Roman concrete
The concrete used in ancient Rome. A mixture of volcanic ash and lime kneaded together with aggregate, it has left structures across many regions that have stood for nearly 2,000 years, beginning with the unreinforced dome of the Pantheon. Why it lasts so long, while modern reinforced concrete deteriorates within decades, was long a mystery. In 2023 a study was published arguing that a mechanism by which the concrete seals its own cracks is at work, and the focus of the debate shifted.
Materials and the conventional explanation
The main materials are lime and "pozzolana", a volcanic ash quarried around Pozzuoli near Naples. Pozzolana reacts with water and lime and hardens, and because it sets even in seawater, it was also used for harbor structures.
The reason for the durability was long attributed to this material, pozzolana, itself. The white lumps of lime mixed into Roman concrete (lime clasts), on the other hand, were regarded as evidence of careless mixing or low-quality raw materials and received little attention.
The self-healing mechanism
In January 2023, a team led by Admir Masic of MIT published the paper "Hot mixing: Mechanistic insights into the durability of ancient Roman concrete" in Science Advances, arguing that these white lime clasts were in fact the key.
The point is a method called "hot mixing". When quicklime (calcium oxide) is mixed in directly instead of slaked lime (calcium hydroxide), it reacts with water and generates intense heat, with temperatures approaching 400°C according to press reports※1. Through this process, lime clasts with a large surface area remain in the mortar.
Then, when a crack forms in the concrete, the fracture preferentially passes through these low-strength lime clasts. When water enters, the lime clast dissolves into a calcium-saturated solution, which recrystallizes as calcium carbonate and fills the crack, or reacts with the pozzolana and reinforces the surroundings. In other words, the structure is as if repair material had been planted in advance at the places where cracks will form.
The research team analyzed samples roughly 2,000 years old taken from the site of Privernum in Italy with electron microscopy and X-ray spectroscopy, and further confirmed this self-healing experimentally by cracking samples made with Roman-style hot mixing, rejoining them, and measuring the amount of leakage while running water through them for 30 days.
Application to the present
If this mechanism is correct, it could be applied to the design of long-lived concrete that needs no repair. Since the cement industry accounts for a considerable share of the world's carbon dioxide emissions, extending the lifespan of structures in itself leads to a reduction in environmental impact. The MIT research team is said to be pursuing the commercialization of concrete made by this method.
At present, however, the results are based on laboratory-scale samples and the analysis of ancient samples. How far the effect holds at the scale of a whole structure, and whether it can be incorporated as is into modern construction, are left to future verification.
To verify
- Debate remains among researchers over whether the main cause of the durability is the pozzolana or the lime clasts
- Whether the ancient Romans used quicklime intending this effect, or whether it came about as a consequence, is unknown
- How close the application to modern concrete is to practical use
References
- Roman concrete - Wikipedia
- Riddle solved: Why was Roman concrete so durable? | MIT News
- MIT and Harvard researchers find secret to "self-healing" Roman concrete | Dezeen
- Relict no more—purposeful inclusion of lime clasts gives Roman concretes self-healing properties | The American Ceramic Society
- Why was ancient Roman concrete so durable? MIT discovers 'self-healing' ingredient | designboom
- Lime Clasts and Heat: The Romans' Secret Sauce | PACA
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