Pont du Guard

This is the Pont du Guard, an aqueduct bridge made in the first century by Romans who used it to supply a colony where the modern French city Nîmes now exists. Think about the most recent modern cement structures that you have seen which have cracks and crumbles, and it will drive home just how sophisticated and durable such structures were.

Historians of engineering have long acknowledged how much the Ancient Romans had mastered the construction of concrete, and there have been many theories about why they were able to produce cement that could last even in waterlogged or earthquake- prone areas. For instance, one type of volcanic ash from an area near the Bay of Naples was shipped all around the Empire and was acknowledged in the Ancient world as being a key ingredient.

But in a paper just published in January 2023, a team of engineering scientists based out of MIT, Harvard, and laboratories in Italy and Switzerland think they have found the most important factor: lime “clasts” or milimeter-width thick particulates made of quicklime (see second photo), which give the concrete the ability to “self-heal”.

Turns out, the lime clasts had been thought to be accidental byproducts of the concrete-making process. But this is because what everyone had got wrong was the temperature at which the cement was made. Rather than having the lime mixed with water at a relatively cool temperature and then mixed with the other ingredients, it turns out they deliberately mixed the lime at hit temperatures, which allowed for a different chemical compound of the lime that produced the “lime clasts.”.

The engineers tested their ideas by cracking different types of concrete and then running water continuously through the cracks. The Ancient cement and modern concrete which contained the special lime particulates came out well ahead: the lime clasts actually reacted with the water and recrystallized, effectively healing the cracks. A control of concrete made without the lime clasts didn’t self repair and water continued to flow through the structure.

Modern cement production makes up about 8% of greenhouse emissions, so we would do well to borrow from the Ancient practice.