MIT study reveals how Roman pozzolanic concrete healed itself for 2,000 years

The ancient Romans were exceptional builders and engineers, and their aqueducts are often cited as the clearest proof. Many of those structures still operate today, thanks in large part to a distinctive building material: pozzolanic concrete, an extraordinarily long-lasting concrete that helped Roman construction endure.

One Roman landmark in particular underlines that durability. The Pantheon remains standing and largely unscathed almost 2,000 years after it was built, and it still holds the record for the world’s largest dome made from unreinforced concrete.

Roman pozzolanic concrete and the traditional explanation

For a long time, the remarkable performance of Roman concrete has been credited mainly to what went into it. Its key component is pozzolana - volcanic ash named after Pozzuoli in Italy, where major deposits exist - combined with lime. Add water, and the ingredients react to create a strong concrete.

The standard account of this pozzolanic concrete also assumes the Romans relied on slaked lime. In that conventional process, limestone is first heated to very high temperatures to produce quicklime (calcium oxide), a highly reactive, caustic powder.

When quicklime is then combined with water, it forms slaked lime (calcium hydroxide), which is less reactive and less caustic, taking the form of a paste. The prevailing view has been that it was this slaked lime that Roman builders blended with pozzolana.

MIT’s 2023 Roman concrete research at Privernum

Research published in 2023 suggests the story is more complicated. An international group led by the Massachusetts Institute of Technology (MIT) reported that the materials - and, crucially, the mixing approach - were not quite what many researchers had assumed.

A major clue lay in small, white pieces of lime embedded in concrete that otherwise looks thoroughly blended. These lime chunks had often been dismissed as evidence of poor workmanship or inferior materials, but MIT materials scientist Admir Masic found that explanation unsatisfying.

"The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me," Masic said back in January 2023.

"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."

To investigate, Masic and colleagues - led by MIT civil engineer Linda Seymour - analysed 2,000-year-old Roman concrete taken from the archaeological site of Privernum in Italy. The team examined the samples using large-area scanning electron microscopy and energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging, focusing on what the lime clasts were and how they might have formed.

“Hot mixing” with quicklime and what it changes

Their results pointed away from the usual slaked-lime-only recipe. The lime clasts they studied did not match what would be expected if slaked lime had simply been mixed with pozzolana and water.

Instead, the researchers concluded Roman concrete was likely produced by combining quicklime directly with pozzolana and water at extremely high temperatures - either on its own or alongside slaked lime. The team refers to this approach as “hot mixing”, and it naturally leaves behind the distinctive lime clasts seen in ancient samples.

"The benefits of hot mixing are twofold," Masic said.

"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."

How lime clasts help Roman concrete heal itself

The same lime clasts may also explain an additional advantage: self-repair.

As cracks develop, they tend to propagate towards the lime clasts because those clasts present a higher surface area than other particles in the surrounding matrix. If water enters a crack, it can react with the lime to produce a calcium-rich solution. As that solution dries, it hardens into calcium carbonate, effectively cementing the crack shut and stopping it from widening.

Evidence consistent with this process has been reported at another 2,000-year-old location: the Tomb of Caecilia Metella, where cracks in the concrete have been found filled with calcite. The mechanism may also help account for the longevity of Roman seawalls built 2,000 years ago, which have endured for centuries despite constant wave action.

To test the hypothesis experimentally, the team produced pozzolanic concrete using both ancient and modern-style recipes that incorporated quicklime. They also created a control concrete made without quicklime, then carried out cracking tests. In those comparisons, the cracked quicklime concrete completely healed within two weeks, while the control samples remained fractured.

The researchers are now working to commercialise this concrete as a more environmentally friendly substitute for modern concretes.

"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," Masic said.

The research has been published in Science Advances.

A version of this article was first published in January 2023.