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I'm no chemist, but it feels like there should have been more than enough motivation to discover the recipe for strong concrete and it should have been discovered ages ago. Why were the details only discovered in 2023?

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Tl;DR: Researchers who were investigating on this topic didn't consider a key component in the concrete mix. They were also puzzled by the mixing technique. These two reason caused the delay in pinpointing the actual reason for the durability of the Roman buildings.

Long Answer:

Researchers indeed have spent decades trying to figure out the secret of the ultradurable construction material in the Ancient Roman buildings that endured especially harsh conditions like weathering, seismic activity and constant volcanic activity. For many years, they have assumed that the key to the ancient concrete’s durability was based on only one ingredient: pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples. This specific kind of ash was shipped all across the Roman empire to be used in construction, and was described as a key ingredient for concrete in accounts by architects and historians at the time. But they forgot to consider one more key material: lime clasts.

An international team of researchers led by the Massachusetts Institute of Technology (MIT) found that not only are the materials slightly different from what we may have thought, but the techniques used to mix them were also different. The smoking guns were small, white chunks of lime called lime-clasts can be found in in the otherwise well-mixed concrete. The presence of these chunks had previously been attributed to poor mixing or materials, but that did not make sense to them. Admir Masic, one of the material scientists quote:

The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me.

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.

This new observation suggested that these tiny lime clasts gave the concrete a self-healing capability. When cracks form in the concrete, they preferentially travel to the lime clasts, which have a higher surface area than other particles in the matrix. When water gets into the crack, it reacts with the lime to form a solution rich in calcium that dries and hardens as calcium carbonate, gluing the crack back together and preventing it from spreading further.

You can find the final observations in their published paper:

Linda M. Seymour et al. Hot mixing: Mechanistic insights into the durability of ancient Roman concrete Sci. Adv.9, (2023). DOI:10.1126/sciadv.add1602

Other references:

  1. https://www.history.com/news/the-secrets-of-ancient-roman-concrete
  2. https://www.sciencealert.com/we-finally-know-how-ancient-roman-concrete-was-so-durable
  3. https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106
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