Friday, July 7, 2017

Why 2,000 Year-Old Roman Concrete Is Better Than What We Produce Today

If you were to take a vacation to the Tuscany Region in Italy and wander to the Mediterranean beaches, you might find this fairly unremarkable structure, or one like it.
You might well say the only remarkable thing about this structure is that it's concrete - manmade - and around 2000 years old.  Exposed to the waves and storms of the Mediterranean sea for two millennia, it has withstood the elements better than (or at least as well as) any modern concrete.  It turns out that the Roman era concrete not only is more durable than modern concrete, it actually gets stronger with age.  Almost 2000 years ago, round A.D. 79, Roman author Pliny the Elder wrote in his Naturalis Historia that concrete structures in harbors, exposed to the constant assault of the saltwater waves, become “a single stone mass, impregnable to the waves and every day stronger.”  He may well have been right.

Concrete is something that we all see but very few think about.  Many use the terms concrete and cement interchangeably, but that's not correct.  Concrete is a mix of sand and rock called aggregate that's bonded together today with Portland cement, a mixture of silica sand, limestone, clay, chalk and other ingredients melted together at blistering temperatures.

The Romans, though, didn't have the techniques for producing Portland cement that we have today.
Romans made concrete by mixing volcanic ash with lime and seawater to make a mortar, and then incorporating into that mortar chunks of volcanic rock, the “aggregate” in the concrete. The combination of ash, water, and quicklime produces what is called a pozzolanic reaction, named after the city of Pozzuoli in the Bay of Naples. The Romans may have gotten the idea for this mixture from naturally cemented volcanic ash deposits called tuff that are common in the area, as Pliny described.
Modern concrete depends on the aggregate not interacting chemically with the Portland cement.  The Roman concrete doesn't utilize that chemistry. It turns out, this might be the reason their concrete gets stronger with time and ours erodes away.

The story of this research is from University of Utah geologist Marie Jackson, who became interested in the Roman concrete formations while on a sabbatical trip.  This led her to study Roman concrete with the tools a physical geologist might use to analyze the microstructures formed in it.  She and her colleagues have found that seawater filtering through the concrete leads to the growth of interlocking minerals that hold the concrete together with increasing cohesion.
In another study of drill cores of Roman harbor concrete collected by the ROMACONS project in 2002-2009, Jackson and colleagues found an exceptionally rare mineral, aluminous tobermorite (Al-tobermorite) in the marine mortar. The mineral crystals formed in lime particles through pozzolanic reaction at somewhat elevated temperatures. The presence of Al-tobermorite surprised Jackson. “It’s very difficult to make,” she says of the mineral. Synthesizing it in the laboratory requires high temperatures and results in only small quantities.

For the new study, Jackson and other researchers returned to the ROMACONS drill cores, examining them with a variety of methods, including microdiffraction and microfluorescence analyses at the Advanced Light Source beamline 12.3.2 at Lawrence Berkeley National Laboratory. They found that Al-tobermorite and a related zeolite mineral, phillipsite, formed in pumice particles and pores in the cementing matrix. From previous work, the team knew that the pozzolanic curing process of Roman concrete was short-lived. Something else must have caused the minerals to grow at low temperature long after the concrete had hardened. “No one has produced tobermorite at 20 degrees Celsius,” she says. “Oh — except the Romans!”
The team concluded that when seawater percolated through the concrete in breakwaters and in piers, it dissolved components of the volcanic ash and allowed new minerals to grow from the highly alkaline leached fluids, particularly Al-tobermorite and phillipsite. This Al-tobermorite has silica-rich compositions, similar to crystals that form in volcanic rocks. The crystals have platy shapes that reinforce the cementing matrix. The interlocking plates increase the concrete’s resistance to brittle fracture.

