An unexpected and ancient manufacturing strategy may hold the key to designing concrete that lasts for thousands of years.
The ancient Romans were masters of engineering, building a vast network of roads, aqueducts, harbors and colossal buildings whose ruins have survived for 2000 years. Many of these structures were built of concrete. Rome’s famous Pantheon, with the world’s largest non-reinforced concrete dome, was consecrated in 128 AD and remains intact, and some of the ancient Roman aqueducts still supply Rome today. increase. On the other hand, many modern concrete structures collapse after decades.
For decades, researchers have studied the properties of this ultra-durable, ancient structure in structures that have withstood particularly harsh conditions, such as docks, sewers, and breakwaters, and in structures built in seismically active areas. We have been trying to unlock the secrets of building materials.
Research teams at MIT, Harvard University, and Italian and Swiss laboratories are now making progress in this area, discovering an ancient concrete manufacturing strategy that incorporates several important self-healing capabilities.Findings will be published today in the journal scientific progressMIT Professor of Civil and Environmental Engineering Admir Masic, former doctoral student Linda Seymour ’14, PhD ’21, and four others in a paper.
For years, researchers have held that the key to the durability of ancient concrete lies in pozzolanic materials, such as volcanic ash from the Pozzuoli region of the Gulf of Naples. This particular type of ash was shipped throughout the vast Roman Empire for use in construction, and was described as an important component of concrete in accounts by architects and historians of the time.
Under close scrutiny, these ancient samples also contain small, unique millimeter-scale, bright white mineral features that have long been recognized as the ubiquitous component of Roman concrete. These white lumps are often called ‘limestones’ and come from lime, another important component of ancient concrete mixes. “Ever since I first started working with ancient Roman concrete, I have always been fascinated by these features,” he says Masic. “They are not found in modern concrete mixes, so why are they present in these ancient materials?”
Previously dismissed as mere evidence of poor mixing practices or low-quality raw materials, new research suggests that these tiny limestone chunks endowed concrete with a previously unrecognized self-healing ability. “The idea that the presence of these limestone lumps was simply due to poor quality control has always haunted me,” says Masic. If they’ve put so much effort into making superior building materials, following all the detailed recipes that have been optimized over time, why are they doing so little to ensure the production of a well-mixed final product? Didn’t we put in the effort?? There must be more to this story.”
Further characterization of these limestone masses using high-resolution multi-scale imaging and chemical mapping techniques developed in Masic’s lab has provided researchers with new insights into the potential functions of these limestone masses. Got an insight.
Historically, when lime was incorporated into Roman concrete, it was thought to first combine with water to form a highly reactive paste-like substance. However, this process alone could not explain the presence of limestone. Masic asked:
Studying this ancient concrete sample, he and his team found that the white inclusions were actually made of various forms of calcium carbonate. And spectroscopic examination indicates that these were formed at extreme temperatures, as expected from the exothermic reaction produced by using quicklime instead of or in addition to slaked lime in the mixture. We provided a clue. The team now concludes that hot mixing is indeed the key to super durability.
“There are two advantages to hot mixing,” says Masic. “First, when the entire concrete is heated to high temperatures, chemical reactions are possible that would not be possible if only slaked lime was used, producing high-temperature-related compounds that would otherwise not form. The elevated temperature accelerates all reactions, resulting in much faster curing and hardening times, enabling much faster construction.”
In the process of mixing at elevated temperatures, limestone develops a characteristically brittle nano-particle structure that is easily broken down to create a reactive calcium source. This could provide important self-healing capabilities, as suggested by the team. As soon as small cracks begin to form in the concrete, they are able to preferentially pass through the limestone mass, which has a large surface area. It can recrystallize as calcium to quickly fill cracks or react with pozzolanic materials to further strengthen composites. These reactions occur naturally, so cracks are automatically repaired before they spread. Previous support for this hypothesis was found by investigation of other Roman concrete samples that showed cracks filled with calcite.
To prove that this is indeed the mechanism responsible for the durability of Roman-era concrete, the team created samples of hot-mix concrete incorporating both ancient and modern formulations and intentionally cracked. and water was poured into the cracks. Sure enough, within two weeks the crack was completely healed and the water stopped flowing. The same concrete mass made without quicklime never cured and water continued to flow through the samples. increase.
“Thinking about how these more durable concrete mixes not only extend the useful life of these materials, but how we can improve the durability of 3D-printed concrete mixes. is exciting,” says Masic.
Through extending functional life and developing lightweight concrete formwork, we hope that these efforts will help reduce the environmental impact of cement production, which currently accounts for about 8% of global greenhouse gas emissions. I’m here. Along with other new formulations, such as concrete that can actually absorb carbon dioxide from the air, another of the Masic lab’s current research interests, these improvements will reduce concrete’s global climate impact. can be useful for
Original: Riddle solved: Why was Roman concrete so durable?
Than: Massachusetts Institute of Technology | Wyss Institute for Biotechnology
