Noblewoman’s tomb reveals new secrets of ancient Rome’s highly durable concrete
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 noblewoman who lived in the first century CE. Lord Byron was among those who marveled at the structure, even referencing it in his epic poem Childe Harold’s Pilgrimage (1812-1818). Now scientists have analyzed samples of the ancient concrete used to build the tomb, describing their findings in a paper published in October in the Journal of the American Ceramic Society.
“The construction of this very innovative and robust monument and landmark on the Via Appia Antica indicates that [Caecilia Metella] was held in high respect,” said co-author Marie Jackson, a geophysicist at the University of Utah. “And the concrete fabric 2,050 years later reflects a strong and resilient presence.”
Like today’s Portland cement (a basic ingredient of modern concrete), ancient Roman concrete was basically a mix of a 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 added gypsum—the better to achieve a smooth, flat surface. But the aggregate used to make Roman concrete was made up fist-size pieces of stone or bricks
In his treatise de Architectura (circa 30 CE), the Roman architect and engineer Vitruvius wrote about how to build concrete walls for funerary structures that could endure for a long time without falling into ruins. He recommended the walls be at least two feet thick, made of either “squared red stone or of brick or lava laid in courses.” The brick or volcanic rock aggregate should be bound with mortar comprised of hydrated lime and porous fragments of glass and crystals from volcanic eruptions (known as volcanic tephra).
Jackson has been studying the unusual properties of ancient Roman concrete for many years. For instance, she and several colleagues have analyzed the mortar used in the concrete that makes up the Markets of Trajan, built between 100 and 110 CE (likely the world’s oldest shopping mall). They were particularly interested in the “glue” used in the material’s binding phase: a calcium-aluminum-silicate-hydrate (C-A-S-H), 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 analyzing the concrete form the ruins of sea walls along Italy’s Mediterranean coast, which have stood for two millennia despite the…
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