Roman Seawater
Concrete Holds the Secret to Cutting Carbon Emissions
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ScienceDaily.com
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| CAPTION: This image shows a drill core of volcanic ash-hydrated lime mortar from the ancient port of Baiae in Pozzuloi Bay. Yellowish inclusions are pumice, dark stony fragments are lava, gray areas consist of other volcanic crystalline materials, and white spots are lime. The inset is a scanning electron microscope image of the special Al-tobermorite crystals that are key to the superior quality of Roman seawater concrete. (Credit: Lawrence Berkeley National Laboratory and University of California at Berkeley) |
The
chemical secrets of a concrete Roman breakwater that has spent the last 2,000
years submerged in the Mediterranean Sea have been uncovered by an
international team of researchers led by Paulo Monteiro of the U.S. Department
of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), a professor
of civil and environmental engineering at the University of California,
Berkeley.
Analysis
of samples provided by team member Marie Jackson pinpointed why the best Roman
concrete was superior to most modern concrete in durability, why its
manufacture was less environmentally damaging -- and how these improvements
could be adopted in the modern world.
"It's
not that modern concrete isn't good -- it's so good we use 19 billion tons of
it a year," says Monteiro. "The problem is that manufacturing
Portland cement accounts for seven percent of the carbon dioxide that industry
puts into the air."
Portland
cement is the source of the "glue" that holds most modern concrete
together. But making it releases carbon from burning fuel, needed to heat a mix
of limestone and clays to 1,450 degrees Celsius (2,642 degrees Fahrenheit) --
and from the heated limestone (calcium carbonate) itself.
Monteiro's team found
that the Romans, by contrast, used much less lime and made it from limestone
baked at 900˚ C (1,652˚ F) or lower, requiring far less fuel than Portland
cement.
Cutting
greenhouse gas emissions is one powerful incentive for finding a better way to
provide the concrete the world needs; another is the need for stronger,
longer-lasting buildings, bridges, and other structures.
"In
the middle 20th century, concrete structures were designed to last 50 years,
and a lot of them are on borrowed time," Monteiro says. "Now we
design buildings to last 100 to 120 years." Yet Roman harbor installations
have survived 2,000 years of chemical attack and wave action underwater.
How the Romans did it
The
Romans made concrete by mixing lime and volcanic rock. For underwater
structures, lime and volcanic ash were mixed to form mortar, and this mortar
and volcanic tuff were packed into wooden forms. The seawater instantly
triggered a hot chemical reaction. The lime was hydrated -- incorporating water
molecules into its structure -- and reacted with the ash to cement the whole
mixture together.
Pozzuoli
Bay defines the northwestern region of the Bay of Naples. The concrete sample
examined at the Advanced Light Source by Berkeley researchers, BAI.06.03, is
from the harbor of Baiae, one of many ancient underwater sites in the region.
Black lines indicate caldera rims, and red areas are volcanic craters. (Click
on image for best resolution.)
Descriptions
of volcanic ash have survived from ancient times. First Vitruvius, an engineer
for the Emperor Augustus, and later Pliny the Elder recorded that the best
maritime concrete was made with ash from volcanic regions of the Gulf of Naples
(Pliny died in the eruption of Mt. Vesuvius that buried Pompeii), especially
from sites near today's seaside town of Pozzuoli. Ash with similar mineral
characteristics, called pozzolan, is found in many parts of the
world.
Using
beamlines 5.3.2.1, 5.3.2.2, 12.2.2 and 12.3.2 at Berkeley Lab's Advanced Light
Source (ALS), along with other experimental facilities at UC Berkeley, the King
Abdullah University of Science and Technology in Saudi Arabia, and the BESSY
synchrotron in Germany, Monteiro and his colleagues investigated maritime
concrete from Pozzuoli Bay. They found that Roman concrete differs from the
modern kind in several essential ways.
One
is the kind of glue that binds the concrete's components together. In concrete
made with Portland cement this is a compound of calcium, silicates, and
hydrates (C-S-H). Roman concrete produces a significantly different compound,
with added aluminum and less silicon. The resulting
calcium-aluminum-silicate-hydrate (C-A-S-H) is an exceptionally stable binder.
