The tomb of a nobleman reveals new secrets of the heavy-duty concrete of ancient Rome

The Tomb of Caecilia Metella is a mausoleum located just outside Rome at the three mile terminal of the Via Appia.
Enlarge / The Tomb of Caecilia Metella is a mausoleum located just outside Rome at the three mile terminal of the Via Appia.

Among Rome’s many popular tourist spots is an impressive 2000-year-old mausoleum along the Via Appia known as the Tomb of Caecilia Metella, a nobleman who lived in the first century AD. Lord Byron was among those who marveled at the structure, even referring to it in his epic poem Childe Harold’s pilgrimage (1812-1818). Now the scientists have analyzed samples of the old concrete used to build the tomb, describing their findings in a published article in October in the Journal of the American Ceramic Society.

“The construction of this very innovative and robust monument and landmark on Via Appia Antica indicates that [Caecilia Metella] was held in high esteem “, said co-author Marie Jackson, geophysicist at University of Utah. “And the fabric of concrete 2,050 years later reflects a strong and resilient presence. “

Like today Portland cement (a basic ingredient of modern concrete), ancient roman concrete was essentially a mixture of semi-liquid mortar and aggregate. Portland cement is typically made by heating limestone and clay (as well as sandstone, ash, chalk, and iron) in a kiln. The resulting clinker is then ground into a fine powder, with just a touch of gypsum added, to better achieve a smooth, flat surface. But the aggregate used to make Roman concrete consisted of pieces of stone or bricks the size of a fist.

In his treatise Architecture (circa 30 CE), the Roman architect and engineer Vitruvian wrote about how to build concrete walls for burial structures that could last a long time without falling into disrepair. He recommended that the walls be at least two feet thick, either of “red squared stone, or of brick or lava laid in layers”. The aggregate of brick or volcanic rock should be bonded with mortar composed of hydrated lime and porous fragments of glass and crystals from volcanic eruptions (known as volcanic tephra).

Portus Cosanus Pier, Orbetello, Italy.  A 2017 study found that the formation of crystals in the concrete used to build the dikes helped prevent the formation of cracks.
Enlarge / Portus Cosanus Pier, Orbetello, Italy. A 2017 study found that the formation of crystals in the concrete used to build the dikes helped prevent the formation of cracks.

Jackson has been studying the unusual properties of ancient Roman concrete for many years. For example, she and several colleagues have analyzed the mortar used in the concrete that makes up the Trajan’s markets, built between 100 and 110 CE (probably the oldest shopping center in the world). They were particularly interested in the “glue” used in the fixing phase of the material: a calcium-aluminum-hydrate silicate (CASH), augmented with crystals of stratlingite. They found that stratlingite crystals blocked the formation and spread of microcracks in the mortar, which could have resulted in larger fractures in the structures.

In 2017, Jackson co-wrote a document analyzing the concrete of the ruins of the dikes along the Italian Mediterranean coast, which have stood for two millennia despite the harsh marine environment. The constant waves of salt water crashing against the walls would long ago have reduced modern concrete walls to rubble, but the Roman dikes appear to have actually gotten stronger.

Jackson and his colleagues discovered that the secret to this longevity was a special recipe, involving a combination of rare crystals and a porous mineral. Specifically, exposure to seawater generated chemical reactions inside the concrete, causing aluminum tobermorite crystals to form from phillipsite, a common mineral found in volcanic ash. The crystals bonded to the rocks, once again preventing the formation and propagation of cracks that would otherwise have weakened the structures.

So naturally, Jackson was intrigued by the tomb of Caecilia Metella, widely considered to be one of the best-preserved monuments of the Appian Way. Jackson visited the grave in June 2006, when she took small samples of the mortar for analysis. Although the day of her visit was quite warm, she recalled that once inside the sepulchral corridor the air was very cool and humid. “The atmosphere was very calm, except for the flight of pigeons in the open center of the circular structure” Jackson said.

