It’s easy to focus on the “ruins” aspect of ancient architecture, and in doing so, overlook the astonishing fact that anything built so long ago remains standing today. While contemporary infrastructure faces numerous threats due to the crumbling of concrete over decades, or even a few years, many of the structures Ancient Romans built some 2,000 years ago remain standing and stable today. This has led many an expert to wonder: What was their secret?
A new study by researchers from the Massachusetts Institute of Technology (MIT), Harvard University, and laboratories in Italy and Switzerland recently published in Science Advances proposes an answer to that question: the technique of “hot mixing.” Lime is a stabilizing element, a calcium-rich inorganic material that is used to mitigate the erosive effect of water against structures like roads and building foundations. The study analyzes the use of quicklime — a heated and more reactive iteration of lime — along with or instead of slaked lime (which is lime that has been cooled) in Ancient Roman mortar construction, and posits that the lime clasts retained by the hot mixing technique allowed for a process of “self-healing” by holding reactive calcium within their matrix.
“By developing these hydraulic mortars, the Romans were able to create a stronger, more durable material that allowed them to build larger, more complex-shaped architectural structures for purposes that were not previously possible, including constructions in the sea,” the researchers wrote.
The team analyzed mortar samples from the ancient city of Privernum, near Rome. Differences in the chemical reactions within hot mixed concrete not only created less brittle, more adaptable mortars, it also reduced curing time for the concrete, which enabled faster a building process. The presence of lime clasts has long been known, but it was frequently attributed to low-quality materials or poor mixing practices.
MIT Professor Admir Masic, the study’s lead author, suspected there was more to the story.
“Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” Masic told MIT News. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”
The results of the study offer more than just a fascinating peek into ancient technology — they also make an explicit point about the lack of durability in Ordinary Portland cement (OPC), a key component of contemporary concrete. The researchers suggest that modern-day building can lower its environmental impact by reducing the need to constantly replace crumbling concrete. According to MIT News, cement production currently accounts for approximately 8% of all global greenhouse gas emissions.
“It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also [improve] the durability of 3D-printed concrete formulations,” said Masic.