Mystery Solved: The Secret Strength of Roman Concrete


| LAST UPDATE 07/16/2023

By Stanley Wickens
Ancient Roman architecture colosseum
John Greim / Contributor via Getty Images

The ancient Romans, renowned for their architectural prowess, have left us awestruck with their long-lasting architectural marvels. Many of these structures, like the still-functional aqueducts and the Pantheon, owe their longevity to a unique construction material: pozzolanic concrete. This exceptionally resilient material has allowed structures like the Pantheon, intact even after nearly 2,000 years, to retain its status as the world's largest dome of unreinforced concrete.

Traditionally, the longevity of Roman concrete has been attributed to its ingredients: pozzolana, a mixture of volcanic ash named after the Italian city of Pozzuoli, and lime. When combined with water, these ingredients react to produce robust concrete. However, recent research led by the Massachusetts Institute of Technology (MIT) suggests that there is more to this story than the mere ingredients. The researchers found intriguing evidence in the form of small, white chunks of lime present in what appeared to be well-mixed concrete. These chunks, earlier thought to be the result of inadequate blending or poor quality materials, puzzled MIT materials scientist Admir Masic. He couldn't reconcile the idea that a civilization known for its meticulous construction methods would leave such a glaring error in their concrete.

Pantheon Roman architecture concrete
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Masic and his team, headed by MIT civil engineer Linda Seymour, thoroughly examined 2,000-year-old samples of Roman concrete from the archaeological site of Privernum in Italy. They subjected these samples to advanced testing methods like large-area scanning electron microscopy and energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging to gain a clearer understanding of the lime clasts. The team's investigation revealed a surprising fact: the lime clasts in their samples didn't conform to the standard understanding of pozzolanic concrete preparation. Instead, it suggested that the Romans likely mixed quicklime directly with pozzolana and water at extremely high temperatures, a process the team dubbed "hot mixing."

This "hot mixing" technique not only hastened the production of concrete but also resulted in the formation of high-temperature-associated compounds, thereby enhancing the strength of the concrete. Additionally, the lime clasts imbued the concrete with an extraordinary self-healing ability. When cracks formed, they gravitated towards the lime clasts due to their higher surface area. Water seeping into these cracks reacted with the lime to create a calcium-rich solution. Upon drying, this solution hardened into calcium carbonate, effectively sealing the crack and preventing further damage—a phenomenon observed in other ancient Roman structures such as the Tomb of Caecilia Metella and seawalls that have withstood the ocean's relentless assault for millennia. The team validated their findings by creating pozzolanic concrete using both ancient and modern recipes with quicklime. The cracked quicklime concrete healed completely within two weeks, whereas the control concrete remained cracked. The team is now focused on commercializing their concrete as an environmentally friendly alternative to current concretes.

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