The ancient Romans were masters of engineering and built vast networks of roads, aqueducts, ports, and huge buildings, whose remains have survived for two millennia. Many of these structures were constructed with concrete. The famous Pantheon in Rome, for example, has the largest unreinforced concrete dome in the world and was inaugurated in the year 128 A.D. It remains intact, and some Roman aqueducts still supply water to Rome. Meanwhile, many modern concrete structures have collapsed within a few decades.
A new study indicates that the tiny limestone clasts found in Roman concrete gave it a previously unknown self-healing capability. Researchers have spent decades trying to uncover the secret of this ancient, ultra-resilient building material, especially in structures that faced particularly harsh conditions, such as docks, sewers, quays, or those built in seismically active areas. Now, an international team of scientists has examined the concrete used by the ancient Romans and believes they’ve found the key: quicklime.
A new study published in Science Advances, authored 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. The durability of concrete had been associated with pozzolanic volcanic ash referenced in Roman texts. For years, it had been believed that the key to the durability of that concrete lay in an ingredient, pozzolanic material, volcanic ash from the Pozzuoli area in the Bay of Naples, Italy, referenced in the accounts of architects and historians of the time.
However, these ancient samples also contain small, distinctive features of bright white minerals on a millimeter scale, long recognized as a ubiquitous component of Roman concretes.
A new interpretation of lime clasts. These white pieces, often called lime clasts, come from lime, another key component of the ancient concrete mixture. Until now, they had been considered mere evidence of careless mixes or low-quality raw materials, according to the authors. However, the new study suggests that these tiny lime clasts gave the concrete a previously unknown self-healing capability.
One of the study’s co-authors, Admir Masic from MIT, highlights that if “the Romans went to such great lengths to create an exceptional building material, why would they put so little effort into ensuring the production of a well-mixed end product?”, so he believed there had to be some reason.
After a more detailed characterization of the calcareous clasts, using high-resolution multi-scale imaging techniques and chemical mapping, the researchers gained new insights into the potential functionality of these remnants. Historically, it had been assumed that when lime was incorporated into Roman concrete, it first combined with water to form a highly reactive paste material in a process known as slaking (slaked lime is calcium hydroxide), but that process alone could not explain the presence of the lime clasts.
The reactive quicklime. Therefore, the team wondered if it was possible that the Romans had used quicklime (CaO, calcium oxide), which is a more reactive form of that material. Studying ancient concrete samples determined that the white particles were formed by 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 using quicklime instead of, or in addition to, slaked lime in the mix.
The hot mix of quicklime, instead of, or in addition to, slaked lime, is key to the super-durability of Roman concrete. The hot mix, according to the team, was actually “the key to the super-durability nature” of concrete due to two factors, Masic explains. On one hand, when the concrete as a whole is heated to high temperatures, it allows for chemistry that would not be possible if only slaked lime were used. Additionally, the temperature increase significantly reduces curing and setting times, allowing for much faster construction.
Verification with test mixes. The team decided to test whether this was the mechanism responsible for the durability of Roman concrete, producing samples of hot-mixed incorporating ancient and modern formulations, cracking them, and running water through them. After two weeks, those openings had completely healed, and water could no longer flow. However, an identical piece of concrete made without quicklime never healed, and water continued to flow through the sample.
The finding could improve 3D printing of concrete and extend the lifespan of materials. Masic believes that “it’s exciting to think about how these more durable concrete formulations could extend not only the lifespan of these materials but also how it could improve the durability of 3D printed concrete formulations.” With an extended lifespan and the development of lighter concrete forms, the authors are confident that these efforts could help reduce the environmental impact of cement production, currently accounting for about 8% of global greenhouse gas emissions.
Together with other new formulations, such as a possible concrete that can absorb carbon dioxide from the air, improvements could be introduced to help reduce the global climate impact of this essential construction material. Source: Sinc Agency
Source: MiMub in Spanish
