The genius of ancient Roman concrete

‘What have the Romans ever done for us?’ is the tongue-in-cheek British question.

In terms of architecture and infrastructure, quite a lot, actually.

Furthermore, in 2023 and 2024, researchers studied the durability of ancient Roman concrete, which has adapted through the millennia without cracking.

On February 1, 2024, ScienceAlert posted a fascinating article, ‘We Finally Know How Ancient Roman Concrete Was Able to Last Thousands of Years’ (emphases mine):

The ancient Romans were masters of building and engineering, perhaps most famously represented by the aqueducts. And those still functional marvels rely on a unique construction material: pozzolanic concrete, a spectacularly durable concrete that gave Roman structures their incredible strength.

Even today, one of their structures – the Pantheon, still intact and nearly 2,000 years old – holds the record for the world’s largest dome of unreinforced concrete.

The properties of this concrete have generally been attributed to its ingredients: pozzolana, a mix of volcanic ash – named after the Italian city of Pozzuoli, where a significant deposit of it can be found – and lime. When mixed with water, the two materials can react to produce strong concrete.

However, MIT researchers delved deeper and uncovered other secrets as to its durability:

… not only are the materials slightly different from what we may have thought, but the techniques used to mix them were also different.

The smoking guns were small, white chunks of lime that can be found in what seems to be otherwise well-mixed concrete. The presence of these chunks had previously been attributed to poor mixing or materials, but that did not make sense to materials scientist Admir Masic of MIT.

“The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” Masic said back in January 2023.

The MIT team used high-tech methods to study the lime clasts:

Masic and the team, led by MIT civil engineer Linda Seymour, carefully studied 2,000-year-old samples of Roman concrete from the archaeological site of Privernum in Italy. These samples were subjected to large-area scanning electron microscopy and energy-dispersive x-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging to gain a better understanding of the lime clasts.

The received wisdom was that the Romans used slaked lime:

First, limestone is heated at high temperatures to produce a highly reactive caustic powder called quicklime, or calcium oxide.

Mixing quicklime with water produces slaked lime, or calcium hydroxide: a slightly less reactive, less caustic paste. According to theory, it was this slaked lime that ancient Romans mixed with the pozzolana.

However, the team found that the lime clasts were inconsistent with that method:

Rather, Roman concrete was probably made by mixing the quicklime directly with the pozzolana and water at extremely high temperatures, by itself or in addition to slaked lime, a process the team calls “hot mixing” that results in the lime clasts.

“The benefits of hot mixing are twofold,” Masic said.

“First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction.”

Here’s the key finding:

And it has another benefit: The lime clasts give the concrete remarkable self-healing abilities.

When cracks form in the concrete, they 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, gluing the crack back together and preventing it from spreading further.

Amazing.

The article says that this explains the durability of ancient Roman seawalls, despite constant battering from salty ocean water.

The researchers decided to experiment:

… the team tested their findings by making pozzolanic concrete from ancient and modern recipes using quicklime. They also made a control concrete without quicklime and performed crack tests. Sure enough, the cracked quicklime concrete was fully healed within two weeks, but the control concrete stayed cracked.

‘What have the Romans ever done for us?’ one may ask.

The MIT team are confident that this ancient method can be brought into the 21st century world:

The team is now working on commercializing their concrete as a more environmentally friendly alternative to current concretes.

“It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete formulations,” Masic said.

The research has been published in Science Advances.

Bring it on, I say.