Noblewoman’s tomb reveals new secrets of ancient Rome’s highly durable concrete

Among the many popular tourist sites in Rome is an impressive 2,000-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 of fist-sized 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 composed 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 from the ruins of sea walls along Italy's Mediterranean coast, which have stood for two millennia despite the harsh marine environment. The constant salt-water waves crashing against the walls would have long ago reduced modern concrete walls to rubble, but the Roman sea walls seem to have actually gotten stronger.

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

So naturally Jackson was intrigued by the Tomb of Caecilia Metella, widely considered to be one of the best-preserved monuments on the Appian Way. Jackson visited the tomb back in June 2006, when she took small samples of the mortar for analysis. Despite the day of her visit being quite warm, she recalled that once inside the sepulchral corridor, the air was very cool and moist. "The atmosphere was very tranquil, except for the fluttering of pigeons in the open center of the circular structure," Jackson said.