Researchers believe they have discovered why Roman concrete remains so durable in comparison to modern materials.
Rome’s famous Pantheon, which has the world’s largest unreinforced concrete dome and was dedicated in AD 128, is still intact, and some ancient Roman aqueducts still deliver water to Rome today.
Despite this, modern concrete structures can start to crumble just a few decades after construction.
Researchers have spent decades trying to figure out the secret of this ultradurable ancient construction material, particularly in structures that endured especially harsh conditions, such as docks, sewers, and seawalls, or those constructed in seismically active locations.
Researchers from MIT, Harvard University, and laboratories in Italy and Switzerland have been discovering ancient concrete-manufacturing strategies that incorporated several key self-healing functionalities.
It was previously assumed that the durability of ancient concrete was due to pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples.
This specific kind of ash was shipped across the Roman empire to be used in construction, and was described as a key ingredient for concrete in accounts by architects and historians at the time.
Under closer examination, these ancient samples contain small, distinctive, millimetre-scale bright white mineral features, which have been long recognised as a ubiquitous component of Roman concretes.
“Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” said MIT professor Admir Masic. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”
Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognised self-healing capability.
Using high-resolution multiscale imaging and chemical mapping techniques the researchers gained new insights into the potential functionality of these lime clasts.
Studying samples of this ancient concrete, the team determined that the white inclusions were made out of various forms of calcium carbonate that were formed at extreme temperatures.
“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,” Masic said.
As soon as tiny cracks start to form within the concrete, they can preferentially travel through the high-surface-area lime clasts. This material can then react with water, creating a calcium-saturated solution, which can recrystallise as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material.
These reactions take place spontaneously and therefore automatically heal the cracks before they spread. Previous support for this hypothesis was found through the examination of other Roman concrete samples that exhibited calcite-filled cracks.
Similar findings were made by a team of University of Utah researchers in 2017 that found that seawater filtering through Roman concrete may have led to the growth of the self-healing minerals.