Why Roman Concrete Stands The Test Of Time Better Than Today’s Concrete

Thousands of people visit Rome daily to visit the city’s ancient structures that have stood the test of time. The Colosseum, a 2000-year-old amphitheater, the Pantheon temple (which has the world’s largest unreinforced concrete dome), and other sites have visitors asking how these ancient marvels could remain so well-preserved over at least two millennia.

Visitors aren’t the only ones asking this question. Modern scientists and engineers also wondered what type of materials their ancient Roman counterparts used to make these buildings so durable when modern concrete structures last for possibly 150 years.

Researchers at the Massachusetts Institute of Technology (MIT), Harvard University, and laboratories in Italy and Switzerland set out on a quest to answer this question. After extensive research, the investigators now believe that “self-healing concrete” is the answer to the mystery.

Over the years, researchers believed the Romans used volcanic ash from the area of Pozzuoli, on the Bay of Naples, to make the concrete strong. The ash was shipped across the Roman Empire to use when constructing buildings. Since ancient architects and historians described volcanic ash as a key ingredient for making concrete, modern researchers attempted to re-create the Roman recipe, but they were unsuccessful.

The research team in this new study, published in January in the journal Science Advances, analyzed 2000-year-old concrete samples that were taken from a city wall at the archaeological site of Privernum in central Italy.

Rather than focus on volcanic ash, the researchers looked at small white chunks in the concrete called “lime clasts.” These white chunks originated from lime, another key ingredient of the ancient concrete mix. Their analysis indicated that the chunks were rich with calcium, and spectroscopic examination provided clues that these had been formed at extreme temperatures.

“Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” Admir Masic, an author of the study, said in a news release. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”

Previous studies disregarded the white chunks as mere evidence of sloppy mixing practices or poor-quality raw materials. However, the new study suggests the white chunks gave the concrete a “previously unrecognized self-healing capability.”

“The idea that the presence of these lime clasts was simply attributed to low-quality control always bothered me,” said Masic, a professor of civil and environmental engineering at MIT. “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”

To generate ideas on how to mix the materials, the team read texts by Vitruvius, a Roman architect, and engineer during the 1st century BC, and Pliny, a renowned Roman historian. The texts spelled out strict specifications for the raw materials, such as making sure that the limestone (the source of the quicklime) is pure, and to mix quicklime with hot ash, and then adding water to produce a lot of heat. The team concluded that this “hot mixing” process—mixing quicklime with volcanic ash and adding water—was the key that made the concrete in Roman structures strong.

“The benefits of hot mixing are twofold,” Masic explained. “First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times since all the accelerated reactions allow for much faster construction.”

But to make sure they were on the right track with the hot-mixing concept, the team produced samples of hot-mixed concrete that involved both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks. Within two weeks, the cracks had completely healed, and the water could no longer flow in the ancient formulation. An identical chunk of concrete using the modern formulation without quicklime never healed, and the water just kept flowing through the sample.

“The results were stunning,” Masic said.

After solving the centuries-old mystery, the team believes the ancient Roman concrete recipe could shake up today’s manufacturing industry. However, Masic said the concrete industry is resistant to change, primarily due to introducing new chemistry into a “tried-and-true” mixture with well-known mechanical properties.” But a major issue is also the cost of using new materials.

Because of these and other reasons, Masic and some of his colleagues created DMAT, a startup to seek seed money to commercially produce the Roman-inspired hot-mixed concrete that could potentially make structures built in the future last for centuries.

“Concrete allowed the Romans to have an architectural revolution,” Masic told CNN Style. “Romans were able to create and turn the cities into something extraordinary and beautiful to live in. And that revolution basically changed completely the way humans live.”

Source Links:

https://www.sciencenews.org/article/chemists-long-lasting-roman-concrete
https://www.cnn.com/style/article/roman-concrete-mystery-ingredient-scn/index.html
https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106