At
ALS beamlines 5.3.2.1 and 5.3.2.2, x-ray spectroscopy showed that the specific
way the aluminum substitutes for silicon in the C-A-S-H may be the key to the
cohesion and stability of the seawater concrete.
Another
striking contribution of the Monteiro team concerns the hydration products in
concrete. In theory, C-S-H in concrete made with Portland cement resembles a
combination of naturally occurring layered minerals, called tobermorite and
jennite. Unfortunately these ideal crystalline structures are nowhere to be
found in conventional modern concrete.
Tobermorite
does occur in the mortar of ancient seawater concrete, however. High-pressure
x-ray diffraction experiments at ALS beamline 12.2.2 measured its mechanical
properties and, for the first time, clarified the role of aluminum in its
crystal lattice. Al-tobermorite (Al for aluminum) has a greater stiffness than
poorly crystalline C-A-S-H and provides a model for concrete strength and
durability in the future.
Finally,
microscopic studies at ALS beamline 12.3.2 identified the other minerals in the
Roman samples. Integration of the results from the various beamlines revealed
the minerals' potential applications for high-performance concretes, including
the encapsulation of hazardous wastes.
Lessons for the future
Environmentally
friendly modern concretes already include volcanic ash or fly ash from
coal-burning power plants as partial substitutes for Portland cement, with good
results. These blended cements also produce C-A-S-H, but their long-term
performance could not be determined until the Monteiro team analyzed Roman
concrete.
Their
analyses showed that the Roman recipe needed less than 10 percent lime by
weight, made at two-thirds or less the temperature required by Portland cement.
Lime reacting with aluminum-rich pozzolan ash and seawater formed highly stable
C‑A-S-H and Al-tobermorite, insuring strength and longevity. Both the materials
and the way the Romans used them hold lessons for the future.
"For
us, pozzolan is important for its practical applications," says Monteiro.
"It could replace 40 percent of the world's demand for Portland cement.
And there are sources of pozzolan all over the world. Saudi Arabia doesn't have
any fly ash, but it has mountains of pozzolan."
Stronger,
longer-lasting modern concrete, made with less fuel and less release of carbon
into the atmosphere, may be the legacy of a deeper understanding of how the
Romans made their incomparable concrete.
This
work was supported by King Abdullah University of Science and Technology, the
Loeb Classical Library Foundation at Harvard University, and DOE's Office of
Science, which also supports the Advanced Light Source. Samples of Roman
maritime concrete were provided by Marie Jackson and by the ROMACONS drilling
program, sponsored by CTG Italcementi of Bergamo, Italy.
Story Source:
The
above story is reprinted from materials provided byDOE/Lawrence Berkeley National Laboratory.
Note: Materials may be edited for content and length. For further
information, please contact the source cited above.
Journal References:
1. Marie D. Jackson, Juhyuk Moon, Emanuele Gotti,
Rae Taylor, Abdul-Hamid Emwas, Cagla Meral, Peter Guttmann, Pierre Levitz,
Hans-Rudolf Wenk, and Paulo J. M. Monteiro.Material and elastic properties
of Al-torbermotite in ancient Roman seawater concrete. Journal of
the American Ceramic Society, 2013
2. Marie D. Jackson, Sejung Rosie Chae, Sean R.
Mulcahy, Cagla Meral, Rae Taylor, Penghui Li, Abdul-Hamid Emwas, Juhyuk Moon,
Seyoon Yoon, Gabriele Vola, Hans-Rudolf Wenk, and Paulo J. M. Monteiro. Unlocking
the secrets of Al-tobermorite in Roman seawater concrete. American
Mineralogist, 2013
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DOE/Lawrence Berkeley
National Laboratory (2013, June 4). Roman seawater concrete holds the secret to
cutting carbon emissions.ScienceDaily. Retrieved June 5, 2013, from
http://www.sciencedaily.com/releases/2013/06/130604135409.htm