Most people think of sea water as corrosive and erosive. But centuries of exposure to seawater has made ancient Roman concrete stronger. Now, researchers know how.

In a new study, researchers detailed the chemical effects of seawater on the minerals and microscale structures inside ancient Roman concrete.

The analysis of geologists at the University of Utah showed seawater encourages the growth of interlocking minerals that bolster cement's cohesive bonds. The phenomenon explains why 2,000-year-old Roman piers and breakwaters are not only still standing, but stronger today than they were 1,000 years ago.

The Romans made concrete by mixing a mortar of volcanic ash, lime and seawater. They added chunks of volcanic rock to the mortar to strengthen and complete their cement mix. The aggregate cement was used in a variety of buildings, as well as marine infrastructure, including sea walls that protected harbors and the boats they sheltered from rough seas.

Portland cement, the variety used most frequently today, isn't all that different from Roman concrete. Both are a combination of mortar and aggregate. But, unlike Roman cement, Portland cement specifically features sand and gravel that won't be reactive with the mortar. Portland cement features inert aggregate -- when aggregate reacts with mortar, disruptive gels can form.

"This alkali-silica reaction occurs throughout the world and it's one of the main causes of destruction of Portland cement concrete structures," University of Utah geologist Marie Jackson said in a news release.

But not all reactive byproducts are destructive. Jackson's research suggests Roman concrete features a variety of reactive mineral compounds that strengthen the microstructures inside cement.

In the latest study, Jackson and her colleagues, including scientists at the Lawrence Berkeley National Laboratory, used advanced imaging technologies called microdiffraction and microfluorescence to observe the effects of seawater on important interlocking minerals in Roman cement.

Their observations suggest volcanic ash is dissolved by seawater filtering through the cement, creating space for new minerals to grow. Scientists determined the minerals are formed from highly alkaline leached fluids. The most common interlocking minerals are Al-tobermorite and phillipsite, both which form platy shapes that reinforce the cement's structural matrices.

The research, detailed in the journal American Mineralogist, proves Roman cement does corrode, but to its benefit -- addition by subtraction.

"We're looking at a system that's contrary to everything one would not want in cement-based concrete," Jackson said. "We're looking at a system that thrives in open chemical exchange with seawater."

Jackson and her colleagues are currently working to create a modern version of the recipe for ancient Roman cement. Volcanic ash and rocks used by the Romans naturally formed cement called tuff, but these rocks aren't readily available in most parts of world.

"Romans were fortunate in the type of rock they had to work with," she says. "They observed that volcanic ash grew cements to produce the tuff. We don't have those rocks in a lot of the world, so there would have to be substitutions made."

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