Researchers have solved a long-standing riddle to explain why Roman concrete is so durable.
There's no denying that the ancient Romans were masters of engineering. They constructed vast networks of roads, ducts, ports, and buildings, whose remains have survived for two millennia. Rome's famous Pantheon, which has the world's largest unreinforced concrete dome, has stood the test of time for nearly 2,000 years and remains intact.
All this is in stark contrast to modern concrete structures that have crumbled after just a few decades. While researchers have spent decades trying to figure out the secrets behind this ultra-durable ancient construction material, a team from Massachusetts Institute of Technology, Harvard University, and labs in Italy and Switzerland has made some progress. Their findings were published in the journal Science Advances in January 2023.
Role of lime clasts
Close examination of these ancient samples revealed that they contain small, distinctive, bright white mineral features, recognised as a ubiquitous component of Roman concretes. Referred to as "lime clasts", these white chunks originate from lime, another key component of the ancient concrete mix.
While these lime clasts have previously been disregarded as evidence of sloppy mixing practices or even poor-quality raw materials, this study suggests that it was these lime clasts that gave the concrete self-healing capabilities that were unrecognised so far.
Quicklime's effect
Researchers wondered if the Romans directly used lime in its more reactive form, quicklime, rather than slaked lime (calcium oxide mixed with water). Studying the samples showed certain inclusions and clues that these would have formed at extreme temperatures. This bode well for their hypothesis as extreme temperatures are expected from exothermic reactions produced by using quicklime, as opposed to, or in addition to, the slaked lime in the mixture. The team concluded that hot-mixing played a pivotal role in the super-durable nature of the concrete.
To prove that this is the case, the team then created samples of hot-mixed concrete that incorporated both modern and ancient formulations. These were then deliberately cracked and water was made to run through the cracks.
The self-healing nature of concrete thus created meant that the cracks completely healed within two weeks due to the reaction with water and water could no longer flow. In the case of an identical slab of concrete made without quicklime, it never healed and the water kept flowing.
The success of these tests have prompted the team to try and commercialise their modified cement material. The extended lifespan of these concrete forms can help reduce the environmental impact of cement production, which currently accounts for nearly 8% of global greenhouse emissions.
Picture Credit : Google