Roman structures, such as the Pantheon, have survived for nearly two thousand years. Researchers have now discovered the secret behind the longevity of Roman concrete. This offers possibilities for modern construction.
The ancient Romans were true master builders. They built roads, aqueducts, harbors and huge buildings, such as the Colosseum and the Pantheon. Large parts still hold up two millennia later. A single millennia-old aqueduct even supplies water to fountains in Rome.
In contrast to modern constructions, these structures made of unreinforced concrete prove to be resistant to climate influences, earthquakes and, on the coast, even direct contact with seawater. This makes the old, Roman building materials the ideal source of inspiration for new, sustainable buildings. But before that a riddle had to be solved: what makes Roman concrete so durable?
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Researchers at the Massachusetts Institute of Technology (MIT) think they have found the answer, they write in Science.
Volcanic material
The Roman concrete mostly consists of volcanoes ejected tuff and other granulates, ie granular solids, bound by a mortar (or grout, used for bricklaying) based on lime and so-called pozzolanas, such as volcanic ash. The idea was that the volcanic material was the secret ingredient. But how this explains the longevity of the Roman concrete was still unclear.
Small white lumps of a few millimeters in size can also be found in the concrete, which you will not find in modern concrete. For a long time they were dismissed as uninteresting irregularities caused by, for example, poorly mixed mortar.
‘The idea that the presence of these lime granules was simply due to poor quality control has always bothered me’, say Admiral Masic, civil and environmental engineer at MIT. “If the Romans put so much effort into making an excellent building material, following all the detailed recipes optimized over many centuries, why should they put so little effort into a well-mixed final product?” According to Masic, there had to be more behind the lime granules.
Self-healing concrete
Masic and colleagues bent over the white lumps, which indeed turned out to consist of calcareous material. A chemical analysis showed that they were formed at high temperatures. That indicates that it is reactive quicklime had been used in the production of the mortar. Mixing quicklime with the volcanic granulates, sand and water causes a chemical reaction in which heat is released. This process is called hot mixing (hot mixing).
That the Romans made mortar by using quicklime instead of, or in combination with slaked lime, could explain both the lime granules and the durability of the concrete. The hot mixing process produces lime granules with a brittle structure. These easily breakable granules, which consist of a reactive material, make the concrete durable due to their self-healing properties.
If cracks appear in the concrete, they will quickly spread to these lime granules because the material breaks easily there. Water can then enter the lime cube through the crack. This causes a reaction whereby the lime crystallizes into calcium carbonate and thus fills the crack. It can also react with pozzolanic materials to form stronger concrete.
Each of these two reactions occurs naturally when cracks form, allowing the concrete to quickly repair itself.
Modern Roman concrete
To test whether this process actually takes place, the researchers simulated the Roman concrete using the hot mixing process. Deliberately caused cracks indeed appeared to be completely repaired within two weeks by pouring some water into them. Concrete made without the quicklime did not recover.
The researchers are now developing this discovery into a commercially available, self-healing concrete. That would last longer and therefore be more sustainable than the current building material.