Riddle solved: Why was Roman concrete so durable?

The ancient Romans were masters of engineering and built huge networks of roads, aqueducts, ports, and massive buildings. The Pantheon, the world's largest unreinforced concrete dome, is one of the structures built with concrete. Modern concrete structures have crumbled over time.

Researchers have spent decades trying to figure out the secret of this ultradurable ancient construction material, particularly in structures that endure especially harsh conditions, such as docks, sewer, and seawalls.

A team of investigators from MIT, Harvard University, and laboratories in Italy and Switzerland discovered ancient concrete-manufacturing strategies that incorporated several key self-healing functions. The findings were published in a paper by MIT professor of civil and environmental engineering Admir Masic and others.

For a long time, researchers assumed that the key to the ancient concrete's longevity was based on volcanic ash from the Bay of Naples. This specific type 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 at the time.

Small, distinctive, millimeter-scale bright white mineral features, which have been long recognized as a ubiquitous component of Roman concretes, are contained in ancient samples. Lime is one of the main components of the ancient concrete mix. Masic has always been fascinated by the features of ancient Roman concrete. These aren't found in modern concrete, so why are they there?

The new study suggests that the tiny lime clasts gave the concrete a self-healing capability. Masic was bothered by the idea that the presence of these lime clasts was due to low quality control. If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been altered over the course of many centuries, why wouldn't they make a well-mixed final product? There needs to be more.

The researchers gained new insights into the potential function of these lime clasts after further characterization using high-resolution multiscale images and chemical mapping techniques.

Historically, it had been assumed that when lime was incorporated into Roman concrete, it was first combined with water to form a paste-like material. The lime clasts can not be accounted for by this process alone. Masic wondered if it was possible that the Romans used quicklime directly.

He and his team studied samples of the ancient concrete and found that the white inclusions were made out of calcium carbonate. As would be expected from the exothermic reaction produced by using quicklime instead of slaked lime, these had been formed at extreme temperatures. The key to the super-durable nature was hot mixing.

The benefits of hot mixing are twofold. When the concrete is heated to high temperatures, they allow chemistries that are not possible if you only use slaked lime. The increased temperature allows for faster construction since all the reactions are accelerated.

The team proposed that the lime clasts could provide a self-healing function by creating an easily fractured and reactive calcium source. When tiny cracks form within the concrete, they can travel through the high- surface area lime clasts. The material can react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate, or react with pozzolanic materials to further strengthen the material. The cracks are healed before they spread because of these reactions. There was previous support for this hypothesis through the examination of other Roman concrete samples.

The team produced samples of hot-mixed concrete that incorporated both ancient and modern formulas, and then ran water through the cracks. The cracks healed within two weeks and the water stopped flowing. A chunk of concrete made without quicklime never healed and the water just kept flowing through it. The modified cement material is being worked on by the team.

Masic says it's exciting to think about how these moredurable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete.

He hopes that the development of lighter-weight concrete forms will help reduce the environmental impact of cement production, which currently accounts for 8 percent of global greenhouse gas emissions. Concrete that can absorb carbon dioxide from the air is one of the new formulas that the Masic lab is working on.

The research team included Janille Maragh at MIT, Paolo Sabatini at DMAT in Italy, and James Weaver at the Wyss Institute for Biologically inspired Engineering at Harvard University. The work was done with the help of a museum in Italy.

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The Massachusetts Institute of Technology provides materials. David wrote the original. The content can be edited for style and length.