Researchers Look at Roman Tomb for Concrete Insight
Researchers from the Massachusetts Institute of Technology and the University of Utah have recently conducted research on a 2,050-year-old Roman tomb for insight into concrete resilience.
For their study, researchers looked at the tomb of first-century noblewoman Caecilia Metella. The research has since been published in the Journal of the American Ceramic Society.
Looking to Ancient Structures
Built using volcanic aggregate, the large cylindrical structure was of interest to lead co-authors of the study, Admir Masic, associate professor of civil and environmental engineering at MIT, and Marie Jackson, research associate professor of geology and geophysics at the University of Utah, for its unusual chemical interactions with rain and groundwater over the course of two millennia.
Due to these design choices, the concrete quality of the tomb may actually exceed its male contemporaries’ monuments.
“Understanding the formation and processes of ancient materials can inform researchers of new ways to create durable, sustainable building materials for the future,” said Masic. “The tomb of Caecilia Metella is one of the oldest structures still standing, offering insights that can inspire modern construction.”
A landmark on the Via Appia Antica, the tomb of Caecilia Metella consists of a rotunda-shaped tower on a square base, measuring roughly 70 feet tall and 100 feet in diameter. The tomb was constructed in about 30 BCE, when the Roman Republic was transforming into the Roman Empire, using a mixture of coarse brick or volcanic rock aggregate bound with mortar made with lime and volcanic tephra (porous fragments of glass and crystals from explosive eruptions).
The crystals, formed from the potassium-rich mineral leucite, dissolved over time, causing the structure to remodel and reorganize the interface between volcanic aggregates and the cementitious binding matrix, improving the cohesion of the concrete.
“Focusing on designing modern concretes with constantly reinforcing interfacial zones might provide us with yet another strategy to improve the durability of modern construction materials,” Masic said. “Doing this through the integration of time-proven ‘Roman wisdom’ provides a sustainable strategy that could improve the longevity of our modern solutions by orders of magnitude.”
Looking at the microstructure of the concrete, researchers discovered that the concrete mixture used for the tomb was similar to the mortar used in the Markets of Trajan 120 years later. The glue of the Markets of Trajan mortar consists of a building block called the C-A-S-H binding phase (calcium-aluminum-silicate-hydrate), along with crystals of a mineral called strätlingite.
What made the tomb’s concrete stronger, however, was the leucite that managed to strengthen the structure over centuries of rainwater and groundwater percolating through the tomb’s walls, reconfigured the C-A-S-H binding phase.
“The X-ray diffraction and Raman spectroscopy techniques allowed us to explore how the mortar had changed,” said Linda Seymour, who participated in this study as a PhD student in the Masic lab at MIT. “We saw C-A-S-H domains that were intact after 2,050 years and some that were splitting, wispy or otherwise different in morphology. X-ray diffraction, in particular, allowed an analysis of the wispy domains down to their atomic structure. We see that the wispy domains are taking on this nano-crystalline nature.”
Stefano Roascio, the archaeologist in charge of the tomb, reported that the study has a great deal of relevance to understanding other ancient and historic concrete structures that use Pozzolane Rosse aggregate.
“The interface between the aggregates and the mortar of any concrete is fundamental to the structure’s durability,” Masic said. “In modern concrete, the alkali-silica reactions that form expansive gels may compromise the interfaces of even the most hardened concrete.
“It turns out that the interfacial zones in the ancient Roman concrete of the tomb of Caecilia Metella are constantly evolving through long-term remodeling. These remodeling processes reinforce interfacial zones and potentially contribute to improved mechanical performance and resistance to failure of the ancient material.”
In addition to Masic, Seymour and Jackson, additional co-authors of the study include Nobumichi Tamura, senior scientist at Lawrence Berkeley National Laboratory. The research is funded, in part, by the U.S. Department of Energy ARPA-e program.
