Team Finds Key to Durability of Roman Concrete

The front of the Roman Pantheon. Credit: R. Abax

“They don’t make them like that anymore,” is a common enough phrase in today’s society. And while typically the speaker is referring to the differences in quality between the same items separated by a few decades, the same logic can be applied to items separated by thousands of years.

For example, Rome’s Pantheon, dedicated in 128 AD as the world’s largest un-reinforced concrete dome, is still intact, as are some ancient Roman aqueducts that continue to deliver water today. Meanwhile, modern concrete structures in the U.S. and other nations have crumbled after only a few decades.

As researchers from MIT, Harvard, and laboratories in Italy and Switzerland recently discovered, it is true “they don’t make concrete like that anymore”—because modern manufacturing methods don’t incorporate self-healing lime clasts.

In addition to volcanic ash from Pozzuoli, ancient concrete samples from Rome have been found to contain millimeter-scale bright white mineral features often referred to as “lime clasts.” Originating from lime, the clasts were thought to be evidence of sloppy mixing practices or poor-quality raw materials.

However, the new study, published in Science Advances, suggests the tiny lime clasts actually play a vital role in ancient concrete—they have a previously unrecognized self-healing capability.

“The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” said corresponding author Adnur Masic, professor of civil and environmental engineering at MIT. “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”

Previous studies suggested that when lime was incorporated into Roman concrete, it was first combined with water to form a highly reactive paste-like material, a process known as slaking. But, using a combination of scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS), powder X-ray diffraction and confocal Raman imaging, Masic and his team discovered that the Romans most likely used lime in its more reactive form, known as quicklime.

According to the study, spectroscopic examination provided clues that the material had been formed at extreme temperatures, as would be expected from the exothermic reaction produced by using quicklime instead of, or in addition to, the slaked lime in the mixture. From this, the team concluded that hot mixing—as the process is called—was actually the key to the durability of ancient Roman concrete.

“The benefits of hot mixing are two-fold,” Masic says. “First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction.”

To prove their theory, the scientists produced samples of hot-mixed concrete that incorporated both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks. Within two weeks, the cracks had completely healed and the water could no longer flow through.

On the contrary, an identical chunk of concrete made without quicklime never healed, with the water continuing to flow through the sample.

As a result of these successful tests, the research team is working to commercialize the hot-mixed-based cement material.

“It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete formulations,” said Masic.