2,050-year-old Roman tomb inspires durable, sustainable building materials for the future

“Understanding the formation and processes of ancient materials can inform researchers of new ways to create durable and sustainable building materials for the future,” says Associate Professor Admir Masic. “The tomb of Caecilia Metella is one of the oldest structures still standing, offering information that can inspire modern construction.” Seen here are the tomb of Cecilia Metella and the ruins of Castrum Caetani in Rome. Credit: Livioandronico2013/Wikimedia Commons

New research on ancient Roman concrete offers insight into the resilience of ancient concrete, inspiring sustainable and sustainable modern constructions.

Concrete often begins to crack and crumble after a few decades of life – but oddly enough this was not the case with many Roman structures. The structures still stand, exhibiting remarkable durability despite conditions that would destroy modern concrete.

A particular structure is the large cylindrical tomb of the first-century nobleman Caecilia Metella. New research from MIT scientists and colleagues published in the Journal of the American Ceramic Society shows that the quality of the concrete in his tomb may exceed that of the monuments of his male contemporaries due to the volcanic aggregate chosen by the builders and the unusual chemical interactions with rain and groundwater that accumulate over two millennia.

The study’s co-lead authors, Admir Masic, an associate professor of civil and environmental engineering at MIT, and Marie Jackson, an associate research professor of geology and geophysics at the University of Utah, teamed up to understand the mineral composition of the old concrete structure. .

“Understanding the formation and processes of ancient materials can inform researchers of new ways to create durable and sustainable building materials for the future,” says Masic. “The tomb of Caecilia Metella is one of the oldest structures still standing, offering information that can inspire modern construction.”

A curiously cohesive concrete

Located on an ancient Roman road also known as the Appian Way, the tomb of Caecilia Metella is a landmark on the Via Appia Antica. It consists of a rotunda-shaped tower that sits on a square base, altogether about 70 feet (21 meters) high and 100 feet (29 m) in diameter. Built around 30 BCE, during the transformation of the Roman Republic into the Roman Empire, ruled by Emperor Augustus, in 27 BCE, the tomb is considered one of the best preserved monuments of the Appian Way.

Caecilia herself was a member of an aristocratic family. She married into the family of Marcus Crassus, who formed a famous alliance with Julius Caesar and Pompey.

Scanning electron microscope image of grave mortar

In this scanning electron microscope image of the tomb mortar, the CASH bond phase appears gray while the volcanic scoria (and leucite crystals) appear light gray. 1 credit

“The construction of this highly innovative and robust monument and landmark on the Via Appia Antica indicates that it was held in great respect,” says Jackson “and the concrete fabric 2,050 years later reflects a strong and resilient presence” .

The tomb is an example of the refined technologies of concrete construction in late Republican Rome. The technologies were described by the architect Vitruvius while the tomb of Caecilia Metella was under construction. Building thick walls of coarse bricks or volcanic rock aggregates bound together with mortar made from lime and volcanic tephra (porous fragments of glass and crystals from explosive eruptions), would yield structures that “over a long period of time do not crumble”.

Vitruvius’ words are proven by the many Roman structures that exist today, including Trajan’s Markets (built between 100 and 110 AD, more than a century after the tomb) and marine structures like piers and piers. breakwater.

What the ancient Romans could not have known, however, was how the crystals of the mineral leucite, which is rich in potassium, in the volcanic aggregate would dissolve over time to beneficially reshape and rearrange the interface. between the volcanic aggregates and the cementitious binding matrix, improving the cohesion of the concrete.

“Focusing on designing modern concretes with constantly reinforced interface areas could provide us with another strategy to improve the durability of modern building materials,” says Masic. “Doing this through the integration of time-tested ‘Roman wisdom’ provides a sustainable strategy that could improve the longevity of our modern solutions by orders of magnitude.”

Linda Seymour ’14, PhD ’21, who participated in this study as a doctoral student in the Masic laboratory at MIT, studied the microstructure of concrete with scientific tools.

“Each of the tools we used added a clue to the processes in the mortar,” says Seymour. Scanning electron microscopy showed the microstructures of mortar building blocks at the micron scale. Energy dispersive X-ray spectrometry has shown the elements that make up each of these building blocks. “This information allows us to quickly explore different areas of the mortar and we can identify basic elements related to our questions,” she says. The trick, she adds, is to hit the exact same building block target with each instrument when that target is only about a hair’s breadth away.

The science behind a particularly strong substance

Within the thick concrete walls of the tomb of Caecilia Metella, a mortar containing volcanic tephra binds large blocks of bricks and lava aggregates. It is similar to the mortar used in the Markets of Trajan 120 years later. The glue in Trajan’s Market Mortar is made up of a building block called the CASH (calcium-aluminum-silicate-hydrate) bonding phase, along with crystals of a mineral called strätlingite.

But the tephra that the Romans used for the Caecilia Metella mortar was more abundant in potassium-rich leucite. Centuries of rainwater and groundwater seeping through the walls of the tomb dissolved the leucite and released the potassium into the mortar. In modern concrete, an abundance of potassium would create expansive gels that would cause microcracking and eventual deterioration of the structure.

In the grave, however, the potassium dissolved and reconfigured the CASH binding phase.

“X-ray diffraction and Raman spectroscopy techniques allowed us to explore how the mortar had changed,” says Seymour. “We saw intact CASH domains after 2,050 years and some that split, were wispy, or had a different morphology. X-ray diffraction, in particular, has allowed an analysis of vaporous domains down to their atomic structure. We see that the vaporous domains take on this nanocrystalline nature,” she says.

The reshaped domains “obviously create robust components of cohesion in the concrete,” says Jackson. In these structures, unlike Trajan’s Markets, little strätlingite is formed.

Stefano Roascio, the archaeologist in charge of the tomb, notes that the study is highly relevant to understanding other ancient and historic concrete structures that use the Pozzolan Rosse aggregate.

“The interface between aggregate and mortar in any concrete is fundamental to the durability of the structure,” says Masic. “In modern concrete, alkali-silica reactions that form expansive gels can compromise the interfaces of even the most hardened concrete.”

“It turns out that the interfacial areas of the ancient Roman concrete of the tomb of Caecilia Metella are constantly changing through long-term remodeling,” says Masic. “These remodeling processes strengthen the interfacial areas and potentially contribute to improving the mechanical performance and fracture toughness of the old material.”

Reference: “Interfacial Zones of Reactive Binder and Aggregates in the Mortar of Concrete from the Tomb of Caecilia Metella, 1C BCE, Rome” by Linda M. Seymour, Nobumichi Tamura, Marie D. Jackson, and Admir Masic, September 16, 2021, Journal of the American Ceramic Society.
DOI: 10.1111/jace.18133

In addition to Masic, Seymour and Jackson, other study co-authors include Nobumichi Tamura, senior scientist at Lawrence Berkeley National Laboratory. The research is funded, in part, by the US Department of Energy’s ARPA-e program.

David C. Barham