The chemical secrets of a concrete Roman breakwater that has spent the last 2,000 years submerged in the Mediterranean Sea have been uncovered by an international team of researchers led by Paulo Monteiro of the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), a professor of civil and environmental engineering at the University of California, Berkeley.
Analysis of samples provided by team member Marie Jackson pinpointed why the best Roman concrete was superior to most modern concrete in durability, why its manufacture was less environmentally damaging – and how these improvements could be adopted in the modern world.
"It's not that modern concrete isn't good – it's so good we use 19 billion tons of it a year," says Monteiro. "The problem is that manufacturing Portland cement accounts for seven percent of the carbon dioxide that industry puts into the air."
Portland cement is the source of the "glue" that holds most modern concrete together. But making it releases carbon from burning fuel, needed to heat a mix of limestone and clays to 1,450 degrees Celsius (2,642 degrees Fahrenheit) – and from the heated limestone (calcium carbonate) itself. Monteiro's team found that the Romans, by contrast, used much less lime and made it from limestone baked at 900˚ C (1,652˚ F) or lower, requiring far less fuel that Portland cement.
Cutting greenhouse gas emissions is one powerful incentive for finding a better way to provide the concrete the world needs; another is the need for stronger, longer-lasting buildings, bridges, and other structures.
"In the middle 20th century, concrete structures were designed to last 50 years, and a lot of them are on borrowed time," Monteiro says. "Now we design buildings to last 100 to 120 years." Yet Roman harbor installations have survived 2,000 years of chemical attack and wave action underwater.
The Romans made concrete by mixing lime and volcanic rock. For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered a hot chemical reaction. The lime was hydrated – incorporating water molecules into its structure – and reacted with the ash to cement the whole mixture together.
Descriptions of volcanic ash have survived from ancient times. First Vitruvius, an engineer for the Emperor Augustus, and later Pliny the Elder recorded that the best maritime concrete was made with ash from volcanic regions of the Gulf of Naples (Pliny died in the eruption of Mt. Vesuvius that buried Pompeii), especially from sites near today's seaside town of Pozzuoli. Ash with similar mineral characteristics, called pozzolan, is found in many parts of the world.
Using beamlines 5.3.2.1, 5.3.2.2, 12.2.2 and 12.3.2 at Berkeley Lab's Advanced Light Source (ALS), along with other experimental facilities at UC Berkeley, the King Abdullah University of Science and Technology in Saudi Arabia, and the BESSY synchrotron in Germany, Monteiro and his colleagues investigated maritime concrete from Pozzuoli Bay. They found that Roman concrete differs from the modern kind in several essential ways.
One is the kind of glue that binds the concrete's components together. In concrete made with Portland cement this is a compound of calcium, silicates, and hydrates (C-S-H). Roman concrete produces a significantly different compound, with added aluminum and less silicon. The resulting calcium-aluminum-silicate-hydrate (C-A-S-H) is an exceptionally stable binder.
At ALS beamlines 5.3.2.1 and 5.3.2.2, x-ray spectroscopy showed that the specific way the aluminum substitutes for silicon in the C-A-S-H may be the key to the cohesion and stability of the seawater concrete.
Another striking contribution of the Monteiro team concerns the hydration products in concrete. In theory, C-S-H in concrete made with Portland cement resembles a combination of naturally occurring layered minerals, called tobermorite and jennite. Unfortunately these ideal crystalline structures are nowhere to be found in conventional modern concrete.
Tobermorite does occur in the mortar of ancient seawater concrete, however. High-pressure x-ray diffraction experiments at ALS beamline 12.2.2 measured its mechanical properties and, for the first time, clarified the role of aluminum in its crystal lattice. Al-tobermorite (Al for aluminum) has a greater stiffness than poorly crystalline C-A-S-H and provides a model for concrete strength and durability in the future.
Finally, microscopic studies at ALS beamline 12.3.2 identified the other minerals in the Roman samples. Integration of the results from the various beamlines revealed the minerals' potential applications for high-performance concretes, including the encapsulation of hazardous wastes.
Environmentally friendly modern concretes already include volcanic ash or fly ash from coal-burning power plants as partial substitutes for Portland cement, with good results. These blended cements also produce C-A-S-H, but their long-term performance could not be determined until the Monteiro team analyzed Roman concrete.
