An unexpected ancient manufacturing strategy may hold the key to designing concrete that lasts for millennia.
From: Massachusetts Institute of Technology
January 6, 2023 -- 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 are published in the journal Science
Advances, in a paper by MIT professor of civil and environmental
engineering Admir Masic, former doctoral student Linda Seymour, 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?"
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 unrecognized self-healing capability. "The idea that the
presence of these lime clasts was simply attributed to low quality control
always bothered me," says Masic. "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."
Upon further
characterization of these lime clasts, using high-resolution multiscale imaging
and chemical mapping techniques pioneered in Masic's research lab, the
researchers gained new insights into the potential functionality of these lime
clasts.
Historically, it had
been assumed 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. But this process alone could not account for the presence of
the lime clasts. Masic wondered: "Was it possible that the Romans might
have actually directly used lime in its more reactive form, known as
quicklime?"
Studying samples of
this ancient concrete, he and his team determined that the white inclusions
were, indeed, made out of various forms of calcium carbonate. And spectroscopic
examination provided clues that these had been formed at extreme temperatures,
as would be expected from the exothermic reaction produced by using quicklime
instead of, or in addition to, the slaked lime in the mixture. Hot mixing, the
team has now concluded, was actually the key to the super-durable nature.
"The benefits of
hot mixing are twofold," Masic says. "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."
During the hot mixing
process, the lime clasts develop a characteristically brittle nanoparticulate
architecture, creating an easily fractured and reactive calcium source, which,
as the team proposed, could provide a critical self-healing functionality. 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 recrystallize 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.
To prove that this was
indeed the mechanism responsible for the durability of the Roman concrete, 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. Sure enough: Within two weeks the cracks had completely healed and the
water could no longer flow. An identical chunk of concrete made without
quicklime never healed, and the water just kept flowing through the sample. As
a result of these successful tests, the team is working to commercialize this
modified cement material.
"It's exciting to
think about how these more durable concrete formulations could expand not only
the service life of these materials, but also how it could improve the
durability of 3D-printed concrete formulations," says Masic.
Through the extended
functional lifespan and the development of lighter-weight concrete forms, he
hopes that these efforts could help reduce the environmental impact of cement
production, which currently accounts for about 8 percent of global greenhouse
gas emissions. Along with other new formulations, such as concrete that can
actually absorb carbon dioxide from the air, another current research focus of
the Masic lab, these improvements could help to reduce concrete's global
climate impact.
The research team
included Janille Maragh at MIT, Paolo Sabatini at DMAT in Italy, Michel Di
Tommaso at the Instituto Meccanica dei Materiali, in Switzerland, and James
Weaver at the Wyss Institute for Biologically Inspired Engineering at Harvard
University. The work was carried out with the assistance of the archeological
museum of Priverno, Italy.
https://www.sciencedaily.com/releases/2023/01/230106144441.htm
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