A modern concrete structure built in or near seawater is expected to decay and crumble within a few decades, but if you travel to the Mediterranean you'll see concrete structures in water that are standing strong after thousands of years.

Ever wonder why?

The simple answer is because ancient Romans used a different mixture to make their concrete — one designed to get stronger as its chemical structure changed.

The more complex answer, explained Marie Jackson, a research associate professor in the department of geology and geophysics at the University of Utah, is because of a unique and rare mineral called aluminous tobermorite. The Romans were able to create aluminous tobermorite by using volcanic ash and lime (calcium oxide) in their concrete mixture.

When the ancient concrete comes in contact with seawater the aluminous tobermorite crystallizes and creates an even stronger substance, Jackson told CBC News.

"It's a rather remarkable discovery because it means that this is a concrete that continues to react and change over time in a very good way," she said, referring to her findings published earlier this week in American Mineralogist.

concrete microscope

This microscopic image shows the lumpy calcium-aluminum-silicate-hydrate (C-A-S-H) binder material that forms when volcanic ash, lime and seawater mix. (Courtesy Marie Jackson)

The Romans didn't stumble upon this substance haphazardly, she said. They studied how rocks reacted when in contact with water before devising the solution for concrete made up of the ash and lime.  

"It's an excellent foundation for the durability of the concrete," Jackson said.  

Modern concrete isn't designed with the same durability in mind.

"It's an inexpensive material that can be mixed and installed very quickly and [has] a great deal of versatility and high compressive strength," Jackson said.

However, modern concrete is not designed to interact with large quantities of water because there are negative effects, she said. 

To learn more about how ancient Romans made this concrete, Jackson's team has worked with Canadian researcher John Oleson of the University of Victoria. Oleson is a classics scholar who has done extensive research on ancient engineering and co-authored a book with Jackson called Building for Eternity: The History and Technology of Roman Concrete Engineering in the Sea.

The book details Oleson's project to recreate Roman concrete and build an underwater of a reproduction of a Roman pier as part of the Roman Maritime Concrete Study (ROMACONS).

Jackson said Oleson has the ratios for the ancient concrete mixture that can create aluminous tobermorite, which isn't commonly found in nature.

"We're trying to recreate this very special mineral that has some very important societal benefits," she said.

Because aluminous tobermorite creates stronger structures, Jackson sees the potential to use it to build waste repositories for hazardous materials.

"The Romans are teaching us how to reproduce very important minerals in ways that we would have never considered before," she said. "Having the mineral in the concrete with longevity would be very valuable."

Jackson's latest discovery builds off previous research published in 2013