Using the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory and core samples from an ancient breakwater near Naples, Italy, a University of California, Berkeley, research team has examined the fine-scale structure and extraordinarily stable binding compound, calcium-aluminum-silicate hydrate of Roman seawater concrete.
The Berkeley Lab equipment has also led to the first experimental determination of a rare hydrothermal mineral, aluminous tobermorite. “Roman concrete has remained coherent and well-consolidated for 2,000 years in aggressive maritime environments," says UC Berkeley Civil and Environmental Engineering Research Engineer Marie Jackson, who, as lead author, describes team findings in June and October 2013 articles for the Journal of the American Ceramic Society and American Mineralogist, respectively. These are the first publications that describe the use of synchroton radiation applications to investigate ancient Roman concrete. "It is one of the most durable construction materials on the planet, and that was no accident.
“Shipping was the lifeline of political, economic and military stability for the Roman Empire, so constructing harbors that would last was critical.” As the Empire and shipping declined, so did the need for seawater concrete. “You could also argue that the original structures were built so well that, once they were in place, they didn't need to be replaced," concludes Jackson, who, along with the CTG Italcementi Group-sponsored Roman Maritime Concrete Study (ROMACONS), sourced core specimens.
Roman concrete’s lime and volcanic ash binder formulation was described around 30 B.C. by Marcus Vitruvius Pollio, an engineer for Octavian, who became Emperor Augustus. Early practitioners packed their lime-ash mortar and rock chunks into wooden molds immersed in seawater, which became an integral part of the mix—with the resulting structures immune to chloride ion exposure.
The UC Berkeley research aims to identify the potential for expanded use of lime and volcanic ash in concrete, potentially offsetting some of the carbon dioxide emissions associated with the high-temperature milling of ASTM C150 portland cement. Lower processing temperatures equate to lime production at substantially less than the CO2 levels of portland cement; volcanic ash is being considered as an alternative to fly ash (see ASTM C168) as market supply conditions allow or dictate.
"The computed bulk modulus of Al-tobermorite based on high-pressure experiments at beamline 12.2.2. of the Advanced Light Source is 55±5 GPa," Jackson explains. "This measured bulk modulus is far higher than experimental measurements of C-A-S-H in alkaline-activated slag concrete, 35±3 GPa, by the same UC Berkeley research group. Until now, researchers have relied on theoretical models to estimate the mechanical properties of tobermorite, so this adds an important constraint on “real” behavior.
"The wide interlayer, 11.49 Å, of the ancient Roman Al-tobermorite double-layer silicate structure likely provides cavities for Na+ and K+ cations derived from the alkali-rich volcanic ash and seawater-saturated lime. This contributes to charge balancing and stability in the maritime environment, which is important for long-term durability. However, the large interlayer spacing also increases compressibility relative to ideal tobermorite with 11.3 Å spacing.
"Even so, this study shows that Al-tobermorite has increased mechanical performance relative to poorly-crystalline C-A-S-H. Romans were able to produce massive seawater concrete structures with Al-tobermorite as the principal crystalline cementitious product. If we could translate Roman expertise to modern concrete structures, then we could conceivably improve the mechanical and material properties of pozzolanic concretes."
The research began with funding from King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, which has an abundance of potentially concrete-grade volcanic ash. In addition to the Berkeley Lab’s ALS, researchers deployed Berlin Electron Storage Ring Society for Synchrotron Radiation in their analyses.
Jackson adds that the release of the Roman concrete investigation findings has spurred other proposals for using ancient Roman principles in innovative cement or concrete research. "We have received numerous messages from folks actively involved in developing products, especially block concrete, in aluminous pozzolanic systems, some of which are autoclaved to produce Al-tobermorite," she says. "We are in the first stages of discussing our research results regarding C-A-S-H and Al-tobermorite with some of these people, to explore how we can apply Roman principles of concrete construction with volcanic pozzolans to specialty concrete products." Other possible avenues opened up by these findings include the study of natural pozzolan from different parts of the world and the utilization of waste products to produce green concrete.