They have stood through the fall of an empire, the carnage of great wars and the foundation of a new country. But quite why structures made using Roman concrete are so durable has remained something of a mystery.
Now researchers say they have discovered one possible explanation: the technique used to make the material may have helped to give it self-healing properties.
“The Pantheon would not exist without the concrete as it was in the Roman time,” said Admir Masic, MIT professor of civil and environmental engineering and the lead author of the paper.
But, he added, despite the Roman author and philosopher Pliny the Elder noting that concrete could become stronger with age, it is unlikely the Romans were aware of the chemistry involved – or just how long the material would endure.
“They knew that was a great material, but they probably didn’t know that it would last thousands of years,” said Masic.
Roman concrete was produced using lumps of volcanic rock and other aggregates held together with a mortar made with ingredients including a pozzolan (such as volcanic ash), a lime source (calcium oxide) and water.
Among previous explanations for the strength of the material, researchers have revealed that concrete from Roman breakwaters and piers contain the minerals aluminous tobermorite and phillipsite that helped to reinforce to concrete.
Now researchers say it appears techniques used to prepare Roman concrete might also help explain why it has stood the test of time.
Writing in the journal Science Advances, Masic and colleagues note that samples of Roman concrete contain small lumps known as lime clasts that are not found in modern structures.
While these have previously been explained as arising from poor mixing of the mortar or other errors, the team suspected there could be other reasons.
They examined a sample of Roman concrete from a wall in the ancient city of Privernum near Rome, revealing that the lime clasts within it contain different forms of calcium carbonate, some of which tend to arise in conditions where water is not freely available.
The team found the clasts were porous with cracks, which also suggested they were formed in a high temperature, low water environment.
The researchers say this suggests the quicklime was not mixed with water before it was added to the other ingredients. Instead, it is likely it was added to the ash and aggregates first, before water was added.
This approach is known as “hot mixing” because of the heat produced. The team add that these high temperatures would not only have helped the mortar to set, but would have reduced the water content in and around the lime clasts, explaining their results.
The team propose the resulting lime clasts could have helped the concrete “self heal”, as water seeping into cracks in the material would dissolve calcium carbonate as it passed through the lime clasts.
The fracture in the concrete could then self-heal either by this calcium-rich fluid reacting with volcanic material, or by recrystallisation of the calcium carbonate. Indeed, the team note calcium carbonate filled cracks have recently been found in Roman concrete.
To test their theory, Masic and colleagues made Roman-inspired concrete, which they mechanically fractured. They then set the pieces 0.5mm apart and exposed them to flowing water over a 30-day period. Samples that contained lime clasts sealed with newly formed calcite but control samples made without lime clasts remained fractured.
Masic said the Roman approach could prove useful in modern construction.
“Roman-inspired approaches, based for example on hot mixing, might be a cost-effective way to make our infrastructure last longer through the self-healing mechanisms we illustrate in this study,” he said.
