On the origin of Roman concrete

The Pantheon cupola in Rome, made by Roman concrete some 2000 years ago. Source: Wikimedia Commons, CC BY-SA 3.0

The Pantheon cupola in Rome, made by Roman concrete some 2000 years ago. Source: Wikimedia Commons, CC BY-SA 3.0

You mix burnt lime, volcanic ash and sand/gravel. Then you have the famed concrete that made the Romans able to build the Pantheon cupola, or to make underwater constructions, like big harbours. And without Roman concrete, we may not have had our modern concrete, based on Portland cement, enabling us to build ever bigger, higher and deeper. But how was Roman concrete discovered?

Now scientists, cited in Sciencedaily.com, in a highly interesting new paper suggest that the Romans got inspiration in making their concrete from observing and interpreting natural processes in the active volcanic fields at Campi Flegrei by Naples, especially by the town of Pozzouli. For here a very similar concrete is made naturally when lime comes in contact with volcanic deposits.

A compelling story, indeed! Observing nature – making a similar, very good material! It is akin to how we think observation of natural agents and processes deep within rain forests will eventually enable us to develop new medical treatments.

Surely important to study and preserve rain forests, but in the case of concrete, it seems to me that the scientist forget a crucial piece of evidence: The ancients made varieties of concrete long before the Romans – at Rhodes and Minoan Thera (Santorini), for example, where volcanic ash is available (see e.g. the great book from the ROMACONS project). It could be that the story of the origin of concrete is rather simple, not involving an awful lot of observation of natural processes and ancient science: You mix your burnt lime with sand – and if the sand contains volcanic ash, you simply get a very strong mortar or concrete. Thus, from experience you will know that when using this particular volcanic sand, then you get a durable mortar that is able to set under water.

And Roman engineers, as well as the great Roman naturalists, among them Seneca, Vitruvius and Pliny the Elder, certainly knew about the properties of the sands across the Roman world. First of all at numerous places in Italy, but also far away, such as in the Rhine Valley, where volcanic ash deposits were hacked up and ground to trass, which resembled the sand of Pozzouli in making durable mortars and concrete.

The story is important, for it raises crucial questions: How was knowledge on key material production obtained in the good old days? By observation and trying to copy nature? Or rather by chance discovery and keen experimentation over centuries and millennia?

References

About Per Storemyr

I work with the archaeology of old stone quarries, monuments and rock art. And try to figure out how they can be preserved. For us - and those after us. For the joy of old stone!
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