Researchers revealed the secret of ‘self-healing’ Roman concrete

So then, how come ancient Roman structures like the Maison Carrée temple (2,021 years old), the Colosseum (1,951 years old), and the Pantheon (1,898 years old) are still standing in front of our eyes? Unlike most modern-day structures, these buildings have been in the middle of battles, storms, earthquakes, world wars, and many other disastrous events.

The Romans didn’t have any high-tech construction machines or technologies. Then how they created these ancient marvels that survived for so long? Well, a team of international researchers has the answer to this question. 

They claim that Romans somehow figured out the recipe to make the world's strongest and most durable concrete. The exact recipe could now help us construct long-lasting, sustainable buildings.  

A "concrete" ingredient was ignored

The study from MIT is not the only research work that attempts to understand the composition of the concrete used by the Romans. Scientists in the past have discovered that ancient Romans used to add volcanic ash to their construction material. 

This fine ash, also known as pozzolanic, was an important ingredient and was only found in Naples's Pozzuoli region. Romans used to transport the ash from Pozzuoli to the construction sites, and add it to the concrete. 

However, according to the researchers, the pozzolanic material isn’t the only thing contributing to the long-lasting nature of ancient Roman structures. The concrete also featured unusual white chunks that many previous researchers considered as marks that may have been left due to poor mixing of concrete. 

This sounds logical at first because concrete is generally prepared by mixing limestone, water, gravel, and various other materials. Nowadays, we have machines to ensure the proper mixing of these materials, but ancient Romans didn’t have access to such machines. So maybe this is why the concrete samples from that time contain white chunks.   

Turns out this was not the case. One of the study authors, and a professor at MIT, Admir Masic said in a news release, "The idea that the presence of these lime clasts was simply attributed to low-quality control always bothered me. If the Romans put so much effort into making an outstanding construction material, 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."

Lime clasts, the strength of Roman concrete

The current study reveals that those white chunks represent “lime clast,” a special material composed of calcium oxide (quick lime). It is highly reactive and dry, and it was mixed with other materials at probably very high temperatures. 

The researchers suggest that apart from the volcanic ash, it is the lime clast and the hot-mixing process that made Roman concrete so durable.

“The benefits of hot mixing are twofold; 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 reactions are accelerated, allowing for much faster construction,” said Professor Masic.

The authors examined the chemical composition and nanostructure of the ancient Roman concrete samples and discovered that the lime clast gave concrete the ability to self-heal. Whenever a crack is formed, the white chunks react with water, causing the recrystallization of calcium carbonate and, eventually, the closing of the crack.  

The researchers even demonstrated this mechanism using hox-mixed concrete samples prepared using both the Roman method and the modern method. They cracked both samples and poured water on the cracks. After two weeks, cracks in the Roman concrete were gone, but concrete prepared with the modern approach couldn’t heal itself.

“Whether the damage occurs within years of construction or centuries thereafter, so long as the lime clasts remain, these self-healing functionalities can persist,” the authors note in their study.  

Roman concrete is stronger and better

Apart from being durable and self-healing, the concrete containing lime clasts is also more sustainable and eco-friendlier. The authors suggest that if we start using Roman concrete, we can reduce our carbon footprint by up to eight percent. 

Professor Masic and his team aim to bring Roman concrete, like modified self-healing concrete, into the market. They also want to test their version of durable concrete in 3D-printing-based construction activities. 

The study was published in the journal Science Advances.

Study Abstract:

Ancient Roman concretes have survived millennia, but mechanistic insights into their durability remain an enigma. Here, we use a multiscale correlative elemental and chemical mapping approach to investigate relict lime clasts, a ubiquitous and conspicuous mineral component associated with ancient Roman mortars. Together, these analyses provide new insights into mortar preparation methodologies and provide evidence that the Romans employed hot mixing, using quicklime in conjunction with, or instead of, slaked lime, to create an environment where high surface area aggregate-scale lime clasts are retained within the mortar matrix. Inspired by these findings, we propose that these macroscopic inclusions might serve as critical sources of reactive calcium for long-term pore and crack-filling or post-pozzolanic reactivity within the cementitious constructs. The subsequent development and testing of modern lime clast–containing cementitious mixtures demonstrate their self-healing potential, thus paving the way for the development of more durable, resilient, and sustainable concrete formulations.