Drilling Into The Secrets Of Roman Concrete

IRA FLATOW: This is Science Friday. I’m Ira Flatow. If you visit Rome– and I hope you do if haven’t been there already– one of the things that stands out is the number of ancient structures that you can still see today. And it’s not just massive slabs of rock. Over 2,000 years ago, the ancient Romans built piers and breakwaters and other structures out of concrete, and some of those structures are still standing.

What did they know about concrete that we don’t because we see concrete crumbling on our roads today. Writing this week in the journal American Mineralogist, researchers unearthed the geology and chemistry secrets of that ancient concrete and how the interactions of seawater with volcanic ash actually make the Roman concrete stronger as it ages. Joining me now is one of the authors of that report. Marie D. Jackson is a geology and geophysics research professor at the University of Utah and an expert on Roman concrete. Welcome to the program, Dr. Jackson.

MARIE D. JACKSON: Hello.

IRA FLATOW: You know, listeners to this show know that concrete is one of my favorite topics. I don’t know why, but I’m fascinated by it. So I am delighted to speak with a fellow concrete geek like yourself.

MARIE D. JACKSON: It’s pretty interesting stuff.

IRA FLATOW: It is interesting stuff. So what’s the secret? What did the Romans know about making their concrete that we don’t do?

MARIE D. JACKSON: Well, the Romans had a very different framework for making the cementing fabric of their concrete. The first step was to mix volcanic ash with lime. That is calcium oxide calcimed from limestone and water. And in the architectural monuments they used freshwater, and in the marine concretes they used seawater. This produced a very potent reaction, called a pozzolanic reaction, that created a really robust framework of coherence in the concrete. That was the first step.

And Vitruvius, in De Architectura, described these materials and processes, in fact, in a lot of detail. We described that in a 2013 American Mineralogist paper. And now we’ve discovered that the mineral cements that form over time come from the corrosion of the volcanic ash that’s left in the concrete as seawater percolates slowly through these very massive structures.

IRA FLATOW: What happens– the concrete changes into– the minerals change?

MARIE D. JACKSON: Yes, it does. And, in fact, Romans expected it to change. And they created a material that thrives on this kind of change, which is very different from the way we make concrete today.

IRA FLATOW: Well, could we take what they’ve done and put it in our concrete?

MARIE D. JACKSON: Many people are working on that now. And, in particular, they’re looking for materials to mix with conventional cement-based concretes that can reduce the amount of cement used in the concrete. This will reduce CO2 emissions and add to the durability of the concrete. And those are indeed called pozzolanic processes.

There’s another step beyond that, and that’s what the Romans recognized is those materials can perhaps react over time to create ongoing mineral cements that reinforce the cementitious fabric over time into a kind of self-healing process.

IRA FLATOW: Now I know that I called you a concrete expert and a geek like– I’m just a geek not the expert, but how did you figure this out? I can see that you are curious about it.

MARIE D. JACKSON: Well, I’m really sort of fascinated by both the architectural concretes in Rome and the harbor concretes. This is in large part due to a drilling program, the ROMACONS drilling program that drilled harbor concretes through the Mediterranean, 11 different harbors. And all of those concretes have the same fabric. Romans shipped this special volcanic ash from the Gulf of Naples all over the Mediterranean. And in the relict particles of lime grows a very unusual mineral called aluminous tobermorite.

We described that in great detail in 2013. And now we found that tobermorite grows through the fabric of the concrete. This is a platy mineral that has some very important industrial processes and industrial applications, but we have a great deal of difficulty making this mineral. It’s the Romans who were the masters of making this very unusual mineral.

IRA FLATOW: How do you think they stumbled on this? And, you know, was it by accident?

MARIE D. JACKSON: Well, they actually wrote about it. Both Pliny the Elder and Seneca write about the volcanic ash deposits of the Gulf of Naples and how these deposits grow mineral cements. So they were actually using a geologic framework to develop specific properties in their concretes. They were very intelligent people. And so what we are actually recognizing is a rock-like process that grows these zeolite and tobermorite cements. In fact, volcanic deposits in seawater, certain ones of them do grow these cements, as well.

IRA FLATOW: Let’s compare our concrete today and the Roman concrete. If we were to build a modern road using their concrete, would it last longer?

MARIE D. JACKSON: Well, you’ve actually brought up a very important point. The way Romans constructed and the way we construct is very different. So the objective I don’t think is to build Roman structures in the modern cement infrastructure, cement and concrete infrastructure, but rather to take the principles of the Romans and apply them to improving our concretes and finding other innovative applications for the kind of mineral growth that they developed.

IRA FLATOW: So they didn’t have to put reinforcement in their concrete like we do rebar and things like that.

MARIE D. JACKSON: They did reinforce their concrete.

IRA FLATOW: They did?

MARIE D. JACKSON: In fact, those very large structures never have been tipped over on the seafloor, even during enormous storm surges. What they did is they made a conglomeratic fabric– that’s what a geologist would call it– of large chunks of rock, either volcanic rock or carbonate rock. And this forms an internal framework in the structure through which they packed this very sophisticated mortar.

IRA FLATOW: Wow, so they did– they were great engineers, great engineers.

MARIE D. JACKSON: Yes, they were.

IRA FLATOW: Well, this is quite fascinating. Are you going to continue? You’re looking at the application of this to nuclear waste storage, are you?

MARIE D. JACKSON: Well, that’s one direction it can go. It can go in many directions. Aluminous tobermorite it is a layered mineral that can capture large cations in its inner layer spacing. And part of what we did in this new study is look at the bonding environments of aluminum and silicon in these crystals.

And they do indeed have the kind of structures that material scientists have determined are really important for that kind of cation exchange. Aluminous tobermorite is a candidate for cesium and strontium cation exchange, and it’s been considered for cementitious barriers around wasting capsulations. But nobody knows how to make aluminous tobermorite-bearing concretes– at least they haven’t until now.

IRA FLATOW: Something to look into. Fascinating, Dr. Jackson– give me any excuse to talk about concrete and not to confuse it with cement like people do.

MARIE D. JACKSON: Well, the Romans never used cement.

IRA FLATOW: There you go. There you go. Well, they weren’t from Portland. Thank you, Dr. Jackson, for taking time to be with us today.

MARIE D. JACKSON: It was a pleasure. Thank you.

IRA FLATOW: You’re welcome. Marie Jackson, research professor in geology and geophysics at the University of Utah.

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