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While modern concrete deteriorates after a few decades of exposure to seawater and ocean climates, many Roman piers and breakwaters have dissolved little in 2,000 years.
University of Utah researcher Marie D. Jackson has discovered the Romans' volcanic secret.
Jackson's team found that the type of concrete used by the Romans a mixture of volcanic ash and lime actually became stronger when immersed in the ocean. The salt water partially dissolves the ash and lime cocktail and new interlocking crystals and minerals form, preventing cracks and making the structures stronger.
Jackson was surprised by the results.
"The ocean percolating through the concrete makes it get better over time," she said. "The Romans actually wrote about it. They were really good natural observers and they noticed that volcanic ash could grow and turn into rock."
Jackson, a research associate professor at the U.'s Department of Geology and Geophysics, co-authored a study about the work published this week in American Mineralogist. Scientists used electron probe microanalysis and spectroscopy to examine samples of Roman concrete and reveal the unexpected crystallization fostered by immersion in seawater.
The Romans' "exact recipe has been lost, but now labs are trying to re-create it" for possible use in construction, Jackson said.
"The Romans were very good at selecting volcanic deposits, and that's something modern concrete manufacturers are doing now, but they need more guideposts to find out what's best for productivity," Jackson said.
The sustainability of using volcanic rock from Nevada in concrete mixes is being studied now, she said. The concrete used in the building of Glen Canyon Dam also contained finely-ground volcanic particles.
"Not the same as the Romans used, but obviously valid and trusted enough for its construction," Jackson said. "I think the study can be very constructive and could be applied to improving sustainability in the service life of cement-based concrete in modern construction."