New research on ancient Roman concrete offers insight into the resilience of ancient concrete, inspires durable and long lasting 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 are still standing, exhibiting remarkable durability despite conditions that would destroy modern concrete.
A special 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 of his tomb can 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, associate professor of civil and environmental engineering at MIT, and Marie Jackson, associate research professor of geology and geophysics at the University of Utah, came together to understand the mineral composition of the old concrete structure.
“Understanding the formation and processes of ancient materials can inform researchers about new ways to create long-lasting and sustainable building materials for the future,” Masic says. “The tomb of Caecilia Metella is one of the oldest structures still standing, offering ideas that can inspire modern construction.”
A curiously coherent 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 rests on a square base, in total approximately 70 feet (21 meters) in height and 100 feet (29 m) in diameter. Built around 30 BCE, during the transformation of the Roman Republic into the Roman Empire, led 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.
“The construction of this very innovative and rugged monument and landmark on Via Appia Antica indicates that it was held with great respect,” Jackson says, “and the fabric of concrete 2,050 years later reflects a strong and resilient presence. “
The tomb is an example of the refined technologies of concrete construction at the end of Republican Rome. The technologies were described by the architect Vitruvius while the tomb of Caecilia Metella was under construction. The construction of thick walls of coarse bricks or aggregates of volcanic rock bonded with mortar based on lime and volcanic tephra (porous fragments of glass and crystals from explosive eruptions) would give rise to structures which “at over time do not fall into disrepair ”.
The words of Vitruvius are proven by the many Roman structures in existence today, including the Trajan’s Markets (built between 100 and 110 AD, more than a century after the tomb) and marine structures like piers and breezes. -blades.
What the ancient Romans could not know, however, is 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 them. volcanic aggregates and the cementitious bond matrix, improving the cohesion of concrete.
“Focusing on designing modern concretes with constantly reinforcing interfacial zones could provide us with another strategy to improve the durability of modern building materials,” Masic says. “Doing this through the integration of proven ‘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 lab 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 the mortar bricks at the micron scale. Energy dispersive x-ray spectrometry has shown the building blocks of each of these building blocks. “This information allows us to quickly explore different areas of the mortar and we could choose some basic items related to our questions,” she says. The trick, she adds, is to hit precisely the same building block target with each instrument when that target is only a hair’s width apart.
The science behind a particularly powerful substance
In the thick concrete walls of the tomb of Caecilia Metella, a mortar containing volcanic tephra binds large mandrels of brick and lava aggregate. It is similar to the mortar used in Trajan’s Markets 120 years later. The glue in Trajan’s Market mortar consists 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 Caecilia Metella’s mortar was more abundant in potassium-rich leucite. Centuries of rainwater and groundwater percolating 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 which would cause microcracks 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 CASH domains that were intact after 2,050 years and some that were dividing, unraveling, or some other different morphology. X-ray diffraction, in particular, allowed an analysis of the filiform domains down to their atomic structure. We see that the wispy domains take on this nanocrystalline nature, ”she says.
The reshaped domains “obviously create strong, cohesive components in the concrete,” says Jackson. In these structures, unlike the Trajan’s Markets, little strätlingite is formed.
Stefano Roascio, the archaeologist in charge of the tomb, notes that the study is very relevant for understanding other ancient and historic concrete structures that use the aggregate of Pozzolane Rosse.
“The interface between the aggregates and the mortar of all concrete is fundamental to the durability of the structure,” explains Masic. “In modern concrete, alkali-silica reactions that form expanding gels can compromise the interfaces of even the most hardened concrete. “
“It turns out that the interfacial areas in the ancient Roman concrete of the tomb of Caecilia Metella are constantly changing through long-term remodeling,” explains Masic. “These remodeling processes strengthen the interfacial areas and potentially contribute to improving the mechanical performance and resistance to failure of the old material.”
Reference: “Reactive Binder and Interfacial Aggregate Zones in Concrete Mortar 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.