Ancient Roman Concrete Secrets Unraveled by Modern Science

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.

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.

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.

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