Tomb of noble women reveals new secrets of ancient Rome’s highly durable concrete

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The tomb of Caecilia Metella is a mausoleum located on the outskirts of Rome, at the three-mile mark of Via Appia.
enlarge / The tomb of Caecilia Metella is a mausoleum located on the outskirts of Rome, at the three-mile mark of Via Appia.

Among the many popular tourist attractions in Rome, there is an impressive 2000-year-old mausoleum located on Via Appia, known as the Tomb of Caecilia Metella. woman.Lord Byron was one of those who marveled at this structure and even quoted it in his epic The Pilgrimage of Childe Harold (1812-1818). Now, the scientists have analyzed the ancient concrete samples used to build the tomb and described their findings in a paper published in the Journal of the American Ceramic Society in October.

“The construction of this very innovative and robust monument and landmark on Via Appia Antica shows that [Caecilia Metella] Said Mary Jackson, a geophysicist at the University of Utah. “The concrete fabric after 2050 reflects a strong and resilient existence.”

Just like today’s Portland cement (the basic component of modern concrete), ancient Roman concrete was basically a mixture of semi-liquid mortar and aggregate. Portland cement is usually 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, and only a little gypsum is added-better to get a smooth, flat surface.But the aggregate used to make Roman concrete is made of fist-sized stones or bricks

In his paper architecture (About 30 AD), the Roman architect and engineer Vitruvius wrote about how to build concrete walls for funeral structures that can withstand a long time without turning into ruins. He suggested that the wall should be at least two feet thick and made of “square red stone or brick or lava paving.” Bricks or volcanic rock aggregates should be bonded with mortar composed of slaked lime and porous glass fragments and volcanic eruption crystals (called volcanic ash).

Portus Cosanus Pier, Orbetello, Italy.  A 2017 study found that crystals formed in the concrete used to build seawalls help prevent cracks from forming.
enlarge / Portus Cosanus Pier, Orbetello, Italy. A 2017 study found that crystals formed in the concrete used to build seawalls help prevent cracks from forming.

Jackson has been studying the unusual properties of ancient Roman concrete for many years. For example, she and several colleagues analyzed the mortar used in the concrete that constitutes Trajan’s Market, which was built between 100 and 110 AD (probably the oldest shopping mall in the world). They are particularly interested in the “glue” used in the bonding phase of the material: a calcium aluminum silicate hydrate (CASH) with added quartz crystals. They found that Stratlingite crystals prevented the formation and spread of microcracks in the mortar, which could lead to larger cracks in the structure.

In 2017, Jackson co-authored a paper analyzing the concrete forms of the ruins of seawalls on the Mediterranean coast of Italy. Despite the harsh ocean environment, these seawalls have stood for two thousand years. The continuous waves of sea water hitting the walls would turn modern concrete walls into rubble long ago, but the Roman seawall seems to have actually become stronger.

Jackson and her colleagues discovered that the secret of longevity lies in a special formula that includes a combination of rare crystals and porous minerals. Specifically, exposure to seawater will cause a chemical reaction inside the concrete, causing the polmontite crystals to form from the common mineral phillipsite found in volcanic ash. The combination of crystals and rocks once again prevented the formation and propagation of cracks, which would otherwise weaken the structure.

Therefore, Jackson is naturally interested in the tomb of Caecilia Metella, which is generally regarded as one of the best-preserved monuments on the Appian Way. Jackson visited the tomb in June 2006 when she collected some mortar samples for analysis. Although the weather was warm on the day of her visit, she recalled that as soon as she entered the corridor of the tomb, the air was very cool and humid. “The atmosphere is very calm, except for pigeons flying in the open center of the circular structure,” Jackson said.

A plaque on the tombstone says
enlarge / A plaque on the tomb reads “To Cecilia Metra, daughter of Quintus Kretikus, [and wife] Crassou”.

