Concrete structures built by the Ancient Romans over two millennia ago have largely endured to this day. In contrast, modern concrete constructions often deteriorate after just a few decades. Researchers at the Massachusetts Institute of Technology (MIT) seem to have uncovered the secret to the longevity of Roman concrete.
Engineering Marvels of Ancient Rome
Ancient Romans were widely renowned for their engineering achievements, including their extensive network of roads and aqueducts, many of which remain intact today. Numerous famous structures, such as the Pantheon in Rome, constructed around 126 CE, showcase their architectural prowess. The Pantheon boasts the world’s largest unreinforced concrete dome and has stood for two millennia.
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Many other ancient Roman buildings have also withstood the test of time in various climatic conditions, including earthquake-prone areas and direct contact with seawater. In contrast, numerous contemporary concrete structures have collapsed or significantly deteriorated after just a few decades.
This stark contrast has long prompted scientists worldwide to ponder: what makes the concrete used by the Romans different and more enduring than modern concrete?
MIT-Led Research Sheds Light on the Mystery
An international group of researchers, led by the Massachusetts Institute of Technology (MIT), appears to have shed light on this mystery by making significant progress in understanding the components of exceptionally durable concrete and revealing its overlooked “self-healing” ability.
For a long time, researchers believed that the durability of ancient concrete was due to the presence of a specific type of volcanic material sourced from the vicinity of Naples. However, after more detailed analyses by the team of experts, it was discovered that these samples also contain a certain percentage of lime, a well-known component of Roman concrete.
“Since I first started working with ancient Roman concrete, I’ve always been fascinated by its characteristics,” says Professor of Civil and Environmental Engineering at the Massachusetts Institute of Technology, Admir Mašić. “These components are not found in modern concrete mixes, so the question arises: why are they present in these ancient materials?” adds Mašić.
Previously, white lime nodules were disregarded as a result of poor mixing or low-quality raw materials. However, a new study suggests that these fine nodules are what imparted the concrete with its self-healing ability. “The idea that the presence of lime is simply attributed to low quality has always troubled me,” says Mašić. “If the Romans put so much effort into creating an extraordinary building material, following all detailed recipes optimized over many centuries, why would they make so little effort to ensure the production of a well-mixed final product? There must be something more to this story,” adds Mašić.
What is process known as “hot mixing”
As the study progressed, researchers wondered if it was possible that the Romans actually directly used lime in its more reactive form, known as quicklime? When he and his team analyzed samples of this ancient concrete, they discovered that the white particles were composed of various forms of calcium carbonate.
Based on spectroscopic analysis, it was concluded that Roman concrete was likely made by mixing calcium carbonate with volcanic material and water at very high temperatures, a process known as “hot mixing.” The team now concludes that “hot mixing” is the key to the super-strong and super-durable nature of concrete.
According to the study, during this process, lime nodules develop a characteristic fragile structure of nanoparticles, creating a easily breakable and reactive source of calcium that could have a crucial self-healing function. When tiny cracks form in the concrete, they travel to the calcium spheres, which have a larger surface area than other particles.
This material can then react with water and form a calcium-rich solution that can dry and harden, effectively sealing the crack. Alternatively, it can react with volcanic materials to further strengthen the composite. These reactions occur spontaneously and thus automatically heal cracks before they spread.
Researchers at the Massachusetts Institute of Technology are considering how this knowledge could be applied today to create more durable buildings, with one potential application being in the field of 3D-printed concrete structures.
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