Ancient Romans were masters of construction and engineering; Perhaps the most famous example of these were aqueducts. And these still-functional marvels are based on a unique building material: Pozzolanic concrete, an incredibly strong concrete that gave Roman structures incredible strength. Even today, one of his structures, the Pantheon, which is almost 2,000 years old, holds the record for the world’s largest unreinforced concrete dome.

The properties of this concrete are often attributed to its ingredients: pozzolana, a mixture of volcanic ash (named after the Italian city of Pozzuoli, where significant deposits can be found) and lime. When these two materials are mixed with water, they can react to create strong concrete. But as it turns out, that’s not the whole story. In 2023, an international research team led by the Massachusetts Institute of Technology (MIT) discovered that not only are the materials slightly different than we thought, but the methods of mixing them are also different.

The smoking guns were small pieces of white lime in what appeared to be well-mixed concrete. The presence of these fragments had previously been attributed to poor mixing or materials, but this didn’t make sense to MIT materials scientist Admir Masic.

“I have always been troubled by the idea that the presence of these lime particles is simply due to poor quality control,” Masich said in January 2023.

“If the Romans put so much effort into creating an extraordinary building material, following all the detailed recipes that had been optimized over centuries, why did they put so little effort into ensuring the production of a well-mixed final product? ? There must be more to this story.”

A team led by Masik and MIT civil engineer Linda Seymour examined 2,000-year-old samples of Roman concrete taken from the archaeological site of Privernum in Italy. These samples were subjected to wide-field scanning electron microscopy and energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging to provide a better understanding of the lime deposit.

One of the questions taken into consideration was the nature of the lime used. The standard understanding of pozzolanic concrete is the use of slaked lime. First, limestone is heated at high temperatures to produce a highly active caustic powder called quicklime or calcium oxide.

Mixing quicklime with water produces slaked lime or calcium hydroxide: a slightly less reactive, less caustic paste. The theory is that it was this slaked lime that the ancient Romans confused with bozolana.

According to the team’s analysis, the lime particles in their samples did not fit this method. Rather, Roman concrete was made by mixing quicklime directly with plaster and water at extremely high temperatures, alone or alongside slaked lime; This was a process the team called “hot mixing” and resulted in lime chips.

“The benefits of hot mixing are twofold,” Masik said.

“First, when aggregate concrete is heated to high temperatures, it forms high-temperature compounds that would not otherwise form, allowing chemical processes that are not possible when using slaked lime alone. Secondly, this higher temperature significantly reduces setting and curing times as all reactions are accelerated, allowing for much faster construction.”

This has another advantage: Lime particles give concrete an excellent self-healing ability.

When cracks form in concrete, they move preferentially towards lime particles, which have a larger surface area than other particles in the matrix. When water enters the crack, it reacts with lime to form a calcium-rich solution, which dries and hardens as calcium carbonate, sealing the crack and preventing it from spreading.

This was seen in concrete at the Tomb of Caecilia Metella, another 2,000-year-old site, where cracks in the concrete were filled with calcite. This may also explain why Roman concrete seawalls built 2,000 years ago remained intact for millennia despite the constant pounding of the ocean.

So the team tested their findings by making pozzolanic concrete using ancient and modern recipes using quicklime. They also made control concrete without quicklime and performed cracking tests. Of course, the cracked lime concrete healed completely within two weeks, but the control concrete remained cracked. The team is now trying to commercialize their concrete as a greener alternative to existing concrete.

“It is exciting to think about how these stronger concrete composites could not only increase the lifespan of these materials, but also the durability of 3D printed concrete composites,” Masik said. The study was published on: Science Developments.