Researchers discovered the key to Roman concrete’s durability, and it has lessons for modern construction
The legacy of the Romans is everywhere. The founders of the United States were obsessed with Rome and drew many parallels to Rome when forming their own government. The Mediterranean world is littered with ruins of the cities the Romans built. Many of their buildings still stand, and not only that, they still function. The Pantheon in Rome, dedicated nearly 2,100 years ago, welcomes visitors every day and its dome remains the largest unreinforced dome in the world. Some of the aqueducts the Romans built carry water to cities to this day. They perfected the arch as an architectural marvel, and the concrete they made has proved extremely durable. It’s only recently that researchers discovered the secret behind that famous Roman concrete.
It has long been assumed that Roman concrete, made from pozzolana (volcanic ash) and quicklime, was mixed with water to form a paste in a process known as slaking. The white flecks commonly found in the finished product were always presumed to be the result of poor mixing or low-quality materials. That assumption never sat well with Admir Masic.
“The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” Masic told MIT News. “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.”
Masic is part of a team from MIT that has been working with researchers from Harvard and labs in Italy and Switzerland to study the durability of Roman concrete. They published their findings in the journal Science Advances.
Working at the archaeological site of Privernum, a Roman era town south of Rome in the Lazio region of Italy, they subjected samples of Roman concrete to a battery of tests. They performed large-area scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging to take a closer look.
They determined that those long-held assumptions about how Roman concrete was made have been wrong all along.
The studies concluded that the telltale white flecks of lime were the result not of poor-quality work or materials, but of hot mixing, combining the quicklime with pozzolana and water at very high temperatures.
“The benefits of hot mixing are twofold,” Masic said. “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.”
To prove their theory, the researchers made concrete two ways. One sample used the quicklime recipes the Romans handed down and the other used modern formulations. Both were hot-mixed. Once they had their concrete made, the researchers cracked the samples. Two weeks later, water flowed right through the cracks in the modern concrete, but the Roman concrete did not let the water pass through.
Because the lime clasts react when exposed to water to recrystallize as calcium carbonate, they quickly filled the cracks. The Roman concrete was self-healing. That explains how Roman aqueducts and seawalls have stood and functioned for two millennia.
Modern Roman Concrete
Just as Roman building techniques that had been lost for centuries informed Renaissance architecture, modern builders can learn a thing or two from Roman concrete. Cement production accounts for 8% of the world’s greenhouse gas emissions, and researchers are working hard to figure out ways of reducing that footprint. Self-healing concrete is a key innovation that could increase the life cycles of buildings.
“It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete formulations,” Masic said.
Yes, we may have found the key to the long-lasting, self-healing concrete in the wisdom of the ancients. It’s yet another part of the Roman legacy.