Roman Seawater Concrete Holds
the Secret to Cutting Carbon Emissions
Berkeley Lab scientists and
their colleagues have discovered the properties that made ancient Roman
concrete sustainable and durable
June 04,
2013
News Release
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.
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. Inset is a scanning electron microscope image of the special
Al-tobermorite crystals that are key to the superior quality of Roman
seawater concrete. (Click on image for best resolution.)
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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.)
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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.
###
Scientific
contacts: Paulo Monteiro,
monteiro@ce.berkeley.edu, 510-643-8251; Marie Jackson,
mdjackson@berkeley.edu, 928-853-7967
“Material and elastic properties of
Al-tobermorite in ancient Roman seawater concrete,” by 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, appears in
the Journal of the American Ceramic Society.
“Unlocking
the secrets of Al-tobermorite in Roman seawater concrete,” by 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, will appear in American
Mineralogist.
The
Advanced Light Source is a third-generation synchrotron light source producing
light in the x-ray region of the spectrum that is a billion times brighter than
the sun. A DOE national user facility, the ALS attracts scientists from around
the world and supports its users in doing outstanding science in a safe
environment. For more information visit www-als.lbl.gov/.
Lawrence
Berkeley National Laboratory addresses the world’s most urgent scientific
challenges by advancing sustainable energy, protecting human health, creating
new materials, and revealing the origin and fate of the universe. Founded in
1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel
prizes. The University of California manages Berkeley Lab for the U.S.
Department of Energy’s Office of Science. For more, visit www.lbl.gov.
DOE’s
Office of Science is the single largest supporter of basic research in the
physical sciences in the United States, and is working to address some of the
most pressing challenges of our time. For more information, please visit the
Office of Science website at science.energy.gov.
