A testament to the inventiveness of Roman engineers, who perfected the use of concrete, the majestic structures of ancient Rome have survived for millennia.
But how did their building materials help keep huge structures like the Pantheon and the Colosseum standing for more than 2,000 years? The Pantheon has the largest unreinforced dome in the world.
Roman concrete has often outlasted its modern counterpart, which can deteriorate in a matter of decades, in many instances. The mysterious ingredient that allowed the Romans to construct elaborate structures in difficult locations like docks, sewers, and earthquake zones has now been identified, according to the researchers of a new study.
The study team, which included scientists from Switzerland, the United States, and Italy, looked at 2,000-year-old concrete samples taken from a city wall at the archaeological site of Privernum in central Italy. These samples are similar in composition to other Roman-era concrete.
They discovered that the concrete could heal cracks that had formed over time thanks to white chunks of concrete called lime clasts. The white chunks had previously been overlooked as evidence of poor raw material quality or careless mixing.
According to the study’s lead author, Admir Masic, an associate professor of civil and environmental engineering at the Massachusetts Institute of Technology, “for me, it was really difficult to believe that ancient Roman (engineers) would not do a good job because they really made careful effort when choosing and processing materials.”
Masic added, “Across the Roman Empire, scholars wrote down precise recipes and imposed them on construction sites.”
The new discovery has the potential to shake up society in the same way that the Romans did in order to make concrete manufacturing more environmentally friendly.
Masic stated, “Concrete enabled the Romans to have an architectural revolution.” The Romans were able to build extraordinary and beautiful cities and transform them into living spaces. Additionally, the human condition was fundamentally altered by that revolution.”
Concrete’s durability and clasts made of lime
Concrete is basically made of artificial stone or rock. It is made by mixing fine aggregate (sand or finely crushed rock) and coarse aggregate (gravel or crushed rock) with water, a binding agent usually made from limestone, and cement.
Roman texts had proposed the utilization of slaked lime (when lime is first joined with water prior to being blended) in the limiting specialist, and that is the reason researchers had accepted that this was the means by which Roman cement was made, Masic said.
The researchers came to the conclusion that the use of quicklime (calcium oxide), the most reactive and hazardous dry form of limestone, rather than or in addition to slaked lime, when mixing the concrete led to the formation of lime clasts.
The lime clasts formed at the extreme temperatures anticipated by the use of quicklime, as revealed by additional analysis, and “hot mixing” was essential to the concrete’s durability.
Masic stated in a press release, “The benefits of hot mixing are twofold.” First, high-temperature-associated compounds that would not otherwise form are produced when the concrete as a whole is heated to high temperatures, allowing chemistry that would not be possible with just slaked lime. Second, because all reactions are accelerated at this higher temperature, curing and setting times are significantly reduced, allowing for much faster construction.”
The team conducted an experiment to determine whether the lime clasts were responsible for Roman concrete’s apparent capacity for self-repair. India’s Meghalaya “living root bridges” gain strength as the trees grow.
They deliberately cracked two concrete samples, one made according to Roman formulas and the other according to modern standards. Water could not pass through the Roman-made chunk of concrete after two weeks, but it did pass right through the concrete made without quicklime.
According to their findings, lime clasts can dissolve into cracks and re-crystallize in water, repairing weathered cracks before they spread. According to the researchers, the capability of self-healing may open the door to the production of modern concrete that is more durable and, as a result, more sustainable. According to the study, such a move would lessen concrete’s carbon footprint, which accounts for up to 8% of global emissions of greenhouse gases.
Researchers had held the belief for many years that the strength of Roman concrete was due to volcanic ash from the Pozzuoli area on the Bay of Naples. This kind of ash was transported across the vast Roman empire to be used in construction. At the time, architects and historians described it as a crucial component of concrete.
Masic stated that lime was previously overlooked, despite the importance of both components.
Science Advances was the journal that published the research.