They have survived the fall of an empire, great wars and the creation of a new country. Roman concrete is used to make structures that are sodurable.

The technique used to make the material might have helped to give it self-healing properties, according to researchers.

The lead author of the paper said that the Pantheon would not exist without concrete.

He said that it was unlikely that the Romans were aware of the chemistry involved in concrete and how long it would last.

They probably didn't know that it would last thousands of years.

Roman concrete was made from volcanic rock and other materials held together with a mortar made with ingredients such as a pozzolan and lime.

Researchers have shown that Roman breakwaters and piers contain minerals that help reinforce concrete.

Techniques used to prepare Roman concrete may help explain why it has stood the test of time.

Lime clasts, which are not found in modern structures, are found in samples of Roman concrete.

The team suspected there could be more than poor mixing of the mortar as the reason for these.

They looked at a sample of Roman concrete from a wall in the ancient city of Privernum near Rome and found that it contained different forms of calcium carbonate.

The team found that the clasts were porous and formed in a low water environment.

The quicklime may not have been mixed with water before it was added to the other ingredients. It is likely that it was added before water was added.

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Because of the heat produced, this approach is called hot mixing. The team said that high temperatures would help the mortar to set, but would also reduce the water content in and around the lime clasts.

The team thinks the lime clasts could have helped the concrete "self heal" by dissolving calcium carbonate when it was wet.

Either the fluid reacting with volcanic material or the recrystallization of the calcium carbonate would self-heal the fractured concrete. There are cracks in Roman concrete filled with calcium carbonate.

Masic and colleagues created Roman-inspired concrete to test their theory. After setting the pieces apart, they exposed them to the water for 30 days. Control samples made without lime clasts were fractured.

The Roman approach could be useful.

He said that Roman-inspired approaches could be a cost-effective way to make our infrastructure last longer.