[Rockhounds] Riddle solved: Why was Roman concrete so durable?

[Kreigh Tomaszewski]

The ancient Romans were masters of engineering, constructing vast networks
of roads, aqueducts, ports, and massive buildings, whose remains have
survived for two millennia. Many of these structures were built with
concrete: Rome’s famed Pantheon, which has the world’s largest unreinforced
concrete dome and was dedicated in A.D. 128, is still intact, and some
ancient Roman aqueducts still deliver water to Rome today. Meanwhile, many
modern concrete structures have crumbled after a few decades.

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.

Now, a team of investigators from MIT, Harvard University, and laboratories
in Italy and Switzerland, has made progress in this field, discovering
ancient concrete-manufacturing strategies that incorporated several key
self-healing functionalities. The findings
<https://www.science.org/doi/10.1126/sciadv.add1602> are published today in
the journal *Science Advances*, in a paper by MIT professor of civil and
environmental engineering Admir Masic, former doctoral student Linda
Seymour ’14, PhD ’21, and four others.

For many years, researchers have assumed that the key to the ancient
concrete’s durability was based on one ingredient: pozzolanic material such
as volcanic ash from the area of Pozzuoli, on the Bay of Naples. This
specific kind of ash was even shipped all across the vast 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 also contain small,
distinctive, millimeter-scale bright white mineral features, which have
been long recognized as a ubiquitous component of Roman concretes. These
white chunks, often referred to as “lime clasts,” originate from lime,
another key component of the ancient concrete mix. “Ever since I first
began working with ancient Roman concrete, I’ve always been fascinated by
these features,” says Masic. “These are not found in modern concrete
formulations, so why are they present in these ancient materials?”

https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106