Enormi possibilità per l’uso moderno di un antico materiale. Il calcestruzzo, il materiale da costruzione inventato e usato per secoli dai romani è più
forte oggi di quando è stato originariamente applicato.
Invece il calcestruzzo moderno esposto all’acqua salina si corrode solo in alcune
decine di anni.
Concrete from ancient Rome for stronger sea walls
Two thousand
years ago, Roman builders constructed vast sea walls and harbor piers. The
concrete they used outlasted the empire — and still holds lessons for modern
engineers, scientists say.
A bunch of half-sunken
structures off the Italian coast might sound less impressive than a
gladiatorial colosseum. But underwater, the marvel is in the material. The
harbor concrete, a mixture of volcanic ash and quicklime, has
withstood the sea for two millennia and counting. What's more, it is stronger
than when it was first mixed.
The Roman stuff
is “an extraordinarily rich material in terms of scientific possibility,”
said Philip Brune, a research scientist at DuPont Pioneer who has studied the
engineering properties of Roman monuments. “It's the most durable building
material in human history, and I say that as an engineer not prone to
hyperbole.”
By contrast,
modern concrete exposed to saltwater corrodes within decades.
The mystery has
been why the ancient material endured. “Archaeologists will say they have the
recipe,” said Marie Jackson, an expert in ancient Roman concrete at
the University of Utah. (Pliny the Elder once wrote an ode to
concrete “that as soon as it comes into contact with the waves of the sea and
is submerged becomes a single stone mass, impregnable to the waves.") But
it's not the complete picture: It's one thing to assemble the ingredients,
another to know how to bake the cake.
Drilling at a marine structure in Portus Cosanus, Tuscany, in 2003. (J.P. Oleson)
To that
end, Jackson and her colleagues peered into the microscopic structures of
concrete samples, extracted from the sea walls and piers as part of a
project called the Roman Maritime Concrete Study. “This rocklike
concrete is behaving, in many ways, like volcanic deposits in submarine
environments,” Jackson said.
Where modern
concrete is designed to ignore the environment, Roman concrete embraces it. As
the scientists report in a study published Monday in the journal American
Mineralogist, Roman concrete is filled with tiny growing crystals. The
crystals, like tiny armor plates, may keep the concrete from fracturing.
The scientists
subjected the concrete samples to a battery of advanced imaging techniques and
spectroscopic tests. The tests revealed a rare chemical reaction, with
aluminous tobermorite crystals growing out of another mineral called
phillipsite. Brune, who was not involved with the study, called the work a
“significant accomplishment.” He likened it to the scientists biting into
a cake of mysterious flavor and determining that the baker used organically
sourced dark chocolate.
In this instance,
the key ingredient proved to be seawater. As seawater percolated within
the tiny cracks in the Roman concrete, Jackson said, it reacted with the
phillipsite naturally found in the volcanic rock and created the tobermorite
crystals.
“Aluminous
tobermorite is very difficult to produce,” she said, and requires very
high temperatures to synthesize small amounts. Cribbing from the ancient Romans
might lead to better production of tobermorite, which is prized for its industrial
applications, she noted.
The Romans mined
a specific type of volcanic ash from a quarry in Italy. Jackson is attempting
to recreate this durable concrete using San Francisco seawater and more
abundant volcanic rocks. She has several samples sitting in ovens and jars in
her lab, which she will test for evidence of similar chemical reactions.
If her effort is
successful, the concrete could yet have a role to play in human history — “if
one was indeed interested in making sea walls” and “forced to protect shoreline
environments,” Jackson said. (In one 2014 study, a team of European
climate scientists predicted that, if the next 90 years follow the trend of the
past 30, the cost of constructing barriers to hold back the sea might rise to
as high as $71 billion per year. The alternative, coastal flooding, could do
trillions of dollars in damage annually.)
Modern sea walls
require steel reinforcements; a future in which “large relic walls of twisted
steel” dot the coast would be “very troubling,” Jackson said. The Romans
didn't use steel. Their reactive concrete was strong enough on its own.
“It's not just a
historical curiosity,” Brune said. “It may yet have a part to play.”
By Ben Guarino, Washington Post
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