Roman tomb reveals secrets of the resilience of ancient concrete

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The mausoleum of the daughter of a Roman nobleman from the first century BCE has recently been researched giving insight into the materials that were used in its construction. The tomb of Caecilia Metella, located 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 further inspection because “it remains very cohesive despite 2050 years of exposure due to infiltration of rainwater, groundwater and high humidity,” says the document.

The science behind the strong substance

The reason behind the strength of the tomb is that it was built from the eruption deposits of the nearby Alban Hills volcano. In the case of the tomb, the binder in the mortar was calcium-aluminum-silicate-hydrate, and the aggregate consisted of alluvial deposits and pozzolanella tephra.

Rocks and ash ejected from a volcano (aka tephra) were used by the ancient Greeks as a cementitious material as early as the middle of the first millennium BCE (nearly 200 to 300 years before Imperial Rome). The material is now known as Pozzolona, ​​after the place in Italy (Pozzuoli), which has one of the main deposits of this variety of volcanic ash.

These have been documented by Roman historians such as Vitruvius (80-15 BC. A contemporary Roman historian-archaeologist, Esther Boise van Deman, refers to this period as “an era in the history of concrete construction”.

The binder material for the grave was produced from the reaction between lime and aggregated tephras. The podium was made up of tuff rock, which is made when volcanic ash solidifies after an eruption, and lava rock, which, as the name suggests, is rock made from post-eruption magma.

Micro morphology studies

The study used scanning electron microscopy, revealing the micromorphologies of the building’s 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, however, could 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 constructed and helped to further strengthen the material. For example, the tomb has been exposed to rain for centuries. This caused the crystals of leucite (part of the tephra aggregate), rich in potassium, to dissolve in the cement matrix and make it rich in potassium.

A similar feature is observed in other Roman structures of the time, for example the Theater of Marcellus and the Markets of Trajan. While the same process produced cracks in modern concrete, the same process strengthened the binder, creating new tissue in the wake.

The lead co-authors of the study published in the Journal of the American Ceramic Society, Admir Masic, said in a statement: “Understanding the formation and processes of ancient materials can inform researchers about new ways to create materials. sustainable and sustainable construction for the future. . “

– The author is an independent scientific communicator. (mail[at]ritvikc[dot]com)



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