A mere 40 years after it was constructed, concrete degradation led to the deadly collapse of a 12-story condominium in Surfside, Florida earlier this year. Meanwhile, the tomb of Caecilia Metella, a Roman noblewoman, is still standing and considered structurally sound, despite being built around 30 BCE.
So why is a 2,051-year-old concrete outlasting its 40-year-old counterpart? Researchers at the University of Utah believe it has to do with the volcanic aggregate ancient Romans used to build the tomb, which is a 70 x 100 ft drum-shaped tower that sits on a square base on the Appian Way.
In a new study, published in the Journal of the American Ceramic Society, scientists used scanning electron microscopy, energy-dispersive X-ray spectrometry, X-ray microdiffraction and Raman spectroscopy to reveal the secrets of ancient Roman concrete in the hopes they could replicate it—thus improving the lifespan of modern concrete at least four-fold.
Today’s concrete is a composite material that comprises aggregate (gravel and sand), cement and water. Portland cement, which is derived from limestone, is most often used as the cement binder for modern concrete. But, based on samples taken by geology professor Marie Jackson, Metella’s tomb contains zero cement.
Instead, ancient Roman builders constructed the thick walls of the tomb with volcanic rock aggregate that was bound with mortar made with hydrated lime and volcanic tephra (porous fragments of glass and crystals from explosive eruptions). Given its volcanic origins, the researchers discovered that the mortar was abundant in potassium-rich leucite.
According to the study, centuries of rainwater and groundwater penetrating the tomb’s walls dissolved the leucite and released the potassium into the mortar. In modern concrete, such a flood of potassium would create expansive gels that would cause microcracking and eventual spalling and deterioration of the structure. In the tomb, however, the potassium dissolved and completely reconfigured the calcium-aluminum-silicate-hydrate (C-A-S-H) binding phase—in a positive way.
It's this unusual chemical reconfiguration that is the essence of strength in Metella’s ancient tomb.
“It turns out that the interfacial zones [between the aggregate and mortar] in the ancient Roman concrete of the tomb are constantly evolving through long-term remodeling," said Admir Masic, associate professor of civil and environmental engineering at MIT. “These remodeling processes reinforce interfacial zones and potentially contribute to improved mechanical performance and resistance to failure of the ancient material.”
Through a Department of Energy ARPA-e project, Masic, Jackson and their colleagues are working to replicate the successes of ancient Roman concrete. Specifically, they want to leverage similar beneficially reactive aggregates in concretes that use engineered cellular magmatics in place of the tephra of the ancient Roman structures. A Roman-like concrete could reduce the energy emissions of modern concrete production and installation by 85%, while also extending the lifespan four-fold.
Rome may not have been built in a day, but it was built for the long-term.