New Research Shows How Natural Chemistry Strengthened Ancient Roman Marine Concrete

While modern marine concrete structures crumble within years, ancient Roman piers and breakwaters endure to this day, and are stronger now than when they were first constructed. New research led by the University of Utah has found that seawater filtering through the concrete leads to the growth of interlocking minerals that lend the concrete added cohesion. The results are published in the journal American Mineralogist.

Portus Cosanus pier, Orbetello, Italy. Image credit: J.P. Oleson.

Portus Cosanus pier, Orbetello, Italy. Image credit: J.P. Oleson.

Romans made concrete by mixing volcanic ash with lime (the product of baked limestone) and seawater to make a mortar, and then incorporating into that mortar chunks of volcanic rock, the ‘aggregate’ in the concrete.

Around 79 CE, Roman naturalist and natural philosopher Pliny the Elder wrote in his 37-volume encyclopedia Naturalis Historia about the natural capacity of volcanic ash to react with water: ‘as soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass (fierem unum lapidem), impregnable to the waves and every day stronger.’

The combination of ash, water, and quicklime produces what is called a pozzolanic reaction, named after the city of Pozzuoli in the Bay of Naples. The Romans may have gotten the idea for this mixture from naturally cemented volcanic ash deposits called tuff that are common in the area.

The conglomerate-like concrete was used in many architectural structures, including the Pantheon and Trajan’s Markets in Rome.

Massive marine structures protected harbors from the open sea and served as extensive anchorages for ships and warehouses.

Modern Portland cement concrete also uses rock aggregate, but with an important difference: the sand and gravel particles are intended to be inert. Any reaction with the cement paste could form gels that expand and crack the concrete.

To learn more about the makeup of Roman cements, University of Utah Professor Marie Jackson and co-authors analyzed samples from Portus Cosanus, Orbetello; Baianus Sinus, Bay of Pozzuoli; Portus Neronis, Anzio; and Portus Traianus, Ostia.

The team’s earlier work at the Advanced Light Source (ALS), an X-ray research center at the Department of Energy’s Lawrence Berkeley National Laboratory, found that crystals of aluminous tobermorite, a layered mineral, played a key role in strengthening the concrete as they grew in relict lime particles.

The new study is helping scientists to piece together how and where this mineral formed during the long history of the concrete structures.

The work ultimately could lead to a wider adoption of concrete manufacturing techniques with less environmental impact than modern Portland cement manufacturing processes, which require high-temperature kilns.

“At the ALS we map the mineral cement microstructures. We can identify the various minerals and the intriguingly complex sequences of crystallization at the micron scale,” Prof. Jackson said.

“Lime (also known as calcium oxide) — exposed to seawater in the Roman concrete mixture — probably thoroughly reacted with volcanic ash early in the history of the massive harbor structures.”

“Previous studies showed how the aluminous tobermorite crystallized in the lime remnants during a period of elevated temperature.”

“Our new findings suggest that after the lime was consumed via these pozzolanic chemical reactions, a new period of mineral growth began.”

The new growth of aluminous tobermorite is often associated with crystals of phillipsite, another mineral.

The minerals form fine fibers and plates that make the concrete more resilient and less susceptible to fracture over time.

“Contrary to the principles of modern cement-based concrete, the Romans created a rock-like concrete that thrives in open chemical exchange with seawater,” Prof. Jackson said.

To understand the long-term chemical processes that occurred in the Roman structures, the team used thin, polished slices of the concrete with an electron microscope in Germany to map the distribution of elements in the mineral microstructures.

The researchers coupled these analyses with a technique at ALS known as X-ray microdiffraction, and a technique at the University of California, Berkeley, known as Raman spectroscopy, to learn more about the structure of crystals in the samples.

“The X-ray beamline where the Roman concrete samples were studied can produce beams focused to about 1 micron, or 1 thousandth of an inch, which is useful for identifying each mineral species and mapping their distribution,” said co-author Dr. Nobumichi Tamura, an ALS staff scientist.

“The beam is almost a hundred times smaller than what can be found in a conventional laboratory.”

The X-ray technique measures an average signal from many tiny mineral grains, providing high resolution and fast data collection.

“We can go into the tiny natural laboratories in the concrete, map the minerals that are present, the succession of the crystals that occur, and their crystallographic properties. It’s been astounding what we’ve been able to find,” Prof. Jackson said.

“This is a concrete that apparently grows aluminum-tobermorite mineral cements over millennia.”

“The study suggests that this process could be useful for modern seawall structures as well as for encasing high-level wastes in cement-like barriers that protect the surrounding environment.”

Prof. Jackson is working with a geological engineer to rediscover the Romans’ complex recipe for concrete. She is mixing seawater from the San Francisco Bay and volcanic rock from the Western U.S. to find the right formula, and is also leading a scientific drilling project to study the production of tobermorite and other related minerals at the Surtsey volcano in Iceland.

“Already, a growing number of concrete manufacturers are exploring the use of volcanic rock and less energy-intensive processes, which could be a win-win for industry and the environment,” Prof. Jackson said.

“In order for Roman concrete recipes to gain more traction, test structures will be needed to evaluate the long-term properties of marine structures built with volcanic rock and measure how they stack up against the properties of steel-reinforced concrete, for example.”

“I think people don’t really know how to think about a material that doesn’t have steel reinforcement.”

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Marie D. Jackson et al. 2017. Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete. American Mineralogist 102 (7); doi: 10.2138/am-2017-5993CCBY

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