https://www.sott.net/article/495845-How-ancient-Roman-
concrete-has-been-able-to-last-thousands-of-years-
finally-revealed
How ancient Roman concrete has been able to last
thousands of years finally revealed
Michelle Starr
Science Alert
Tue, 29 Oct 2024 20:48 UTC
The ancient Romans were masters of building and
engineering, perhaps most famously represented by the
aqueducts. And those still functional marvels rely on a
unique construction material: pozzolanic concrete, a
spectacularly durable concrete that gave Roman
structures their incredible strength.
Even today, one of their structures - the Pantheon,
still intact and nearly 2,000 years old - holds the
record for the world's largest dome of unreinforced
concrete.
The properties of this concrete have generally been
attributed to its ingredients: pozzolana, a mix of
volcanic ash - named after the Italian city of
Pozzuoli, where a significant deposit of it can be
found - and lime. When mixed with water, the two
materials can react to produce strong concrete.
The ancient Romans were masters of building and
engineering, perhaps most famously represented by the
aqueducts. And those still functional marvels rely on a
unique construction material: pozzolanic concrete, a
spectacularly durable concrete that gave Roman
structures their incredible strength.
Even today, one of their structures - the Pantheon,
still intact and nearly 2,000 years old - holds the
record for the world's largest dome of unreinforced
concrete.
The properties of this concrete have generally been
attributed to its ingredients: pozzolana, a mix of
volcanic ash - named after the Italian city of
Pozzuoli, where a significant deposit of it can be
found - and lime. When mixed with water, the two
materials can react to produce strong concrete.
Based on the team's analysis, the lime clasts in their
samples are not consistent with this method. Rather,
Roman concrete was probably made by mixing the
quicklime directly with the pozzolana and water at
extremely high temperatures, by itself or in addition
to slaked lime, a process the team calls "hot mixing"
that results in the lime clasts.
"The benefits of hot mixing are twofold," Masic said.
"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."
And it has another benefit: The lime clasts give the
concrete remarkable self-healing abilities.
After casting, the Roman-inspired hot-mixed concrete
samples were mechanically fractured and then re-mated
(with a gap of 0.5 ± 0.1 mm) and preconditioned for our
crack-healing studies (A). Using an integrated flow
circuit (B), water flow through the sample over the
course of 30 days was documented with a flow meter.
Compared to the lime clast–free control (orange line),
after 30 days, water flow through the lime clast–
containing sample (blue line) ceased (C), and
examination of the cracked surface revealed that it had
been completely filled with a newly precipitated
mineral phase (D and E), which was identified as
calcite from Raman spectroscopy measurements (F).
When cracks form in the concrete, they preferentially
travel to the lime clasts, which have a higher surface
area than other particles in the matrix. When water
gets into the crack, it reacts with the lime to form a
solution rich in calcium that dries and hardens as
calcium carbonate, gluing the crack back together and
preventing it from spreading further.
This has been observed in concrete from another 2,000-
year-old site, the Tomb of Caecilia Metella, where
cracks in the concrete have been filled with calcite.
It could also explain why Roman concrete from seawalls
built 2,000 years ago has survived intact for millennia
despite the ocean's constant battering.
So, the team tested their findings by making pozzolanic
concrete from ancient and modern recipes using
quicklime. They also made a control concrete without
quicklime and performed crack tests. Sure enough, the
cracked quicklime concrete was fully healed within two
weeks, but the control concrete stayed cracked.
The team is now working on commercializing their
concrete as a more environmentally friendly alternative
to current concretes.
"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,"
Masic said.
The research has been published in Science Advances.
https://www.science.org/doi/10.1126/sciadv.add1602?
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