A group of scientists from the Massachusetts Institute of Technology in the United States explained, using the example of a Roman tomb, why ancient concrete is still more tenacious than modern concrete.

The authors of the new work examined the tomb of Cecilia Metella – it is a round tower made of large stones of volcanic origin, supported by concrete. During construction, materials from a solidified lava flow were used.

Today these walls are over two thousand years old, but the concrete remains very durable. The authors of the new work decided to find out why. Scientists have determined that tephra was used in the construction of the tomb – these are deposits of material thrown into the air by a volcano and then settled to the ground. The stones used by the builders contained a lot of leucite, a magmatic mineral rich in potassium.

Over time, groundwater washed the walls of the tower, they dissolved leucite and released potassium into the solution. In modern concrete, the abundance of potassium would lead to the formation of expanding gels that would cause microcracks and structural collapse, but this did not happen with the tomb.

To find out why, the authors studied the solution using X-ray diffraction and Raman spectroscopy. As a result, they realized that the concrete was changing unevenly. This means that dissolved potassium in different places changed the structure of compounds in different ways.

X-ray diffraction made it possible to analyze the altered areas down to their molecular structure – and it became clear that these areas have acquired a nanocrystalline nature, reducing the average size of the “grain” in concrete. Researchers are confident that it is these altered areas that enhance the adhesion of the particles that make up such concrete.

Also, for the strength of the structure, the most important was the area where the concrete was connected to the stone. The fact is that today in concrete there are chemical reactions between alkaline components and silica, which is in the composition of sand. The result is gels that expand and destabilize the structure.

And in ancient concrete, as a result of the reaction, nanocrystalline structures appear: they even increase the strength. As a result, such ancient structures only become stronger over time, this helps them withstand external destructive factors.