A plaque on the grave reads
Enlarge / A plaque on the tomb reads “To Caecilia Metella, daughter of Quintus Creticus, [and wife] of Crassus “.

Carole Raddato / CC BY-SA 2.0

Little is known about Caecilia Metella, the nobleman whose remains were once buried in the tomb, other than that she was the daughter of a Roman consul, Quintus Caecilius Metellus Creticus. She got married Marcus Licinius Crassus, whose father (of the same name) was part of First triumvirate, with Julius Caesar and Pompey the Great. It was probably his son, also named Marcus Licinius Crassus, because why allow historians to easily follow the family genealogy? – who ordered the construction of the mausoleum, probably built sometimes between 30 and 10 BC.

A marble sarcophagus housed in the Farnese Palace would come from the tomb of Caecilia Metella, but it was probably not that of the nobleman since it dates from between 180 and 190 AD. In addition, cremation was a more common funeral custom at the time of the lady’s death, so historians believe that the cella of the tomb probably once contained a funeral urn, rather than some sort of sarcophagus.

It is the structure of the tomb itself that is of most interest to scientists like Jackson and his colleagues. The mausoleum is perched on top of a hill. There is a cylindrical rotunda atop a square podium, with an adjoining castle at the rear which was built in the 14th century. The exterior bears a plaque with the inscription “To Caecilia Metella, daughter of Quintus Creticus [and wife] of Crassus. “

Lava overlying volcanic tephra in the tomb substructure.
Enlarge / Lava overlying volcanic tephra in the tomb substructure.

Marie jackson

The foundation is built partly on tuff stone (volcanic ash compacted under pressure) and lava rock from an ancient flow that once covered the area around 260,000 years ago. The podium and rotunda are both made up of several layers of thick concrete, surrounded by travertine blocks as a frame as the layers of concrete formed and hardened. The walls of the tower are 24 feet thick. Originally there would have been a conical mound of earth at the top, but it was later replaced by medieval ramparts.

To take a closer look at the microstructure of the tomb’s mortar, Jackson teamed up with her MIT colleagues Linda Seymour and Admir Masic, as well as Nobumichi Tamura of the Lawrence Berkeley Lab. Tamura analyzed the samples at Advanced light source, which helped them identify both the many different minerals in the samples and their orientation. The ALS beamline produces powerful x-ray beams the size of a micron, which can penetrate through the entire thickness of samples, by Tamura. The team also imaged the samples under a scanning electron microscope.

They discovered that the mortar in the tomb was similar to that used in the walls of the Trajan’s markets: volcanic tephra of Pozzolana Rosse pyroclastic flow, binding together large pieces of brick and lava aggregate. However, the tephra used in the tomb mortar contained much more of the potassium-rich leucite. Over the centuries, rainwater and groundwater seeped through the walls of the tomb, which dissolved the leucite and released the potassium. It would be a disaster in modern concrete, producing microcracks and severe structural deterioration.

This obviously did not happen with the grave. But why? Jackson et al. determined that the potassium in the mortar in turn dissolved and effectively reconfigured the CASH binding phase. Some parts remained intact even after more than 2,000 years, while other areas appeared more hazy and showed signs of division. In fact, the structure looked somewhat like that of nanocrystals.

Scanning electron microscope image of the tomb mortar.
Enlarge / Scanning electron microscope image of the tomb mortar.

Marie jackson

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

The more scientists learn about the precise combination of minerals and compounds used in Roman concrete, the closer we come to the possibility of replicating these qualities in today’s concrete, for example by finding a suitable substitute (such as coal fly ash) to extremely rare volcanic rock. the Romans used. This could reduce energy emissions from concrete production by up to 85% and significantly improve the life of modern concrete structures.

“Focusing on the design of modern concretes with constantly reinforcing interfacial zones could provide us with another strategy to improve the durability of modern building materials” said Masic. “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. “

DOI: Journal of the American Ceramic Society, 2021. 10.1111 / jace.18133 (About DOIs).

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