Materials

Just add seawater: Ancient Roman concrete gets stronger over time

An ancient Roman pier is still standing in a bay in Italy, and researchers have studied samples of the concrete to explore the secrets of its long-lasting strength
An ancient Roman pier is still standing in a bay in Italy, and researchers have studied samples of the concrete to explore the secrets of its long-lasting strength
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This scanning electron microscope image shows how Al-tobermorite crystals form in volcanic ash, using a similar principle to how the Roman concrete gets its added strength
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This scanning electron microscope image shows how Al-tobermorite crystals form in volcanic ash, using a similar principle to how the Roman concrete gets its added strength
A microscope image of a mix of volcanic ash, lime and seawater, which has resulted in the growth of Al-tobermorite crystals
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A microscope image of a mix of volcanic ash, lime and seawater, which has resulted in the growth of Al-tobermorite crystals
The researchers on the project, Nobumichi Tamura (left), Marie Jackson (middle) and Camelia Stan (right)
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The researchers on the project, Nobumichi Tamura (left), Marie Jackson (middle) and Camelia Stan (right)
An ancient Roman pier is still standing in a bay in Italy, and researchers have studied samples of the concrete to explore the secrets of its long-lasting strength
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An ancient Roman pier is still standing in a bay in Italy, and researchers have studied samples of the concrete to explore the secrets of its long-lasting strength

Leave modern concrete structures in the ocean, and a few decades later they'll be in need of replacing, or at least a serious patch job. Meanwhile, ancient Roman concrete is still standing strong after thousands of years, and not only does it resist damage, but the salt water actually makes it stronger. X-ray examinations have found the key to the mixture's incredible longevity, which could help improve modern recipes.

In recent years, we've electrified concrete to melt snow, made it more fire-resistant, added bacteria to make it self-heal any cracks that form, and have found ways to "program" it to be stronger. But it seems that the Romans were way ahead of their time, with a superior method for making concrete that could stand the test of time.

Although the complete recipe has been lost over the millennia, studies of samples have shown that volcanic ash, lime and seawater are the main ingredients. But the real magic seems to happen when those ingredients interact with the environment – specifically the saltwater incessantly pounding on the surface.

Researchers from Berkeley Lab and the University of Utah took samples of ancient concrete from 2,000-year-old harbor structures in Orbetello, Italy, and studied them in the X-ray research center at Berkeley Lab's Advanced Light Source (ALS) to try to find clues to its durability.

"At the ALS we map the mineral cement microstructures," says Marie Jackson, lead researcher on the study. "We can identify the various minerals and the intriguingly complex sequences of crystallization at the micron scale."

The team found that when seawater seeps into the concrete, it dissolves the lime inside. Normally, this kind of corrosion would destroy modern concrete in a matter of years, but it actually strengthens the Roman stuff by allowing crystals of Al-tobermorite and phillipsite to grow, plugging the holes.

A microscope image of a mix of volcanic ash, lime and seawater, which has resulted in the growth of Al-tobermorite crystals
A microscope image of a mix of volcanic ash, lime and seawater, which has resulted in the growth of Al-tobermorite crystals

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

Reviving this long-lost, ingenious technique would certainly be handy today, but the exact formula is still unknown. The researchers are experimenting with different combinations of seawater and volcanic rock to try to unlock its secrets, which could be useful for building longer lasting seawalls, dams and piers.

The research was published in the journal American Mineralogist and the researchers describe their work in the video below.

Sources: Berkeley Lab, University of Utah

How seawater strengthens Roman concrete

8 comments
Pete0097
It make sense. Look at coquina. It is just sand and shells cemented together with dissolved shell fragments. The seawater is what dissolves the shells. The big problem with seawater and concrete are the critters that get into the concrete and start the erosion process.
owlbeyou
This is very interesting but incomplete. No mention of the effects of corroding rebar in modern cement construction. Also the significance of freezing temperatures that the Romans, Greeks and Egyptians never had to deal with.
habakak
More importantly, how did the Romans figure this out? Trial and error? Pure luck?
Gregg Eshelman
Makes sense that if the salt is put into the concrete as it's mixed that more salt coming in from seawater wouldn't cause damage. What's interesting about this past winter here in Washington and Payette Counties, Idaho (other than the record snowfall depth) is how much damage it did to concrete that had weathered many decades of winters. Sidewalks all over have spalled and concrete parking blocks have exploded like I haven't seen since ones that were submerged for days by the New Year's Day flood of 1997. The parking blocks look to have failed due to saturation of water and internal pressure from swelling of rusting rebar - and that being non-structural they can be made from any sort of mix. But a 50 year old sidewalk that has only mildly weathered over time, then in one winter falls apart??? What was so different about the 2016/2017 winter?
chase
I think old tech from times past is very fascinating. I have to ditto the comment that posed the question. Was it intentional or blind luck. I'd like to think a mixture of both. Or perhaps some artist made a mixture for a statue and... Over time they saw the same results. Look at how Pyrex came about. By happen stance. What don't understand is why it's so difficult to replicate. Modern science should be able to easily determine exact percentages of compounds. A computer could narrow down known material properties for the area. And give the best odds for a correct mixture that comes closest to the percentages with those materials. It's a pretty cool finding though. I'm sure the Romans are pretty pleased with themselves at this discovery. And smiling in their graves.
neutrino23
Probably there was a fair amount of luck involved. They figured out how to make something strong. I doubt they knew this would last so long. They certainly didn't have SEM/EDS to figure out the microstructure. I suppose the problem in reverse engineering this is that it is not a simple mixture. The different starting ingredients react together to form new minerals so looking at what you have now you have to figure out which minerals were there to start with and which are reaction products. Then for the reaction products you need to figure out the precursors for them.
Nik
As Romans only had men and horses to move materials, they would have used whatever was readily available locally, so its my guess that the longevity of mix they arrived at, was mainly luck. Salt seawater was on the spot, fresh water would have had to have been carried to the site, increasing labour costs. How would they have known of its longevity when they mixed it, are there much older examples elsewhere? There 'hardeners' that can be added to concrete to improve setting time, and make it harder, which are mainly salt, so the knowledge seems to exist in the modern trade, but it may be the cost factor that prevents its general use, or, ignorance by those specifying the mix for the jobs.
J Scott
Read "Concrete - 7000 Year History" by Reece Palley. Wonderfully opinionated, yet talks in detail about roman concrete. You might enjoy it!
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