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Friday, December 03, 2021

Roman tomb reveals secrets of ancient concrete resilience

The reason behind the tomb’s strength is that it was constructed from the deposits of the eruptions of the nearby Alban Hills volcano.

Written by Ritvik Chaturvedi | Bengaluru |
October 29, 2021 7:59:29 pm
Cecilia_metellaThe Tomb of Cecilia Metella and Castrum Caetani ruins in Rome. (Wikimedia Commons)

The mausoleum of a first-century BCE Roman nobleman’s daughter has recently been the subject of research providing insights into the materials that went into its construction. The Tomb of Caecilia Metella, situated on the outskirts of Rome, is closely associated with the much larger context of the archaeological site of Capo di Bove. The site corresponds to the period of Imperial Rome (27 BCE to 476 CE).

Architecturally, the structure of the Tomb of Caecilia Metella consists of a cylindrical dome on a square podium. “The concrete of the cylindrical wall’ deserved a closer inspection because ‘it remains highly cohesive despite 2050 years of exposure due to infiltration of rainwater, groundwater and high humidity,” the paper states.

Science behind the strong substance

The reason behind the tomb’s strength is that it was constructed from the deposits of the eruptions of the nearby Alban Hills volcano. In the case of the tomb, the binding agent in the mortar was calcium-aluminum-silicate-hydrate, and the aggregate consisted of alluvial deposits and Pozzolanelle tephra.

Rocks and ash ejected out of a volcano (aka tephra) were used by ancient Greeks as cementing material as early as the mid-first millennium BCE (nearly 200-300 years before Imperial Rome). The material is now known as Pozzolona, after the place in Italy (Pozzuoli), that has one of the primary deposits of this variety of volcanic ash.

These have been documented by Roman historians like Vitruvius (80-15 BCE), who, while referring to these structures, said they “over a long passage of time do not fall into ruins”. A contemporary Roman historian-archaeologist, Esther Boise van Deman, refers to the period as “an Epoch in the history of concrete construction”.

The binding material of the tomb was produced from the reaction between lime and aggregate tephra. The podium was made of tuff rock, which is made when volcanic ash is solidified after an eruption, and lava rock, which, as the name suggests, is a rock made of post-eruption-magma.

Micro morphology studies

The study employed scanning electron microscopy, revealing the micro morphologies of the building structure and X-ray diffraction to study the chemical composition as well as the structure of the material.

The key to the durability of the structure could, however, be the interface between the aggregate and the mortar.

The aggregates, derived from volcanic tephra in this case, continued to remain reactive long after the structure was built and contributed to further strengthening of the material. For instance, the tomb was exposed to rain for centuries. This caused the leucite crystals (part of tephra aggregate), rich in potassium, to dissolve into the cementing matrix and make it rich in potassium.

A similar characteristic is seen in other Roman structures from the time, e.g. the Theatre of Marcellus and the Markets of Trajan. While the same process would produce cracks in modern concrete, the same process strengthened the binder, creating a new fabric in the wake.

Lead co-authors of the study published in the Journal of the American Ceramic Society, Admir Masic, said in a release: “Understanding the formation and processes of ancient materials can inform researchers of new ways to create durable, sustainable building materials for the future.”

– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)

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