The Curia of Pompey is famous for being the site where Julius Caesar was stabbed to death on the ides of March in 44 BCE. It is of great interest to tourists, historians, and archaeologists alike. After analyzing mortar samples collected from the curia, researchers from Italy and Spain have confirmed an earlier hypothesis that the structure was constructed in three distinct phases, according to a recent paper published in the journal Archaeometry.

In ancient Rome, a curia was a structure where members of the senate would meet. The great Roman general Gnaeus Pompeius Magnus (Pompey) built this particular curia as a memorial to his own military achievements. A large theater section contained a temple, a stage, and seating on one end; a large porticus (housing the general’s art and books) surrounded a garden in the middle; and the Curia of Pompey was at the opposite end.

During Julius Caesar’s reign, the Roman senators temporarily met in the Curia of Pompey after their usual Curia on the Comitium burned down in 52 BCE. (Followers of an assassinated tribune named Publius Clodius Pulcher set it on fire while cremating his body.) Caesar’s planned replacement (Curia Julia) was under construction as a replacement meeting site when the ruler met his own brutal demise at the base of the Curia of Pompey. The senators who killed him thought assassination was the only way to preserve the republic, but the murder ultimately lead to the republic’s collapse.

While the theater complex would survive for centuries, the Curia of Pompey did not remain open. After the assassination, the curia was walled up (and possibly set on fire) just 11 years after its opening. A latrine was later built on the site. The curia was buried under more recent construction as Rome expanded and wasn’t excavated until the 1930s as part of Italian dictator Benito Mussolini’s razing of parts of modern Rome to unearth ancient historical sites. In addition to the Curia of Pompey, those efforts revealed four temples. The remains of the structure are still visible in an area of Rome called Largo di Torre Argentina.

The suggestion that the curia had been built in phases is not new. An earlier archaeological study analyzed the rock layers (strata) at the site and found that the curia’s construction began around 55 BCE using pozzolane rosse (pink pozzolana) extracted from volcanic deposits near the city’s center. Around 19 BCE, during the reign of Augustus Caesar, there was a second construction phase using pozzolane rosse extracted from a site further away. The third and final construction phase took place during the medieval period.

Fabrizio Marra of the National Institute of Geophysics and Volcanology in Rome and his fellow co-authors sought additional confirmation of this hypothesis from the perspective of archaeometry. Specifically, they wanted to conduct chemical analysis of the mortar (concrete) used to build the curia to determine which quarries supplied the building materials for each phase of construction. The team also analyzed samples from three basins at the site: two located on the western side of Largo di Torre Argentina, and the third on the northern side of Pompey’s curia.

As we’ve reported previously, ancient Roman concrete was basically a mix of a semi-liquid mortar and aggregate. Portland cement (a basic ingredient of modern concrete) 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 (0.6 m) and 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).

“A large number of papers in the last 15 years have shown the exceptional care with which Roman constructors produced mortar and concrete,” Marra et al. wrote. Scientists 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). In 2017, the same team analyzed 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 researchers found that the secret to that longevity was a special recipe involving a combination of rare crystals and a porous mineral.

And just last year, scientists analyzed samples of the ancient concrete used to build a 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. The scientists discovered that the tomb’s mortar was similar to that used in the walls of the Markets of Trajan: volcanic tephra from the pozzolane rosse pyroclastic flow, binding together large chunks of brick and lava aggregate. However, the tephra used in the tomb’s mortar contained much more potassium-rich leucite.