Scientists Figure Out Why Roman Buildings Have Survived For So Long

Now it seems that a large team of researchers from the United States, Switzerland, and Italy has solved the puzzle. Using concrete samples from Privernum, an ancient Roman city and archaeological site, the researchers determined that lime clasts allowed cracked walls to “heal”—and thus extend their own longevity—over time.

The Romans made their concrete in largely the same way we do now, using aggregate (like sand, gravel, or volcanic tuff), water, and a binding agent. Scientists previously assumed the Romans used volcanic ash as their binding agent. Historic texts indicate that ash from the area of Pozzuoli was shipped around the Roman empire and used in various forms of construction. Any white chunks found throughout the concrete were thought to be the product of poor mixing or low-quality raw material.

But large-area scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) has helped researchers identify their mistake. Rather than an indication of poor quality, the white spots within the Romans’ concrete served as an independent healing mechanism for cracks that formed in the buildings’ walls. The Romans used quicklime with or in place of the more traditional slaked lime (lime mixed with water) when they made their binding agents. Because quicklime is more reactive, it produced an exothermic reaction that facilitated what’s known as “hot mixing.”

Original Roman lime clasts (top) vs modern recreation. (Image: Seymour et al/Science Advances DOI 10.1126/sciadv.add1602)

Hot mixing didn’t just reduce the concrete’s curing and setting times. It also produced high-temperature-associated compounds that slaked lime couldn’t have formed. These included a nanoparticulate architecture responsible for a brittle and reactive calcium source. Any small cracks that started to form in the concrete would “travel” through the lime clasts, which would react with water to produce a calcium-saturated solution. When this solution recrystallized, it would fill the cracks.

The researchers sought to test Roman lime clasts’ effect on modern concrete, which actually requires more repair (and emits far more carbon during manufacturing) than its predecessor. In a paper published last week in Science Advances, they describe the process by which they combined Ordinary Portland cement (OPC) with ash, sand, and water. They then added quicklime to some samples while leaving the others as-is. The researchers cracked all of their samples, ran water through the cracks, and then left the samples alone for two weeks.

Just as the Romans intended, the samples containing lime clasts had completely “healed” by the end of the two-week period. Water could no longer flow through where the cracks had once been. Meanwhile, samples made without quicklime did not heal and continued to allow water to flow through their cracks.

The researchers now intend on commercializing quicklime-equipped concrete. “One method to reduce cement’s carbon footprint…is to improve the longevity of concrete through the incorporation of self-healing functionalities,” they write. “The resulting extended use life, combined with a reduction in the need for extensive repair, could thus reduce the environmental impact and improve the economic life cycle of modern cementitious constructs.”

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