MIT researchers uncover self-healing secret of ancient Roman concrete's durability

Researchers from the Massachusetts Institute of Technology (MIT) and Harvard University announced a breakthrough in understanding the durability of ancient Roman concrete. El Confidencial reported that the team, led by Professor Admir Masic, uncovered that the Romans employed a technique called "hot mixing," which could explain the remarkable resilience of structures like the Pantheon.

For decades, scientists have sought to unravel the secret behind Roman concrete's longevity. According to Science Alert, modern analyses revealed that the Romans' mixing techniques were more sophisticated than previously thought. Masic and his team studied concrete samples from the archaeological site of Privernum in Italy, dating back nearly 2,000 years, using advanced methods like scanning electron microscopy and X-ray spectroscopy.

One of the key findings was the presence of small, white chunks of lime, known as clasts, within the concrete. These lime clasts were initially considered signs of poor workmanship. However, Masic challenged this notion. "The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me," he said, according to Science Alert. By examining the clasts with special microscopes, the team concluded that they originated from quicklime added directly during the mixing process—a method known as hot mixing.

Science Alert reported that hot mixing involves adding quicklime (calcium oxide) to the mixture instead of slaked lime (calcium hydroxide), generating temperatures of nearly 400 degrees Celsius. This process not only produced high-temperature compounds that wouldn't form otherwise but also reduced curing and setting times, allowing for faster construction. "The advantages of hot mixing are remarkable," Masic explained.

The lime clasts resulting from hot mixing imparted self-healing properties to the concrete. When cracks formed, they preferentially traveled to the lime clasts due to their higher surface area. Upon contact with water, the reactive lime released calcium-rich solutions that hardened into calcium carbonate, naturally sealing the cracks. This phenomenon helped maintain the structural integrity of Roman concrete over centuries, as noted by Scienze Notizie.

The team tested this theory by creating concrete samples using ancient recipes with quicklime and controls without it. Science Alert reported that when they deliberately cracked the samples and exposed them to water, the quicklime concrete healed completely within two weeks, while the control samples remained cracked. "It's exciting to think about how these more durable concrete formulations can not only extend the life of these materials but also improve the durability of 3D-printed concrete formulations," Masic stated, according to Science Alert.

Understanding the self-healing mechanism of Roman concrete opens avenues for developing more sustainable and durable modern materials. El Confidencial highlighted that modern concrete, based on Portland cement developed in the 19th century, is less durable and more polluting than its ancient counterpart. The manufacturing of concrete contributes approximately 8% of global greenhouse gas emissions, as reported by The New York Times [https://www.nytimes.com/2024/10/19/science/concrete-roman-construction.html].

Researchers like Masic are hopeful that incorporating ancient techniques could reduce the environmental impact of modern concrete. "Using ancient techniques could extend the lifespan of modern concrete and reduce its environmental impact," El Confidencial reported. DMAT, a company founded by Masic, aims to integrate the principles of Roman concrete chemistry into modern applications. The New York Times noted that DMAT sells an additive claiming to seal cracks in concrete, potentially reducing reliance on Portland cement.

Not all experts agree on the centrality of hot mixing in the Romans' self-healing concrete. The New York Times reported that Marie Jackson, a geologist at the University of Utah, believes the key lies in the materials mixed with lime, such as pozzolana—a type of volcanic ash. Jackson and her collaborators have tested their hypotheses by creating modern analogues of Roman concrete. "The way Romans chose the materials actually blocked the propagation of fractures. They were the masters," Jackson stated.

Further research by Jackson's team involved building concrete arches, submerging them in seawater, and observing their strength over time. The New York Times reported that the arches could withstand two to three times more force after 50 days underwater. Their findings suggest that the formation of minerals like strätlingite contributes to the concrete’s long-term resilience.

The quest to decode Roman concrete also extends to extraterrestrial applications. IFLScience reported that researchers explored the possibility of using similar techniques to create building materials on Mars. By combining Martian regolith—the dust abundant on the planet's surface—with human serum albumin from blood plasma, they proposed creating a concrete known as AstroCrete. "Although it is a bit strange, blood can be utilized to create strong concrete or bricks for onsite construction on Mars," the team stated, according to IFLScience.

The idea capitalizes on resources available on Mars or within astronauts themselves. Urea, extracted from tears, sweat, or urine, could be used to enhance the concrete's tensile strength. "The production process is simple. Aggregates (Martian regolith) bind together through contact with human serum albumin," the team explained.

While AstroCrete presents a novel solution for Martian habitats, it highlights the broader potential of ancient techniques to solve modern challenges. Researchers continue to study Roman concrete not only to preserve historical structures but also to inspire sustainable practices today. As Masic reflected, understanding these time-tested methods could lead to "greener and more durable modern versions of concrete," according to The New York Times.

This article was written in collaboration with generative AI company Alchemiq