Scientists studying Roman concrete used 2,000 years ago to construct buildings, bridges, and other structures that possess a remarkable ability to last through the centuries have found evidence that ancient Rome’s artisans used advanced technology to create the material.
The researchers also characterized several reactive materials, including volcanic ash from Mount Vesuvius, that were used to reinforce Roman concrete’s self-healing ability, including several bridges and other structures that are still in use today.
“With this paper, we wanted to clearly define a technology and associate it with the Roman period in the year 79 C.E.,” explained MIT Associate Professor Admir Masic.
How ‘Hot Mixing’ Gave Roman Concrete its Self-Healing Properties
According to a press release announcing the research, the first person to document the process of making Roman concrete was Vitruvius in his 1st-century BCE text “De architectura.” Although the ancient text is considered the first known book to discuss architectural theory, the process it describes for mixing Roman concrete has created a modern-day controversy.
According to Vitruvius, the Romans added water to lime to create a concrete predecessor. After this step was completed, other ingredients, such as volcanic ash, were added to make the final concrete product. Unfortunately for the ancient scholar, a 2023 study of Roman concrete by Professor Masic and his collaborators showed that this mixture would not have resulted in the robust, long-lasting material we see today.
“Having a lot of respect for Vitruvius, it was difficult to suggest that his description may be inaccurate,” Professor Masic said.
Instead, the original study found that lime fragments, volcanic ash, and other dry ingredients were mixed separately before water was added. Once the mixed dry materials and water were stirred, they would produce heat.
Described by Masic’s team as “hot mixing,” the process traps and preserves the lime as small, gravel-like features. Because lime is highly reactive, these preserved ‘clasts’ can fill in cracks and re-dissolve. Scientists believe this self-healing property is among the key reasons Roman concrete has persevered for over 2,000 years.
New Analysis Finds Vitruvius Was Wrong
To confirm or refute Viruvius’ account, Masic’s team gained access to an ancient concrete wall that was in the process of being built. The site also contained fully completed buttresses and a structural wall, the latter including examples of mortar repairs still visible after two millennia.
“We were blessed to be able to open this time capsule of a construction site and find piles of material ready to be used for the wall,” Masic said of the ancient Pompeian site.
After performing several tests, the researchers determined that the ancient walls and buttresses offered “the clearest evidence yet” that jot mixing was responsible for the strength and durability of Roman concrete. For example, the samples collected at the site contained the lime clasts Masic’s team had described in 2023.
The team also discovered a dry raw-material pile containing intact fragments of quicklime that had been pre-mixed with other dry materials. The researchers note that premixing the dry ingredients before adding the water to generate a heat reaction is a “critical first step” in hot-mixed concrete production.
Next, the team performed an isotopic analysis of the selected samples, including the volcanic ingredients in the dry mix used in Roman concrete. These tests included analysis of a type of volcanic ash called pumice.
As suspected, the analysis revealed a chemical reaction between the pumice particles and the surrounding pore solution. This process, which the team said occurred over time, created new mineral deposits in the concrete, enhancing its strength and durability. Masic noted the results of these tests allowed his team to follow the critical carbonation reactions that occur during hot mixing over time, “allowing us to distinguish hot-mixed lime from the slaked lime originally described by Vitruvius.”
“These results revealed that the Romans prepared their binding material by taking calcined limestone (quicklime), grinding them to a certain size, mixing it dry with volcanic ash, and then eventually adding water to create a cementing matrix,” the professor explained.
The professor said it is possible that Vitruvius was misinterpreted, since the ancient text does mention the production of “latent heat” during the cement mixing process. Still, he said the excitement of discovery is slightly tempered by the discovery that the ancient architect may have been wrong.
“The writings of Vitruvius played a critical role in stimulating my interest in ancient Roman architecture, and the results from my research contradicted these important historical texts,” he said
Translating Ancient Technology into Modern Construction Techniques
When discussing the potential implications of his team’s findings, the team noted that modern concrete also uses calcium, one of the ancient material’s ingredients, “so understanding how it reacts over time holds lessons for understanding dynamic processes in modern cement as well.”
“There is the historic importance of this material, and then there is the scientific and technological importance of understanding it,” Masic said. “This material can heal itself over thousands of years, it is reactive, and it is highly dynamic. It has survived earthquakes and volcanoes. It has endured under the sea and survived degradation from the elements.”
To further explore the potential benefits of Roman concrete mixing and fabrication methods for modern construction applications, Masic founded a new company, DMAT. The professor said he was motivated to do so because the knowledge of these ancient builders is relevant to modern applications: “Because Roman cement is durable, it heals itself, and it’s a dynamic system.”
“The way these pores in volcanic ingredients can be filled through recrystallization is a dream process we want to translate into our modern materials,” he explained. “We want materials that regenerate themselves.”
“We don’t want to completely copy Roman concrete today,” the professor added. “We just want to translate a few sentences from this book of knowledge into our modern construction practices.”
The study “An unfinished Pompeian construction site reveals ancient Roman building technology” was published in Nature Communications.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.