This image above 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.
"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.
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.
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.
What does it mean for studio casting? It means that we can extend the exterior lives of our art castings by using a mix of less Porland cement and more fly ash. It is certainly worth experimenting with, as perhaps 2000 years in the future a team of archiologists will uncover one of your cast pieces only to wonder how well it was presevered.