Scientists excavating the ruins of Pompeii have discovered a construction site "frozen in time" by the eruption of Mount Vesuvius in 79 AD, shedding light on the ingredients and methods behind the durable, self-healing concrete with which the ancient Romans revolutionized architecture.
The site represents a construction project that was underway when the eruption buried Pompeii in volcanic ash and rock. Researchers found rooms with unfinished walls and piles of pre-mixed dry materials, as well as measuring and weighing tools, ready to prepare concrete.
"Studying this was really like traveling back in time and standing next to workers as they mixed and placed their concrete," said Admir Mašić, a professor of civil and environmental engineering at the Massachusetts Institute of Technology (MIT) and lead author of the study published in the journal Nature Communications.
Concrete, as an indispensable building material, helped the Romans build structures such as stadiums like the Colosseum, domed temples like the Pantheon, public baths and other large buildings, aqueducts and bridges that had never existed before. Since concrete could harden underwater, it was also essential for the construction of arches and breakwaters.
The precise method by which they made their concrete has been the subject of debate, as recent archaeological discoveries appear to contradict descriptions in treatises by the 1st century BC Roman architect and engineer Vitruvius, Reuters reports.
Discoveries in Pompeii have shown that the Romans used a technique called "hot-mixing," in which a material called quicklime—dried limestone that has been preheated—is directly combined with water and a mixture of volcanic rock and ash, producing a chemical reaction that naturally heats the mixture. This differs from the method described by Vitruvius, who wrote about a century earlier.
"Pompeii preserves buildings, materials, and even works in progress, exactly in the condition they were in when the eruption occurred. Unlike completed structures that have gone through centuries of repairs and exposure to the weather, this site records the construction process as it happens," said Mašić.
"For the study of ancient technologies, it simply has no parallel," he added. "Its remarkable preservation offers a true 'snapshot' of Roman building practices in action."
The building under construction combined living quarters with a bakery with ovens, grain washing basins, and storage. Evidence at the site suggests that Vitruvian's technique, known as quicklime, was not used for the construction of the walls.
This method may have already been obsolete by the time of this project in Pompeii.
"Imagine what a difference of a hundred years can mean for construction technology. A good analogy might be early telephone connections. In the 1920s and 1930s: rotary dials, copper power lines. In the 2020s: smartphones using packet switching of digital signals and wireless networks," said Mašić.
The hot-mixing technique contributed to the self-healing properties of concrete, which chemically "repairs" cracks. Concrete contains white residues of lime used in preparation, so-called "lime inclusions", which can dissolve and recrystallize, closing cracks caused by water penetration.
The Romans industrialized concrete, starting in the 1st century BC and 1st century AD.
"This allowed builders to erect massive monolithic structures, complex vaults and domes, as well as harbors with concrete that hardens underwater. Concrete fundamentally expanded what could be built and the way cities and infrastructure were imagined," said Mašić.
A new understanding of Roman concrete could also be relevant for contemporary architects.
"Modern concretes generally do not have built-in self-healing capabilities, which is increasingly important as we seek longer-lasting infrastructure with lower maintenance costs," said Mašić.
"So, while the ancient process itself is not a direct replacement for modern standards, the principles we discovered could influence the design of the next generation of durable, low-carbon concretes."
Bonus video:
