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Why Ancient Roman Concrete Lasted Thousands of Years

Pantheon Rome
Pantheon Rome. Credit: Flickr / Mr Gs Travels CC BY-NC-SA 2.0

The ancient Romans were great builders, especially known for their aqueducts. These are still standing today because they were made with a special kind of concrete called pozzolanic concrete. This type of concrete is especially strong and is the reason why Roman structures are so durable.

One of their buildings, the Pantheon, is almost two thousand years old and still in good condition. It has the world’s largest dome made of concrete with no extra support.

The unique qualities of this concrete come from pozzolana, a blend of volcanic ash found near the Italian city of Pozzuoli, and lime. When these two ingredients get mixed with water, they react and create durable concrete.

Not only the material but also the process of construction was different

In 2023, a team of researchers around the world, led by the Massachusetts Institute of Technology (MIT), discovered that not only were the materials used by the Romans in the production of concrete different but so was their method for mixing them.

Researchers found evidence in the form of small, white lime chunks in what appeared to be well-mixed concrete. Previously, people believed those chunks were due to poor mixing practices or materials, but MIT’s materials scientist, Admir Masic, was not convinced by such an explanation.

“The idea that the presence of these lime clasts was simply attributed to low-quality control always bothered me,” Masic said in a study published 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?” Masic wondered.

“There has to be more to this story,” he concluded.

Analysis of 2,000-year-old samples of Roman concrete

Masic and the team, led by MIT civil engineer Linda Seymour, took a close look at ancient Roman concrete samples from the Privernum archaeological site in Italy, dating back two thousand years.

They used advanced techniques, such as large-area scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging to examine the lime chunks more closely.

One critical question pertained to the kind of lime the Romans used. Usually, people thought pozzolanic concrete had slaked lime. Typically, when limestone is heated at really high temperatures, it yields a strong powder called quicklime or calcium oxide.

When quicklime is then mixed with water, slaked lime, also known as calcium hydroxide—a paste that is a bit less reactive and not as strong—is produced. The idea was that the ancient Romans mixed this slaked lime with pozzolana, according to the theory.

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