We Finally Know How Ancient Roman Concrete Was So Durable

The Romans were masters of engineering and building. Pozzolanic concrete is a spectacularly durable concrete that gave Roman structures their incredible strength.

The world's largest dome of unreinforced concrete can be found in the Pantheon, a structure that has been around for nearly 2,000 years.

The ingredients of this concrete include pozzolana, a mix of volcanic ash, and lime. Strong concrete can be created when water and materials react together.

That is only part of the story. A team of researchers led by the Massachusetts Institute of Technology found that the materials are slightly different from what they were thought to be.

There were small white chunks of lime that could be found in the well-mixed concrete that the smoking guns were found in. The presence of these chunks had been attributed to poor mixing, but that didn't make sense to materials scientist Admir Masic.

The idea that low quality control was to blame for the presence of these lime clasts always bothered me.

If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been altered over the course of many centuries, why wouldn't they make a well-mixed final product? There needs to be more.

The samples of Roman concrete were carefully studied by Masic and the team. To gain a better understanding of the lime clasts, these samples were subjected to a number of different methods.

The nature of the lime was one of the questions that was asked. Slaked lime is used in pozzolanic concrete. A caustic powder called quicklime is produced when limestone is heated to a high temperature.

Slaked lime is a slightly less caustic paste that is produced when quicklime is mixed with water. It is believed that the Romans mixed the slaked lime with the pozzolana.

The lime clasts in their samples are not in line with the method. Roman concrete may have been made by mixing the quicklime directly with the pozzolana and water at high temperatures, or by adding slaked lime to the mix.

The benefits of hot mixing are twofold.

When the concrete is heated to high temperatures, they allow chemistries that are not possible if you only use slaked lime. The increased temperature allows for faster construction since all the reactions are accelerated.

The self-healing abilities of the lime clasts make it an excellent choice.

Cracks in the concrete preferentially travel to the lime clasts, which have a higher surface area than other particles in the matrix. When water gets into the crack, it reacts with the lime to form a solution rich in calcium that dries and hardens as calcium carbonate, preventing it from spreading further.

There are cracks in the concrete at the Tomb of Caecilia Metella, which is 2,000 years old. It could explain why the seawalls built 2,000 years ago are still standing today.

The team made pozzolanic concrete from ancient and modern recipes. They made a control concrete with no quicklime. The control concrete remained cracked even after the cracked quicklime concrete was fully healed.

The team is trying to make their concrete more eco-friendly.

Masic says it's exciting to think about how these moredurable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete.

The research was published in a journal.