Research suggests that ‘self-healing’ Roman concrete could help modern construction

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They survived the fall of an empire, the carnage of great wars, and the rise of a new state. But why structures made of Roman concrete are so durable remains a mystery.

Now, scientists say they have discovered one possible explanation: The technique used to make the material may have helped give it self-healing properties.

“The Pantheon wouldn’t exist without concrete, as it did in Roman times,” said Admir Masic, an MIT professor of civil and environmental engineering and lead author of the paper.

However, he added that despite the fact that the Roman writer and philosopher Pliny the Elder noted that concrete could get stronger with age, it was unlikely that the Romans were aware of the chemistry involved – or how long the material would last.

“They knew it was a great material, but they probably didn’t know it would last for thousands of years,” Masic said.

Roman concrete was made from pieces of volcanic rock and other aggregates held together by a mortar made from ingredients including pozzolana (such as volcanic ash), a source of lime (calcium oxide) and water.

Among previous explanations for the strength of the material, researchers revealed that the concrete from the Roman breakwaters and pillars contained the clay minerals tobermorite and phillipsite, which helped strengthen the concrete.

Now, scientists say it appears the techniques used to prepare Roman concrete may 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 clumps called calcareous clasts, which are not found in modern structures.

While it had previously been explained that they were due to poor mortar mixing or other errors, the team suspected there might be other causes.

They examined a sample of Roman concrete from a wall in the ancient city of Privernum near Rome, revealing that the limestone clasts contained therein contain various forms of calcium carbonate, some of which tend to form in conditions where water is not freely available.

The team found that the clasts were porous with cracks, which also suggested they 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 that it was added to the ash and aggregates first before the water was added.

This approach is known as “hot mixing” due to the heat generated. The team adds that these high temperatures would not only help the mortar set, but would also reduce the water content in and around the limestone clasts, explaining their results.

The team suggests that the resulting calcareous clasts may have helped the concrete to ‘self-heal’, as water seeping into cracks in the material would dissolve the calcium carbonate as it passed through the calcareous clasts.

The crack in the concrete could then self-heal, either by reaction of this calcium-rich fluid with the volcanic material, or by recrystallization of the calcium carbonate. Indeed, the team notes that calcium carbonate-filled cracks have recently been found in Roman concrete.

To test their theory, Masic and colleagues created Roman-inspired concrete that they mechanically cracked. They then placed the pieces 0.5 mm apart and exposed them to flowing water for a period of 30 days. Samples containing calcareous clasts sealed with newly formed calcite, while control samples made without calcareous clasts remained fractured.

Masic said the Roman approach could prove useful in modern construction.

“Roman-inspired approaches, based on hot-mixing, for example, could be a cost-effective way to extend the life of our infrastructure through the self-healing mechanisms we have illustrated in this study,” he said.

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