Jackson says that this corrosion-like process would normally be a bad thing for modern materials. “We’re looking at a system that’s contrary to everything one would not want in cement-based concrete,” she says. “We’re looking at a system that thrives in open chemical exchange with seawater.”
Unfortunately, the Roman recipes for concrete have been lost to the ages.  Dr. Jackson and her colleagues continue to research possible ways of recreating the recipes.  She is now working with geological engineer Tom Adams to develop a replacement recipe, using materials from the western U.S. The seawater in her experiments comes from the Berkeley, California, marina, collected by Jackson herself.

So why is 2000 year-old Roman concrete better than modern concrete?  It seems pure luck.  They were in an area where the chemistry of their volcanic ash with their lime and their rocks would work properly.  It might not be replicable with those ingredients from everywhere in the world, but they observed, took notes and experimented, eventually making the most of what they had.

Roman concrete wouldn't be applicable everywhere.  It's weaker in compression than modern concrete, for example, but would shine in areas exposed to saltwater.  It's conceivable that a return to the Roman recipes, or something researchers could come up with based on the Romans' work, could make docks, piers or all sorts of structures in salt water or lagoons much more durable than modern concrete would.  It's also possible this research will lead to ways to make something that is better than both our modern concrete and the Roman version.


10 comments:

  1. Replies
    1. X2 - very interesting. Thank you.

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  2. "OK, but outside of agriculture, sanitation, law and order and invincible concrete...what have the Romans ever done for us?"

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  3. Started a nasty creation called a "Senate".

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  4. I had always heard that Roman concrete was superior to modern concrete but that no one knew why or how to reproduce it.
    Excellent post, it answered quite a few questions.
    Thanks!

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  5. The Washington (com)Post had to add their little climate change ideology to the article.

    We can use it to stop rising sea levels!!

    https://www.washingtonpost.com/news/speaking-of-science/wp/2017/07/04/ancient-romans-made-worlds-most-durable-concrete-we-might-use-it-to-stop-rising-seas/?utm_term=.7ad92fd552d8

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  6. In all fairness to our concrete companies and engineers we can make extremely strong long lasting concrete. But there are other priorities including cost and convenience. You can easily substitute thickness and rebar reinforcement to compensate for a cheaper and more forgiving concrete mix. Also we don't generally build to last 2000 years and we tear down old buildings and structures after 50-100 years. A really good mix of concrete would cost more and in many cases have to be prepared on site instead of while driving to the site and it would have to be poured and in place within 30 minutes or less rather than a couple of hours of work time. Everything in life is a compromise.

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    1. As long as you bring this up, I had some misgivings with this article, but found it very interesting on balance. I took a "mandatory elective" course in materials science way back in my junior year, and one of things I got out of it is that concrete continues to harden for a long time. ISTRC they said 100 years. Saying Roman concrete continued to harden didn't surprise me. I'm sure that the rate of hardening has to go down and at some point it will be as hard as it ever gets. I find it really difficult to believe Roman concrete is still hardening after 2000 years.

      Second, when I was a larval engineer, a wise old graybeard told me, "engineering is the art of compromise", and that is an incredibly wise statement. No design of anything is perfect for all purposes. A car designed for racing isn't the ideal configuration for hauling the kids to soccer or for high gas mileage commuting. A boat designed for fishing 6" deep saltwater flats isn't going to be comfortable in rough seas offshore. And so on. I have no doubt the common mixes of concrete are a compromise.

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  7. I saw a scholarly paper awhile back about this subject. It involved a USGOV, study and if I recall correctly the construction of Flaming Gorge dam.

    The real secret to it was not so much the recipe, modern concrete is basically the same. Rather it is the water.

    Specifically the old days they used much less water and packed the concrete as a kind of paste.

    This allowed the concrete to.be packed more densely. In turn it made the crystal structure more interlocking because it's more dense.

    Downside of course is that it takes longer to cure and won't pour Into forms like modern recipes. It also won't pour out of a cement truck.

    One could duplicate this method by mixing a ready mix with only enough water to make it have a clay consistently and packing into a form. Between 7-12% is my estimation based on my experience with compressed earth blocks that use Portland cement and Sandy clay soil instead of aggregate.

    Come and Make It blog

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