes adapted to local environments and resources (ports, temples, roads, thermal baths). This finding only confirms it; but, in a calculated way, it is used to take advantage of the expectation that this material generates in the public imagination.
And if it weren’t for making invisible the excellent work of contemporary researchers, it wouldn’t be a problem either. Because what is evident is that we do not make “Roman-style concrete” not because of ignorance; we don’t do it because we don’t want tobecause it does not serve the world we want to build.
Image | Andy Holmes
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