How 1,900-year-old latrines explain why Roman concrete lasts

Twenty-seven kilometers east of Rome are the remains of a communal latrine whose concrete stood for nearly 2,000 years. It has survived the empire that established it, centuries of alteration and even Italy’s third consecutive failure to qualify for the World Cup.

This is an impressive performance for a bathroom, especially a shared bathroom.

Today, this humble latrine, which was part of the residence of Emperor Hadrian vast 2nd century villa at Tivoli, is helping scientists unravel one of engineering’s favorite mysteries: why certain Roman concretes have endured for millennia. A study published this week in Scientific advances offers the clearest picture yet of how the material continued to change and strengthen long after it was cast.

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Researchers have long attributed the remarkable durability of Roman concrete to ingenious ancient chemistry. Builders mixed lime with volcanic ash, triggering mineral reactions that persisted as the concrete aged. “We can imagine the situation as if the Romans used volcanoes to improve their concrete, while we instead used high-temperature cement kilns,” explains Maria Juengerwho studies cement and concrete materials at the University of Texas at Austin and was not involved in the research.

In 2023, researchers at MIT and elsewhere proposed that the bright white chunks scattered throughout Roman concrete — known as lime clasts and long dismissed as evidence of incomplete mixing — could help explain the nature of the material. self-healing properties. When cracks form, water dissolves calcium-rich materials from the clasts, which then recrystallize as calcium carbonate, sealing the fracture.

Studying the chemistry of ancient concrete requires a sample that no one has repaired or restored along the way – a rare commodity in ruins tended by generations of restorers.

The researchers had a particular advantage.

“No one is restoring latrines,” says Paulo JM Monteiroa civil engineer at the University of California, Berkeley and lead author of the new study. “Thus the material remained intact for nineteen centuries, quietly conducting an experiment that no living person could initiate.”

Monteiro and his colleagues, led by Xiaohong Zhu of Beijing University of Technology, used high-resolution x-ray imagingelectron microscopy and chemical analyzes to map carbonate minerals inside ancient concrete on scales down to tens of nanometers. This process is called carbonation, in which carbon dioxide from the air seeps into the concrete and reacts with calcium-rich compounds, leaving behind calcite, a hard crystalline mineral. The team’s analyzes reveal calcite woven through the material, filling the pores and binding its components together.

“Calcite had already been suspected as an important binding phase in concrete from the Roman interior,” says Monteiro. “What’s new is that we can now see how it ties together.”

The study indeed gives promotion to carbonates.

“This reinforces the idea that carbonates are more dynamic in these systems and play a fundamental, not marginal, role,” explains I admire Masicthe MIT materials scientist whose group led the work on lime clasts.

It is less clear whether this knowledge can improve modern concrete.

“The elephant in the room is steel,” Juenger says. Unlike Roman concrete, most modern concrete is reinforced with steel bars. Fresh concrete is sufficiently alkaline too protect the metal from rustbut carbonation gradually lowers its pH and weakens this protection. “The same reaction that quietly strengthened Roman concrete poses a slow threat to ours,” says Monteiro.

At the same time, engineers are increasingly interested in controlled carbonationwhich can trap carbon dioxide in mineral form – no small feat for an industry whose key ingredient, cement, accounts for around 8 percent of global carbon emissions. The paper’s authors warn against expecting rapid climate gains following a response that, at Hadrian’s Villa, took centuries. “Modern engineers are therefore faced with a delicate balancing act between durability and durability,” says Monteiro. “We hope our techniques can help optimize this balance.”

Back in Tivoli, the long-standing latrine experiment continues.