Jan Bartek – AncientPages.com – One of the mysteries that scientists have long tried to solve is why Roman concrete often lasted thousands of years, but ours decomposes in mere decades. Scientists have now discovered that an unexpected ancient manufacturing strategy may hold the key to designing concrete that lasts for millennia.
The ancient Romans were masters of engineering, building vast networks of roads, aqueducts, harbors and massive buildings whose remains have survived two millennia. Many of these structures were built of concrete: Rome’s famous Pantheon, which has the largest unreinforced concrete dome in the world and was dedicated in AD 128, is still intact, and some ancient Roman aqueducts still supply water to Rome today. Meanwhile, many modern concrete structures have disintegrated after a few decades.
Caption: Large-scale elemental map (calcium: red, silicon: blue, aluminum: green) of a 2 cm fragment of ancient Roman concrete (right) collected from the archaeological site of Privernum in Italy (left). At the bottom of the image, the calcium-rich limestone spike (in red) that is responsible for the unique self-healing properties of this ancient material is clearly visible. Credit: MIT
Researchers have spent decades trying to figure out the secret of this ultra-durable ancient building material, especially in structures that have withstood particularly harsh conditions, such as docks, sewers and seawalls, or those built in seismically active locations.
Now, a team of researchers from MIT, Harvard University, and labs in Italy and Switzerland have made progress in this area, discovering ancient concrete-making strategies that included several key self-healing features. The findings are published in the journal Science Advances, in an article by MIT professor of civil and environmental engineering Admir Masic, former doctoral student Linda Seymour ’14, PhD ’21, and four others.
For many years, scientists assumed that the key to the durability of ancient concrete was based on a single ingredient: a pozzolanic material, such as volcanic ash from the Pozzuoli region of the Bay of Naples. This specific type of ash was even transported throughout the vast Roman Empire to be used in construction, and was described as a key component of concrete in the accounts of architects and historians of the time.
On closer examination, these ancient samples also contain small, distinctive, millimeter-scale bright white mineral elements that have long been recognized as a ubiquitous component of Roman concretes. These white pieces, often referred to as “lime spikes”, come from lime, another key ingredient in ancient concrete mixes. “Ever since I first started working with ancient Roman concrete, I have always been fascinated by these properties,” says Masic. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”
Previously thought to be just evidence of sloppy mixing practices or poor quality raw materials, a new study suggests that these tiny limestone clasts have given concrete a previously unknown ability to self-repair. “The idea that the presence of these lime clasts was simply attributed to poor quality control has always troubled me,” says Masic. “If the Romans put so much effort into producing an excellent building material according to all the detailed recipes that had been optimized over many centuries, why would they put so little effort into ensuring that a well-mixed final product was produced? ? There has to be more to this story.”
After further characterizing these limestone clasts using high-resolution multiscale imaging and chemical mapping techniques pioneered in Masic’s research lab, the researchers gained new insights into the potential functionality of these limestone clasts.
Historically, it was thought that when lime was incorporated into Roman concrete, it was first mixed with water to form a highly reactive paste-like material, in a process known as slaking. But this process alone could not explain the presence of calcareous clasts. Masic wondered, “Could it be possible that the Romans actually directly used lime in its more reactive form, known as quicklime?
By studying samples of this ancient concrete, he and his team discovered that the white inclusions were actually made of different forms of calcium carbonate. And spectroscopic examination gave clues that these were formed at extreme temperatures, as would be expected from an exothermic reaction induced by the use of quicklime instead of or in addition to slaked lime in the mixture. Hot mixing, the team now concluded, was actually the key to the ultra-resistant nature.
“The benefits of hot mixing are twofold,” says Masic. “First, when all the concrete is heated to high temperatures, it allows for chemical processes that are not possible if you only use slaked lime, creating compounds associated with high temperature that would not otherwise be formed. Second, this elevated temperature significantly reduces curing and setting times because all reactions are accelerated, allowing for much faster construction.”
The multiple arches of the Pont du Gard in Roman Gaul (present-day southern France). Source
During the hot-stirring process, the limestone clasts develop a characteristically brittle nanoparticle architecture, creating an easily breakable and reactive source of calcium that the team proposed could provide a critical self-healing function. Once small cracks start to form in the concrete, they can preferentially pass through the high-surface lime clasts. This material can then react with water to create a calcium-saturated solution that can recrystallize as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material. These reactions occur spontaneously and therefore automatically heal cracks before they spread. Previous support for this hypothesis was found through examination of other Roman concrete samples that showed cracks filled with calcite.
To prove that this was indeed the mechanism responsible for the durability of Roman concrete, the team made samples of hot-mixed concrete that contained both ancient and modern formulations, deliberately cracked them, and then let water through the cracks. Sure enough: Within two weeks the cracks had completely healed and the water could no longer flow. An identical piece of concrete made without quicklime never cured and water continued to flow through the sample. As a result of these successful tests, the team is working to commercialize this modified cementitious material.
“It’s exciting to think about how these more durable concrete mixes could not only extend the life of these materials, but also how they could improve the durability of 3D printed concrete mixes,” says Masic.
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Through extended service life and the development of lighter concrete forms, he hopes these efforts could help reduce the environmental impact of cement production, which currently accounts for about 8 percent of global greenhouse gas emissions. Along with other new formulations, such as concrete that can actually absorb carbon dioxide from the air, another current research focus of Masic’s lab, these improvements could help reduce concrete’s impact on the global climate.
The study was published in the journal Science
Written Jan Bartek – AncientPages.com Staff writer