The secrets of the mortar that built an empire

Updated:11/22/2020 01: 55h

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Although it is not known who first discovered or used concrete, it probably his birth took place 12,000 years ago when remains of burned limestone reacted with oil shale.

It is easy to imagine our ancestors next to a fireplace, chatting and telling stories, the fire would come into contact with calcareous stones, plaster and clay. The high temperatures caused the stone to carbonate and turn into dust.

Surely, the last ingredient arrived hours later in the form of a downpour, the drizzle caused the dust and the stones to become a solid and compact amalgam, forging the first cement in history. A gift from nature.

Then everything would come rolled. Those primitive men seized the opportunity and they decided to build the floor of their homes by joining limestone, sand, gravel and water, by way of a very rudimentary cement.

From the Nabataeans to the Colosseum

It took a long time for the Nabataeans – the inhabitants of present-day Syria and Jordan – to use it to build architectural structures, some of which are still preserved today. Later, moving along the winding timeline, it was the turn of the Egyptians, who used a mortar of lime and plaster to build the famous pyramids of Gizeh.

However, it was the Romans those who used concrete on a large scaleIt was they who used it in works such as the Colosseum, Trajan’s market, the Pantheon of Agrippa or the Alcántara bridge, in Hispania. With this mortar they “built” their empire.

It is said that Octavian Augustus found a brick Rome and left a marble one, although it would be much more accurate to say that the Rome that yielded was made of concrete.

The long life of these buildings makes us suspect that the builders Romans knew perfectly how to dose the components of the mixture and the use of additional techniques to improve the strength of the material.

However, despite the fact that many Romans mentioned concrete in their writings – from Pliny the Elder to Vitruvius, passing through Cato the Censor – its exact recipe has not reached us.

The secret is in the dough

For centuries Roman concrete was an inscrutable secret. It was necessary to resort to fluorescence and microdifraction to reveal one of the best kept mysteries of the Roman Empire.

Now we know that they achieved this by mixing volcanic ash with lime – calcium oxide – and sea water. With this mixture they achieved a mortar to which they later incorporated volcanic rock -Puzzolans-, achieving what is known as a pozzolanic reaction.

Finally, the holes in the lime were filled by tobermorite crystals, while the seawater filtered through the cracks in the rock, reacting with the remains of the volcanic ash and contributing to the creation of more crystals. Ultimately, the resulting Roman concrete had a very rock-like consistency. With the help of concrete, Rome became the Eternal City.

To finish an etymological curiosity. The word “concrete” has its origin in the resemblance to a sponge cake that was prepared with almonds, flour, milk and eggs and that was known by the name of “formigó”. I leave it there…

M. Jara

Pedro Gargantilla is an internist at the Hospital de El Escorial (Madrid) and author of several popular books.

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