The Secret of Roman Concrete

The Colosseum or Rome, constructed out of Roman Concrete. Courtesy Megan Sauter.

From the Pantheon of Rome to the port of Caesarea Maritima, Roman buildings have stood the test of time, thanks to one specific material: Roman concrete. Yet despite decades of research, the secret of Roman concrete has remained tantalizingly allusive. But not anymore, suggests a study published in the journal Science Advances. According to its authors – led by researchers from MIT, – one key component of this ancient material gave it a special property, self-healing.

Self-Healing Concrete

Thanks to ancient descriptions of the material by Pliny the Elder and Vitruvius, as well as modern research, we know that Roman concrete was made with volcanic rock and other aggregates bound by a mortar of volcanic ash, lime, and water. While this formula is not remarkably different from the one we use today, the longevity of the material might have to do with the precise technique used in its preparation and known as “hot mixing.” The team was turned onto this possibility by the ubiquity of lime clasts throughout Roman concrete. These chunks of unincorporated lime have often been interpreted as the result of sloppy mixing or low-quality materials.

“The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” said Admir Masic, the lead author of the study, in a press release. “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”

The Pantheon of Rome is an unreinforced dome constructed out of Roman concrete. Courtesy Photo Companion to the Bible.

Indeed, the Romans have long been thought of as master builders and engineers, whose constructions have stood the test of time as few others have done. The Pantheon of Rome, for example, remains the world’s largest unreinforced concrete dome, despite being nearly 2,000 years old. In contrast, modern concrete begins to degrade after only a few decades. So, what is the secret of Roman concrete?

While studying a concrete sample from the wall of the ancient Roman city of Privernum, the team noticed that the lime clasts within the concrete showed signs of having formed in high temperature and low humidity conditions. This could not be explained by the traditional process of lime slaking, where the lime is hydrated in water to create a paste before being added to the other mortar materials. Instead, the presence of these lime clasts can only be explained by the process of hot mixing, where the lime is added directly to the mortar mixture and only then water is incorporated. This would lead to an endothermic reaction, creating a high heat and low humidity environment within the mortar.

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

Overlooking the artificial harbor of Caesarea Maritima which was constructed largely out of Roman concrete. Courtesy Nathan Steinmeyer.

Although often considered an unwanted byproduct, the unfused lime can work as a lasting source of calcium within the concrete, long after it has already been set. When incorporated with water, through a crack or other defect in the concrete, the lime clast reacts to create a calcium-rich solution that can operate as glue to fill in holes and cracks. As such, the concrete can self-heal small cracks within itself even centuries or millennia after it was created. Although this process had previously been noticed within Roman concrete, its causes remained unknown.

To double-check their results, the team prepared several samples of concrete using their theorized method for Roman concrete. Then they deliberately cracked the material and began to run water through the cracks. After two weeks, the team’s new Roman concrete was completely sealed.

Despite knowing of the incredible power of their concrete, it is unlikely that the Romans ever understood the chemical science behind it or its longevity. “They knew that was a great material, but they probably didn’t know that it would last thousands of years,” Masic told The Guardian.

The Secret of Roman Concrete and Eco-Friendly Construction

With the final secret of Roman concrete potentially in hand, the team is now working to commercialize their new cement recipe. While it has the potential to provide higher-quality and longer-lasting concrete, it can also significantly mitigate the environmental toll of the production of concrete. Currently, cement production accounts for about eight percent of global greenhouse gas emissions. By drastically lowering the frequency at which concrete needs to be replaced, as well as requiring only a fraction of the energy to make, this new concrete would sharply reduce the quantity of yearly emissions by the concrete industry. Perhaps the most instantly noticeable benefit of such material, though, would simply be in the reduced number of potholes on roads across the globe.