Why are ancient monuments so durable and strong?
We have all marveled at the structural brilliance of ancient Roman monuments, from the colosseum to the Pantheon to the Marcus Aurelius Column, and wondered how they have survived the test of time and are still standing strong. The Pantheon, which has the world’s largest unreinforced concrete dome, was made in 128 AD and is still intact amid modern concrete structures developing cracks within a few years.
Researchers have now found several key self-healing functionalities that were incorporated into manufacturing strategies to build these Roman structures. Researchers had already identified that the durability of the concrete was due to pozzolanic material, or volcanic ash that was brought from Pozzuoli, in the Bay of Naples. However, there is more to it.
A team led by researchers from the Massachusetts Institute of Technology (MIT) has identified small, distinctive, millimeter-scale bright white mineral features in the samples. These lime clasts originate from lime, another key component of the ancient concrete mix. These lime clasts gave the concrete self-healing capacity.
“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,” MIT professor of civil and environmental engineering Admir Masic, author of the paper said.
The study published in Science Advances states that the development and testing of modern lime clast–containing cementitious mixtures demonstrate their self-healing potential. The study could pave the way for the development of more durable, resilient, and sustainable concrete formulations.
It was earlier believed that when lime was incorporated into Roman concrete, it was first combined with water to form a highly reactive paste-like material, in a process known as slaking. The team has now determined that white inclusions were made out of various forms of calcium carbonate and had been formed at extreme temperatures, as would be expected from the exothermic reaction produced by using quicklime.
The team produced samples of hot-mixed concrete that incorporated both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks. They found that within two weeks the cracks had completely healed and the water could no longer flow.
“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,” Masic added.
The lime clasts develop a characteristically brittle nanoparticulate architecture, creating an easily fractured and reactive calcium source, which, as the team proposed, could provide critical self-healing functionality.