Among the numerous tourist attractions in Rome, the 2,000-year-old catacombs along the Via Appia, known as the Tomb of Caecilia Metella, is one of the most impressive resting places in the world.
Historians believe that this is where the body of a noblewoman lived in the first century AD. And material scientists scrape the walls of the tomb to learn about the ancient concrete used in the construction of the giant structure. The research report was published in the Journal of the American Ceramic Society last October.
“Particularly, the construction of the project is advanced, robust and becomes a landmark on Via Appia Antica [bà Caecilia Metella] was highly respected,” said co-author Marie Jackson, a geophysicist at the University of Utah. “Even though the concrete layer is 2,050 years old, it still shows its strength and durability“.
Much like modern cement, ancient Roman concrete was also a simple mixture of semi-liquid mortar and aggregate*. We often make cement by putting in a kiln of limestone and clay (or sandstone, ash, chalk or iron), to produce a fine powder. However, the aggregates that make up Roman concrete are made up of fist-sized stones and bricks.
In his treatise de Architecture (drafted in AD 30), the Greek engineer and architect Vitruvius wrote about how to build the mausoleum walls so that the building would stand the test of time. He said the wall of the resting area must be at least half a meter thick, made of “square redstone blocks, or bricks, or layered lava“. Aggregates made of bricks or volcanic rock are mixed with mortars composed of limestone or volcanic debris (which exists as glass fragments or crystals).
Geophysicist Jackson has been studying the unusual properties of Roman concrete for many years. She and her colleagues analyzed the mortar used in the construction of Trajan’s Market, completed around AD 100, and perhaps the oldest shopping mall in history.
The team specifically focused on the “glue” that holds the building materials together: it’s a complex of calcium, aluminum, silicate and hydrate compounds, strengthened by crystals of strätlingite. Analysis shows that the strätlingite crystals prevent the formation of cracks in the mortar, which can turn into large cracks that damage structures.
In 2017, Ms. Jackson co-authored a study analyzing the concrete for a breakwater along the Italian coast, a structure that has survived for two millennia against the extremely harsh conditions of the salty sea. Constant waves of water hitting the wall will certainly cause modern concrete technology to soon collapse, yet the ancient Roman breakwaters seem to get stronger with each wave.
Mrs. Jackson and her colleagues discovered the secret formula for maintaining the longevity of the wall: It is a combination of rare crystals and a pitting mineral. When the compound comes into contact with seawater, a chemical reaction occurs, causing the mineral phillipsite (commonly found in volcanic ash) to turn into aluminum tobermorite crystals. The crystal adheres firmly to the rock wall, further preventing cracks from forming.
With the rich premise of research related to ancient works, it is obvious that researcher Jackson was deeply interested in Caecilia Metella’s Tomb. In June 2006, during Mrs. Jackson’s first visit to the mausoleum, she took back samples of mortar for analysis. According to the researcher, that summer day had a high temperature, but the air in the tomb was very cool and humid.
Historians don’t know much about the noble woman left in the ancient tomb, other than that Caecilia Metella was the daughter of the Roman consul Quintus Caecilius Metellus Creticus. She married Marcus Licinius Crassus, son of Marcus Licinius Crassus, one of the three leaders of the First Triumvirate, alongside Julius Caesar and Pompey the Great.
Most likely, it was Caecilia Metella’s son, Marcus Licinius Crassus, who ordered the construction of the grand mausoleum for his mother; Archaeological traces indicate that the mausoleum was completed sometime between 30 and 10 BC. There is one more thing worth noting in the above story: the three generations of grandfather, father and grandson were named Marcus Licinius Crassus that may confuse historians.
At the Palace of Farneses there is a marble coffin believed to have been taken from the Tomb of Caecilia Metella, however it is highly likely that it does not contain the body of a noble woman, as the age of the stone coffin falls to approx. 180-190 AD. During the time when Caecilia Metella was still alive, cremation was very popular, so it is likely that in the heart of Caecilia Metella’s Tomb, there was an ashes containing the remains of a respected woman.
The construction context is interesting, but the architectural structure of the mausoleum makes researcher Jackson and his colleagues excited. The mausoleum is located on top of a hill, the structure consists of a circular dome placed on a square step, next to the mausoleum is a castle built in the 14th century. Outside the mausoleum is a stone inscription:
To study more closely the microstructure of mausoleum mortar, Ms. Jackson enlisted the help of experts at the Massachusetts Institute of Technology (MIT) and Lawrence Berkeley Laboratory. Using Advanced Light Source (ALS) technology, the Berkeley team has discovered a variety of minerals present in concrete. X-rays with the width of a bacterium have penetrated through ancient mortar samples, yielding valuable scientific data.
They found that the mausoleum’s mortar was similar to the material that made up the Trajan Market wall: volcanic debris from the eruption was used in bonding large bricks with lava aggregates. However, the crumbs used in the construction of Caecilia Metella’s Tomb contain many potassium-rich leucid minerals. Over the centuries, rainwater and groundwater have seeped into the mausoleum walls, dissolving leucites to produce potassium. In modern concrete walls, this compound will inevitably cause cracks, making the wall structure unstable.
But apparently, Caecilia Metella’s Tomb is still standing until now. According to Ms. Jackson, the potassium build-up in the mortar once again strengthened the bonds between the calcium, aluminum, silicates and hydrates present in the wall. Some parts of the wall are still intact after 2,000 years, although some places have begun to show signs of cracking. In fact, the mausoleum has a structure similar to a nanocrystal.
“As it turns out, the contact surface [giữa cốt liệu và vữa] in the concrete of ancient Rome that built Caecilia Metella’s Tomb continuously evolves through the ability to sustain permanent reconstruction”, said Professor Admir Masic from MIT. “The reconstruction process reinforced the contact surfaces and, in turn, could help improve the building performance and corrosion resistance of the ancient material.“.
The deeper they dig into what made up Roman concrete, the closer scientists are to recreating the legendary building material, to improve the concrete of modern times. The breakthrough could reduce the energy used in concrete production, while increasing the lifespan of today’s buildings.
“Focus on designing concrete with exposed surfaces [giữa cốt liệu và vữa] Self-reinforcing will help us improve the durability of modern building materials,” concluded researcher Masic. “Thanks to the application of time-tested ‘Roman wisdom’, it is possible to improve the lifespan of modern buildings several times over.“.
According to ArsTechnica