Why was Roman concrete so durable?

The ancient Romans were masters of engineering, constructing vast networks of roads, aqueducts, ports, and massive buildings, whose remains have survived for two millennia. Many of these structures were built with concrete: Rome’s famed Pantheon, which has the world’s largest unreinforced concrete dome and was dedicated in A.D. 128, is still intact, and some ancient Roman aqueducts still deliver water to Rome today. Meanwhile, many modern concrete structures have crumbled after a few decades.

For many years, researchers have assumed that the key to the ancient concrete’s durability was based on one ingredient: pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples. This specific kind of ash was even shipped all across the vast 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. Now, a team of investigators from MIT, Harvard University, and laboratories in Italy and Switzerland, has made progress in this field, discovering ancient concrete-manufacturing strategies that incorporated several key self-healing functionalities. Under closer examination, these ancient samples also contain small, distinctive, millimeter-scale bright white mineral features, which have been long recognized as a ubiquitous component of Roman concretes. These white chunks, often referred to as “lime clasts,” originate from lime, another key component of the ancient concrete mix. “Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” says Admir Masic, MIT professor of civil and environmental engineering. “These are not found in modern concrete formulations, so why are they present in these ancient materials?” Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognized self-healing capability. “The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” says Masic. “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.”

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Source: https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106