The Pantheon, almost 2000 years old, is still intact: what is the secret of the strength of ancient Roman concrete

The nearly 2000-year-old Pantheon, the "Temple of All the Gods" in Rome, holds the record for the world's largest unreinforced concrete dome. The ancient Romans were masters of construction and engineering.

According to scientists, the functional wonders of the Romans were based on a unique building material – pozzolanic concrete, which gave the structures incredible strength. An international team of researchers has determined that the secret of the strength of the ancient Roman material lies not only in its components but also in the method of mixing them, ScienceAlert reports.

The properties of pozzolanic concrete, named after the Italian city of Pozzuoli, are attributed to its ingredients: pozzolana and a mixture of volcanic ash. When mixed with water, these materials react to form strong concrete.

However, an international team of researchers led by the Massachusetts Institute of Technology (MIT) found that not only do the materials play a role in strength, but the methods of mixing them also differ.

According to the scientists, small white pieces of lime were found in ancient Roman concrete, which had previously been attributed to poor mixing.

"The idea that the presence of these lime fragments was simply attributed to poor quality control has always bothered me," said materials scientist Admir Masik of MIT.

"If the Romans put so much effort into creating an outstanding building material, following all the detailed recipes that had been optimized over many centuries, why did they put so little effort into ensuring that they produced a well-mixed end product?" the expert wondered.

Masik and a team of experts led by MIT civil engineer Linda Seymour carefully studied 2,000-year-old samples of Roman concrete from the archaeological site of Provernum in Italy.

These samples were subjected to state-of-the-art microscopic scanning and various X-ray studies to better understand the lime fragments.

According to the researchers, the standard understanding of pozzolanic concrete is that it uses slaked lime. Limestone is heated at high temperatures to produce a highly active, caustic powder called quicklime or calcium oxide.

Mixing quicklime with water is known to produce slaked lime or calcium hydroxide: a slightly less reactive, less caustic paste. According to theory, it was this slaked lime that the ancient Romans mixed with pozzolana.

However, based on the analysis, the lime fragments in their samples do not correspond to this method. Rather, Roman concrete was made by mixing quicklime directly with pozzolana and water at extremely high temperatures, either alone or in addition to the slaked lime. The team calls this process "hot mixing," which results in the formation of lime fragments.

Admir Masik emphasized that the benefits of hot mixing are twofold. "First, when the overall concrete is heated to high temperatures, it allows chemical processes that are not possible if you use only slaked lime, forming high-temperature compounds that would not otherwise form," he says.

"Secondly, this increased temperature significantly reduces the curing and setting time because all the reactions are accelerated, which allows for a much faster construction process," the materials scientist added.

Lime fragments also give concrete an excellent self-healing ability, experts say. When cracks form in concrete, they preferentially move to the lime fragments, which have a larger surface area than other particles in the matrix.

When water enters the crack, it reacts with the lime to form a calcium-rich solution that dries and hardens as calcium carbonate, sealing the crack and preventing it from spreading further, the experts explain.

The research team is now working on commercializing its concrete as a more environmentally friendly alternative to the current one. "This option can increase not only the service life of these materials, but can also increase the durability of concrete compositions printed on a 3D printer," Admir Masik summarized.

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