The tomb of a noble woman reveals new secrets of the highly durable concrete of ancient Rome


The Tomb of Cecilia Metella is a mausoleum located on the outskirts of Rome at the three-mile marker of Via Appia.
Enlarge / The Tomb of Cecilia Metella is a mausoleum located on the outskirts of Rome at the three-mile marker of Via Appia.

Among the many popular tourist sites in Rome is an impressive 2000 year old mausoleum along the Via Appia known as the Tomb of Caecilia Metella, a noble woman who lived in the 1st century AD Lord Byron was one of those who marveled at the structure, even making reference to it in his epic poem. Childe Harold Pilgrimage (1812-1818). Now scientists have analyzed samples of the ancient 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 great respect “, said co-author Marie Jackson, a geophysicist in the University of Utah. “And the concrete fabric 2050 years later reflects a strong and resistant presence.”

Like today portland cement (basic ingredient of modern concrete), old Roman concrete it was basically a mixture of semi-liquid mortar and aggregates. 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 plaster, best for a smooth, flat surface. But the aggregate used to make the Roman concrete consisted of fist-sized pieces of stone or bricks.

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

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

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

In 2017, Jackson co-authored a paper analyze the concrete of the ruins of the levees along the Mediterranean coast of Italy, which have stood for two millennia despite the harsh marine environment. The constant waves of salt water crashing against the walls would have long ago reduced modern concrete walls to rubble, but the Roman walls seem to have grown stronger.

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

So naturally, Jackson was intrigued by the Tomb of Cecilia Metella, considered one of the best-preserved monuments on the Appian Way. Jackson visited the grave in June 2006, when he took small samples of the mortar for analysis. Although the day of his visit was quite warm, he recalled that once inside the burial corridor, the air was very cool and humid. “The atmosphere was very calm, except for the flapping of the pigeons in the open center of the circular structure”, Jackson said.

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

Carole Raddato / CC BY-SA 2.0

Almost nothing is known about Cecilia Metella, the noble woman whose remains were once buried in the tomb, other than that she was the daughter of a Roman consul. Quintus Caecilius Metellus Creticus. She married Marcus Licinius Crassus, Whose father (of the same name) was part of the First triumvirate, With Julius Caesar Y Pompey the great. It was probably his son, also named Marcus Licinius CrassusBecause why make it easier for historians to follow the family genealogy? Who ordered the construction of the mausoleum, probably built sometimes between 30 and 10 BC. C.

A marble sarcophagus located in the Palazzo Farnese is purportedly from the Tomb of Caecilia Metella, but it was probably not from the noblewoman, as it dates from between 180 and 190 AD In addition, cremation was a more common funerary custom at the time of the death of the lady, which is why historians believe that the grave cella probably once had a funerary urn, rather than some kind 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 attached castle to the rear that was built sometime in the 14th century. The exterior has a plaque with the inscription “To Caecilia Metella, daughter of Quintus Creticus [and wife] of Crassus “.

Lava over volcanic tephra in the substructure of the tomb.
Enlarge / Lava over volcanic tephra in the substructure of the tomb.

Marie jackson

The base is built in part on tuff rock (volcanic ash that has been compacted under pressure) and lava rock from an ancient flow that once covered the area about 260,000 years ago. The podium and rotunda are composed of several layers of thick concrete, surrounded by travertine blocks as a frame while the concrete layers are formed and hardened. The tower’s walls are 24 feet thick. Originally there would have been a conical earth mound on top, but it was later replaced by medieval battlements.

To take a closer look at the microstructure of the grave mortar, Jackson partnered with colleagues at MIT Linda Seymour and Admir Masic, as well as Nobumichi Tamura of the Lawrence Berkeley Lab. Tamura analyzed the samples at the Advanced light source, which helped them to identify both the different minerals contained in the samples and their orientation. The ALS beamline produces powerful micron-sized X-ray beams, which can penetrate through the entire thickness of samples, according to Tamura. The team also imaged the samples with scanning electron microscopy.

They found that the mortar in the tomb was similar to that used on the walls of the Trajan’s Markets: volcanic tephra of Pozzolane Rosse pyroclastic flow, joining large pieces of brick and lava aggregate. However, the tephra used in the grave mortar contained much more potassium-rich leucite. Over the centuries, rainwater and groundwater seeped through the walls of the tomb, dissolving the leucite and releasing potassium. This would be a disaster in modern concrete, producing microcracks and serious deterioration of the structure.

That obviously did not happen with the grave. But why? Jackson et al. determined that the potassium in the mortar dissolved in turn and effectively reconfigured the CASH binding phase. Some parts remained intact even after more than 2000 years, while other areas appeared dimmer and showed some signs of division. In fact, the structure looked a bit like nanocrystals.

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

Marie jackson

“It turns out that the interfaces in the ancient Roman concrete of Caecilia Metella’s tomb are constantly evolving through long-term remodeling.” said Masic. “These remodeling processes reinforce the interfaces 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 are to being able to reproduce those qualities in today’s concrete, such as finding an appropriate substitute (such as coal fly ash) for extremely rare volcanic rock. . the Romans used. This could reduce the energy emission from concrete production by up to 85 percent and significantly improve the service life of modern concrete structures.

“Focusing on designing modern concretes with constantly reinforcing interfaces could provide us with another strategy to improve the durability of modern building materials,” said Masic. “Doing this through the integration of time-tested ‘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).


arstechnica.com

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