Ancient Roman Concrete Secrets Unraveled by Modern Science

Wikimedia Commons. Photo by Rabax63

Ever since their construction, the monuments and structures of ancient Rome have amazed and astounded their viewers with their grandeur and the ingenuity of their engineering. Throughout its history, Rome conquered much of modern Europe, northern Africa, and the Middle East, encompassing nearly 1.7 million square miles of territory at its height. While this conquest certainly brought about no shortage of violence and colonial oppression for countless peoples, it also brought about the diffusion of many Roman technologies such as aqueducts, roads, and, perhaps the most significant of all Roman creations, concrete.

Credit: Carol Hagen, UC Berkley

A cross-section of Roman concrete. Multiple inclusions such as pozzolanic materials and white lime clast chunks are clearly visible in the mixture.

Any casual tourist in a region of past Roman occupation will likely come across the concrete ruins of an empire that still persist after thousands of years of human activity and weather. For centuries, many thought the secrets of what made these Roman structures so durable were lost. Buildings like the Pantheon in Rome are emblematic of this inquiry, still standing very nearly just as it was when it was constructed around 117 CE. The Pantheon remains to this day the world’s largest unreinforced concrete dome; a feat that no modern construction company could hope to replicate. It has not only survived the tides of humanity ebbing and flowing around it but has also withstood countless earthquakes which have leveled the buildings and other structures surrounding it even in more modern times. 

Earlier work by scholars and engineers thought that the unique coffering within the dome’s interior, the thinning of the dome thickness leading up to the oculus, as well as the stress-relieving arches built into the walls of the rotunda were the reasons behind this engineering marvel. Later, more sophisticated analyses of the concrete used to construct the dome found that broken bits of pottery were added to the concrete mixture to help lighten the load of the material while stretching its use.

Credit: Wikimedia.

A view of the pantheon from the front with the dome looming behind. Though constructed around 117 CE, the building maintains the inscription that would have adorned the first pantheon constructed around 29-19 BCE before it burnt down. The inscription reads: Marcus Agrippa, son of Lucius, made this in his third consulship.

 

Even with these properties considered, however, the sheer durability of the concrete has puzzled generations of scholars and laypersons alike. Other structures of Roman origin bring to light similar questions, such as the impressive aqueducts scattered across the Mediterranean (some still in use today), and even entire cities and settlements still left standing. So, what is it that makes Roman concrete so incredibly strong? 

Recent research published just this year may finally have our answers. A collaborative team between MIT, Harvard University, and labs in Italy and Switzerland released new research that has unlocked an exciting new aspect of what made Roman concrete so durable: it is self-healing.

Credit: Wikimedia.

A view of the Pantheon’s dome from across Rome. The dome remains the world’s largest made of unreinforced concrete. 

 

Prior to the release of this paper, it was assumed that the answer to Roman concrete’s durability was based on the inclusion of pozzolanic ash. When mixed with seawater, this volcanic material created a concrete that was incredibly strong as well as hydrophobic, allowing the Romans to construct their impressive structures both on land and in water. This was only part of the story, however. Included in the Roman concrete formula are also small white chunks known as lime clasts. These lime clasts were often ascribed to careless mixing techniques or poor base materials, when, in fact, they were the secret ingredient all along.

When Romans mixed their concrete, it was traditionally thought that they first incorporated lime with water to create a reactive paste-like material (a process called slaking), however, such a process could not account for the presence of lime clasts. Through analyzing many concrete examples, researchers discovered that the lime clasts were, indeed, composed of calcium carbonates which only could have been made at extreme temperatures. Now, it is thought that the Romans were actually using quick lime, lime's more reactive sibling, mixed with water or a lime/water mixture to produce an exothermic reaction that can account for the presence of lime clasts as well as the concrete’s strength. This heated process, researchers concluded, allows for both more intense chemical reactions to produce a more durable concrete as well as allows for significantly reduced setting and curing times, thus facilitating quicker construction.

Credit: Wikimedia.

A view of the Pantheon’s dome from inside. The recesses within the dome are known as coffers while the hole at the top is called the oculus. Both features are thought to help relieved stress on the building by lightening the dome’s weight. 

But how does this make concrete self-healing? The lime clasts have the answer. When produced in high-temperature settings, the lime clasts take on a more brittle composition wherein an easily fractured and highly reactive calcium source can be obtained. Thus, if cracks in the concrete expose these lime clast chunks and water is allowed to reach them, a chemical reaction between the two can create a calcium-saturated solution which can then crystallize into calcium carbonate or even react further with the included pozzolanic materials quickly resealing and healing the crack. 

The research team responsible for this discovery replicated these results within their labs and has highlighted how this information is important not only for our understanding of ancient engineering but how it could also benefit modern construction methods to produce more durable and stronger infrastructure today. The potential ramifications could impact not only the quality of our own buildings and roads but could also possibly play a role in helping cut down the environmental impact of cement production, an industry that accounts for about 8% of global greenhouse gas emissions. 

The research teams involved in this project include Janille Maragh (MIT), Paolo Sabatini at DMAT (Italy), Michel di Tommaso at the Instituto Meccanica dei Materiali (Switzerland), and James Weaver at the Institute for Biologically Inspired Engineering (Harvard University). The research was also supported by the Archaeological Museum of Priverno (Italy). The original research publication can be found in Seymour, L. M. et al. 2023. “Hot Mixing: Mechanistic insight into the durability of ancient Roman concrete,” Science Advances 9 (1), eadd1602 (2023). https://www.science.org/doi/epdf/10.1126/sciadv.add1602 

 

About the Author

Danielle Vander Horst

Dani is a freelance artist, writer, and archaeologist. Her research specialty focuses on religion in the Roman Northwest, but she has formal training more broadly in Roman art, architecture, materiality, and history. Her other interests lie in archaeological theory and public education/reception of the ancient world. She holds multiple degrees in Classical Archaeology from the University of Rochester, Cornell University, and Duke University.

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