Anyone visiting Italy can’t help but marvel at structures built 2,000 years ago that remain standing and, in some cases like the aqueducts in Rome, are still functioning.
Researchers affiliated with the Advanced Light Source at DOE’s Lawrence Berkeley National Laboratory think they’ve figured out why materials used to build those edifices endure. The researchers believe there are lessons to be learned from those materials that might change the way modern concrete is made.
Analyzing Roman concrete that has been submerged in seawater for millennia, researchers uncovered a mortarlike substance made from lime and volcanic ash that, upon contact with seawater, triggered a warm chemical reaction. The resulting concrete included a highly stable binder consisting of calcium, aluminum, silicate, and hydrates, known by its acronym C-A-S-H.
In the October edition of American Mineralogist, the researchers attribute the longevity and cohesion of Roman seawater concrete to the use of pozzolanic pyroclastic rock, the preparation of the lime, and the Romans’ construction techniques. A key ingredient was crystalline Al-tobermorite, an aluminum-tinged mineral structure that gave the C-A-S-H binder’s ribbons of silica tetrahedra more symmetry than poorly crystalline compounds, and made the Roman concrete super durable.
What also separates Roman seawater concrete from modern concrete made from Portland cement is that the latter requires significantly higher temperatures: 2,642 degrees F to produce Portland cement’s basic ingredients, known as clinker; versus 900 degrees F to produce lime.
This is important because while Portland cement is widely used—25 billion tons are produced globally every year—the manufacturing process accounts for 7 percent of the world’s annual carbon dioxide emissions.
Partially substituting fly ash for Portland cement reduces CO2 levels. But ash comes from coal mining, which has its own environmental issues. The supply of fly ash is only about 800 million tons annually, and the ash isn’t always suited for blended cements or concrete mixtures.
Volcanic pozzolan, on the other hand, is found throughout the world, including the western U.S., says Marie Jackson, a research geoscientist who is on the team that’s examining Roman seawater concrete for clues about how concrete with volcanic rock could be produced in ways less harmful to the environment.
Portland cement isn’t going away, of course. It still has many pluses, not the least of which being that it hydrates quicker than a similar product made with volcanic pozzolan.
But Paulo Monteiro, the research team’s leader, says there’s interest among manufacturers about blending pozzolan with Portland cement to reduce the cement content in the concrete mix. He cautions that the cost of pozzolan varies “greatly” among regions. But Jackson thinks cost should to take into account “the whole state of repair. Conventional Portland cement concretes are not particularly durable materials.”