A surprising manufacturing method from the past could be the key to creating concrete that lasts for thousands of years.
Researchers have finally discovered the reason for the Pantheon’s longevity; the 1,898-year-old Roman structure has remained standing while many modern concrete structures reinforced with steel have crumbled within decades.
The Romans were skilled engineers who built extensive networks of roads, aqueducts, ports, and large structures that have stood for thousand of years and can be seen today.
The ancient Romans were masters of using concrete in their engineering projects. The Pantheon, a famous Roman building with the world’s largest unreinforced concrete dome, was completed in 128 AD and is still standing strong. In fact, some ancient Roman aqueducts continue to function, delivering water to Rome to this day.
In contrast, many modern concrete structures have collapsed or severely degraded within a short time frame.
For decades, scientists have been trying to understand the secret behind the durability of ancient concrete, particularly in structures that faced harsh conditions such as docks, sewers, and seawalls, or those built in earthquake-prone areas.
Now, researchers from MIT, Harvard, and Italian and Swiss labs have made strides in this direction by uncovering ancient concrete-manufacturing processes that featured essential self-healing features. Admir Masic, a professor of civil and environmental engineering at MIT, former PhD student Linda Seymour, and four other authors report the results in a paper published today in the journal Science Advances.
For a long time, scientists believed that pozzolanic material, such as volcanic ash from the Pozzuoli region on the Bay of Naples, was the basis for the longevity of the old concrete. This particular kind of ash was even brought all the way across the huge Roman empire to be utilized in building, and was identified as a major element for concrete in reports written at the time by architects and historians.
Upon further examination, the ancient samples were found to contain small, distinctive, millimeter-scale bright white mineral features, which are a well-known characteristic of Roman concretes. These white fragments, often known as “lime clasts,” come from lime, another essential ingredient in the old concrete mix.
“Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” adds Masic. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”
Before, people thought that these tiny lime clasts were just signs of poor mixing or low-quality raw materials. However, new findings indicate that these tiny lime clasts gave the concrete a previously unknown self-healing capability.
“The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” explains Masic. “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”
It was formerly believed that lime was added to Roman concrete after being mixed with water to create a highly reactive paste-like substance, a procedure known as slaking. But the existence of the lime clasts could not be explained by this mechanism alone.
Masic questioned whether it was “possible that the Romans might have actually directly used lime in its more reactive form, known as quicklime?”
When he and his team looked at samples of this old concrete, they saw that the white parts were, in fact, made of different kinds of calcium carbonate. And spectroscopic analysis showed that these were made at very high temperatures, which is what you would expect if you used quicklime instead of or in addition to slaked lime in the mixture. The team has now come to the conclusion that the key to the super-durable nature was hot mixing.
Masic says that hot mixing has two advantages.
First, when the whole batch of concrete is heated to high temperatures, it enables chemistries that would not be feasible if just slaked lime was employed, resulting in the formation of high-temperature-associated compounds that would not otherwise occur.
Second, since all the reactions are expedited by the higher temperature, curing and setting periods are greatly shortened, enabling much quicker construction.
During the mixing process, the lime forms a brittle nanoparticulate structure that is prone to cracking. This creates a highly reactive calcium source that can provide a self-healing functionality when cracks form in the concrete. As soon as cracks begin to appear, they can easily travel through the high-surface-area lime. This material then reacts with water to create a calcium-saturated solution, which 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 repair the cracks before they can spread. Previous research on other Roman concrete samples also found evidence to support this hypothesis, as they contained calcite-filled cracks.
To confirm that this was the reason for the durability of Roman concrete, the team created samples of hot-mixed concrete with both ancient and modern formulations, intentionally cracked them, and then ran water through the cracks. As expected, within two weeks the cracks had completely sealed and the water could no longer pass through. A sample of concrete made without quicklime did not heal and the water continued to flow through it. Based on the success of these tests, the team is now working to bring this modified cement material to market.
“It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete formulations,” adds Masic.
He thinks that these initiatives will effectively reduce the environmental effect of cement production, which presently accounts for around 8% of global greenhouse gas emissions, by extending functional lifetime and creating lighter-weight concrete forms. Along with other new formulas, like concrete that can actually absorb carbon dioxide from the air, which is another area of research in the Masic lab right now, these changes could help to lessen the effect of concrete on the global climate.
Source: 10.1126/sciadv.add1602
Image Credit: Getty