Editorial comment
What have the Romans ever done for us? Apart from sanitation, medicine, education, wine, public order, irrigation, roads, aqueducts, public health, and providing one of the key formative cultural influences behind Western civilisation…
Register for free »
Get started now for absolutely FREE, no credit card required.
Well, you can now add self-healing cement that results in concrete structures capable of lasting millennia to that list. It’s been known for a long time that there must be something special about the cement used by the Romans – after all, the Pantheon in Rome still features the world’s largest unsupported concrete dome, a record it has held for just shy of two thousand years. Through countless earthquakes, almost every kind of weather, war, and civil unrest, the dome of the Pantheon has endured it all.
So, what’s the secret? It had long been thought that the special properties of Roman concrete had something to do with volcanic ash from the Bay of Naples. The Romans certainly thought highly of the ingredient and considered it important enough to ship it to construction projects across the empire – no mean feat considering that the fastest methods of cargo transport at the time were either an ox cart or sailing vessel.
However, the ash may well have been a red herring with contemporary Roman architects inadvertently guiding modern investigations in the wrong direction. According to researchers at MIT, Harvard, and laboratories in Italy and Switzerland, the true secret lies in another unique characteristic of Roman cement: lime clasts.
The presence of lime clasts in Roman concrete (and their absence from the modern product) had been previously explained away as a side-effect of primitive mixing methods and poor quality raw materials. However, as MIT professor of civil and environmental engineering Admir Masic, says: “The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me. If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimised over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product?”
The researchers ultimately concluded that the lime clasts were formed by the use of quicklime in the concrete mixture, rather than the slaked lime used in modern concrete. The quicklime produces an exothermic reaction and the subsequent ‘hot mixing’ process is key to the formation of lime clasts. The clasts are brittle and break easily, but react with water when broken and recrystallise as calcium carbonate. This recrystallising process happens spontaneously, automatically repairing cracks as they propagate through the concrete, thus providing the extraordinary durability demonstrated by the Pantheon’s dome.
The research team is already looking at commercialising the discovery and hopes this breakthrough in understanding will lead to new forms of lighter, more durable concrete, the extended lifespan of which would have a positive impact on the cement industry’s carbon footprint. Maybe we’ll soon be able to add ‘solving climate change’ to the list of Roman accomplishments?