Carol Radato/CC BY-SA 2.0

Few people knew that Caecilia Metella was a noblewoman who was buried in a tomb, except that she was the daughter of Quintus Caecilius Metellus Creticus, the consul of Rome. She is married to Marcus Licinius Crassus and Julius Caesar and Pompey the Great, whose father (same name) is the first Member of the Trinity. It is probably her son-also called Marcus Licinius Crassus, because why does it make it easier for historians to trace the family lineage? ——He ordered the construction of a mausoleum, which may sometimes be built between 30 and 10 BC.

The marble sarcophagus in Farnese Palace is said to come from the tomb of Caecilia Metella, but it may not belong to the noblewoman, as it dates back to between 180 and 190 AD. In addition, when this lady died, cremation was a more common funeral custom, so historians believe that the cellar of the tomb may have placed an urn instead of some kind of sarcophagus.

Scientists like Jackson and her colleagues are most interested in the structure of the tomb itself. The mausoleum is located on a hill. On the square podium is a cylindrical rotunda, behind which is a castle built sometime in the 14th century. There is a plaque on the outside that says: “To Quintus Kretikus’ daughter Caecilia Metella (Caecilia Metella) [and wife] Crassou. “

Lava covered the volcanic ash at the bottom of the tomb.
enlarge / Lava covered the volcanic ash at the bottom of the tomb.

Mary Jackson

The foundation is partly built on tuff (volcanic ash compacted under pressure) and ancient flowing lava that once covered the area about 260,000 years ago. Both the podium and the rotunda are composed of several layers of thick concrete, surrounded by travertine blocks as a frame, while the concrete layer is formed and hardened. The wall thickness of the tower is 24 feet. Initially there was a cone-shaped mound at the top, but it was later replaced by medieval battlements.

To take a closer look at the microstructure of the tombstone, Jackson collaborated with MIT colleagues Linda Seymour and Admir Masic and Nobumichi Tamura of Lawrence Berkeley Lab. Tamura analyzed the samples on advanced light sources, which helped them identify the many different minerals contained in the samples and their orientation. According to Tamura, the ALS beamline produces a powerful X-ray beam about a micrometer in size, which can penetrate the entire thickness of the sample. The team also used scanning electron microscopes to image the samples.

They found that the mortar of the tomb was similar to the mortar used on the walls of Trajan’s Market: volcanic ash from the Pozzolane Rosse pyroclastic flow, which glued together large bricks and lava aggregates. However, the volcanic ash used in the grave mortar contains more potassium-rich leucite. For centuries, rain and groundwater seeped into the walls of the tomb, dissolving leucite and releasing potassium. This will be a disaster for modern concrete, causing micro-cracks and severe structural deterioration.

This obviously did not happen to the grave. but why?Jackson Wait. Make sure that the potassium in the mortar sequentially dissolves and effectively reconfigures the CASH binding phase. More than 2000 years later, some parts are still intact, while other parts are more slender, showing signs of splitting. In fact, this structure is somewhat similar to the structure of nanocrystals.

Scanning electron microscope image of tomb mortar.
enlarge / Scanning electron microscope image of tomb mortar.

Mary Jackson

“It turns out that the interface area in the ancient Roman concrete of the tomb of Caecilia Metella has been continuously developed through long-term renovation,” Masic said. “These modifications have strengthened the interface area and may help improve the mechanical properties and failure resistance of ancient materials.”

The more scientists know about the precise combination of minerals and compounds used in Roman concrete, the closer we will be to reproducing these qualities in today’s concrete-such as finding suitable substitutes for extremely rare volcanic rocks (such as pulverized coal). Gray) used by the Romans. This can reduce energy emissions from the production of concrete by as much as 85% and significantly increase the service life of modern concrete structures.

“Concentrating on designing modern concrete with continuously enhanced interface areas may provide us with another strategy to improve the durability of modern building materials,” Masic said. “Doing this by integrating the tried and tested’Roman Wisdom’ provides a sustainable strategy that can increase the lifespan of our modern solutions by orders of magnitude.”

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

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