They discover the secret of why Roman concrete was so durable

Science Writing, 6 Jan. The ancient Romans were masters of engineering and construction, as their aqueducts, buildings and ports have survived, in many cases, for two centuries. A team of scientists has examined the concrete they used and believes they have found the key: quicklime.

Researchers have been trying to unlock the secret of this ultra-durable ancient building material for decades, especially in structures that endured particularly harsh conditions, such as piers, culverts and levees, or those built in seismically active locations.

A new study published by Science Advances signed by researchers from the Massachusetts Institute of Technology (MIT), Harvard University and laboratories in Italy and Switzerland has discovered ancient concrete manufacturing strategies that incorporated several key functionalities.

For years, it had been believed that the key to the durability of this concrete was based on an ingredient, the pozzolanic material, a volcanic ash from the Pozzuoli area, in the Bay of Naples (Italy), to which the stories referred. of architects and historians of the time.

However, those ancient samples also contain tiny, distinctive millimeter-scale brilliant white mineral features, long recognized as a ubiquitous component of Roman concretes.

These white bits, often referred to as “lime clasts”, come from lime, another key component of the old concrete mix.

Such remains have until now been considered mere evidence of careless mixing or poor-quality raw materials, MIT explains in a statement.

However, the new study suggests that these tiny lime clasts gave concrete a hitherto unknown self-healing ability.

One of the signatories to the research, Admir Masic of MIT, noted that if “the Romans put so much effort into making an exceptional building material, why would they put so little effort into ensuring the production of a well-mixed final product? “That’s why I thought there had to be a reason.

Following a more detailed characterization of the limestone clasts, using high-resolution multiscale imaging and chemical mapping techniques, the researchers gained new insights into the potential functionality of these limestone clasts.

Historically, it had been assumed that when lime was incorporated into Roman concrete, it first combined with water to form a highly reactive pasty material in a process known as slaking, but that process alone could not explain the presence of the clasts. of lime

So the team wondered if the Romans might have used quicklime, which is a more reactive form of the material. Studying samples of old concrete, they determined that the white particles were indeed made up of various forms of calcium carbonate.

Spectroscopic examination provided indications that they had formed at extreme temperatures, as would be expected from the exothermic reaction produced by the use of quicklime instead of, or in addition to, slaked lime in the mix.

Hot mixing, according to the team, was actually “the key to the super-durable nature” of the concrete due to two factors, Masic explained.

On the one hand, when the concrete as a whole is heated to high temperatures, it allows a chemistry that would not be possible if only slaked lime were used, producing compounds associated with those temperatures that would not otherwise form.

In addition, the increase in temperature significantly reduces curing and setting times, as all reactions are accelerated, allowing for much faster construction.

The team decided to prove that this was the mechanism responsible for the durability of Roman concrete by producing hot-mix samples incorporating ancient and modern formulations, cracking them, and running water through them.

After two weeks those openings had fully healed and water could no longer flow, however an identical piece of concrete made without quicklime never cured and water continued to flow through the specimen.

Masic considered that “it is exciting to think about how these more durable concrete formulations could extend not only the useful life of these materials, but also how the durability of 3D printed concrete formulations could be improved.”

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