Why has Roman concrete lasted so long?

No single ingredient explains it. Roman concrete used volcanic ash and lime, and cracks in it can fill with new calcite, which gives it a kind of self-healing. It was also unreinforced, so it avoided the steel corrosion that is the main durability problem in modern structures. The comparison is not a simple contest, because the two materials fail in different ways. [1] [2]

For many years, researchers assumed the key was pozzolanic material, such as volcanic ash from Pozzuoli near the Bay of Naples. According to [livescience.com](https://www.livescience.com/archaeology/romans/how-is-roman-concrete-still-standing-after-2-000-years), the silica and alumina in the ash react with lime and water at ambient temperatures to produce a stronger, longer-lasting concrete, and the mixture can harden underwater. Roman builders typically combined volcanic tuff and other coarse aggregate with a lime-and-ash mortar, with formulations tailored to each use, as described in [science.org](https://www.science.org/doi/10.1126/sciadv.add1602). [3] [4] [2]

A 2023 study led by Admir Masic's group at MIT argued that the Romans used hot mixing, adding quicklime rather than only slaked lime. This leaves small white lime clasts in the mortar. Cracks tend to run through these brittle, reactive clasts. Water then dissolves calcium from them, and it recrystallizes as calcium carbonate that seals the crack. In the team's test, deliberately cracked hot-mixed samples stopped leaking within two weeks, while a sample made without quicklime never healed, as reported by [news.mit.edu](https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106). Live Science notes calcite-filled cracks in the Tomb of Caecilia Metella as an example of this, according to [livescience.com](https://www.livescience.com/archaeology/romans/how-is-roman-concrete-still-standing-after-2-000-years). [2] [4] [3]

Researchers emphasize different mechanisms. The traditional view credits the pozzolanic reaction between ash and lime. The MIT group credits hot mixing and the lime clasts it leaves behind. A later Science Advances study of a latrine at Hadrian's Villa agrees that the pozzolanic reaction is of fundamental importance. It argues, though, that long-term carbonation also substantially improves durability, and that calcite cement formed from unhydrated calcium oxide is the primary binding phase, with only a minor amount of calcium-aluminum-silicate-hydrate. A further line of work, summarized by [science.org](https://www.science.org/content/article/why-modern-mortar-crumbles-roman-concrete-lasts-millennia), points to seawater reacting with volcanic ash to grow rare minerals such as aluminum tobermorite in marine structures. These explanations are not mutually exclusive, and the search results do not say that specialists have settled on one. [1] [2] [5]

Live Science cites an engineer's estimate that Portland cement structures last roughly 75 to 100 years depending on environment. The Guardian notes that modern Portland-cement concrete is not meant to change after hardening, so reactions with it cause damage. The Hadrian's Villa authors, however, caution that Roman survival does not imply superior durability under all conditions. In modern infrastructure the main challenge is reinforcement corrosion, while Roman concrete had no steel. They also warn against overstating direct parallels to modern structural uses. [3] [6] [1]