Researchers from the US, Switzerland and Italy claim to have discovered the secret to the longevity of Roman concrete.
The team looked at whether, as has been proposed, it owes its endurance to the use of volcanic ash from the city of Pozzuoli, close to Naples. They used high-resolution imaging and chemical mapping to establish that the material contains lumps of lime that give it a self-healing ability.
These more durable concrete formulations could expand not only service but also the durability of 3D-printed concreteAdmir Masic, MIT professor of civil and environmental engineering
Researchers believe the Romans used a hot mixing technique that caused “clasts” to develop. This means that when cracks form and fill with water, the lime reacts by saturating it with calcium carbonate that then crystallises, filling the crack.
The team succeeded in recreating the hot-mixed concrete technique, then proving that the resulting material had the ability to repair its own cracks. A similar test was undertaken without quicklime but this did not self-heal.
The team contained members from the Massachusetts Institute of Technology, Harvard, the Instituto Meccanica dei Materiali from Switzerland and Italy’s Archeological Museum of Priverno.
Admir Masic, MIT professor of civil and environmental engineering, said: “Ever since I first began working with ancient Roman concrete, I’ve been fascinated by these [clasts]. These are not found in modern concrete formulations, so why are they present in these ancient materials?
“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 research is published in the journal Science Advances.