Their analyses showed that the Roman recipe needed less than 10 percent lime by weight, made at two-thirds or less the temperature required by Portland cement. Lime reacting with aluminum-rich pozzolan ash and seawater formed highly stable C A-S-H and Al-tobermorite, insuring strength and longevity. Both the materials and the way the Romans used them hold lessons for the future.
"For us, pozzolan is important for its practical applications," says Monteiro. "It could replace 40 percent of the world's demand for Portland cement. And there are sources of pozzolan all over the world. Saudi Arabia doesn't have any fly ash, but it has mountains of pozzolan."
Stronger, longer-lasting modern concrete, made with less fuel and less release of carbon into the atmosphere, may be the legacy of a deeper understanding of how the Romans made their incomparable concrete.
More information: "Material and elastic properties of Al-torbermotite in ancient Roman seawater concrete," by Marie D. Jackson, Juhyuk Moon, Emanuele Gotti, Rae Taylor, Abdul-Hamid Emwas, Cagla Meral, Peter Guttmann, Pierre Levitz, Hans-Rudolf Wenk, and Paulo J. M. Monteiro, appears in the Journal of the American Ceramic Society.
"Unlocking the secrets of Al-tobermorite in Roman seawater concrete," by Marie D. Jackson, Sejung Rosie Chae, Sean R. Mulcahy, Cagla Meral, Rae Taylor, Penghui Li, Abdul-Hamid Emwas, Juhyuk Moon, Seyoon Yoon, Gabriele Vola, Hans-Rudolf Wenk, and Paulo J. M. Monteiro, will appear in American Mineralogist.
Amazing how our modern technological world still can learn how to make things better using knowledge gleaned from historically "less advanced" peoples!
In the old world, knowledge was power, just as today. However, each tradesman would develop and test his own techniques, find what worked well, and keep that a secret. There is no doubt that many things were lost to history, knowledge that died with the person who discovered it. We share information much more freely now, and we use things like patents and copyrights to protect the original creator from having his work stolen. Back then, though, there were no such things, and keeping your knowledge a secret was the key to your livelihood, and maybe even your survival. I give praise to those people who spent countless years testing different ways to do things, generation after generation, eventually prodicing things like steel, glass, concrete, and other materials that built our society.
Today's concrete fails almost always due to rusting of steel reinforcing bars. Stainless & composite rebars are available but expensive. We need to use more Roman technology: compression-based structures that don't need rebar.
We need to use more Roman technology: compression-based structures that don't need rebar.
We are using compression based technology. Our bridges only span as far as they do because of the tensioned steel cables that compress the concrete. There's no other way it can withstand bending, so unless you want to build like the Romans did and have pillars every few feet, better think of a material that takes tension like steel and doesn't rust.
There's no other way it can withstand bending, so unless you want to build like the Romans did and have pillars every few feet, better think of a material that takes tension like steel and doesn't rust.
The selection of reinforcing material other than steel has added difficulty: The thermal expansion rates need to match. Concrete and steel have quite similar values, so they work good together, but not perfectly which can be seen from structures that have to work at greatly varying temperatures. For example here in Finland outside structures might be at -30C at winter and at +30C at summer, making 60C degrees variation. That can break the structures after repeated exposure.
How on earth do you suppose they discovered this marvel? I would just love to know the timeline and the series of accidents involved.
Bundles of very long carbon nanotubes? :-) Anybody know if corrosion of some kind would eat those up were it possible to create such things?
How on earth do you suppose they discovered this marvel?
Let's just say they had quite a long time to figure it out (and a lot of opportunities to experiment in various parts of the world with various resources at their disposal).
Trial and error can lead to some amazing results if you do it long enough (and not just a few discoveries are due to 'happy accidents') .
It is with great respect that we have the privileged to study the past through historic materials that are still performing. Technology as it advances in other fields seems to be discovering breakthroughs daily (computer industry), but in our industry of building construction we are humbled to find evidence of better ways to formulate and design materials from others that went before us.
Many thanks to the authors / researchers of this article and the supporting laboratory work to confirm the findings of Roman concrete.
we are humbled to find evidence of better ways to formulate and design materials from others that went before us.
That they didn't have our tools (for modelin, simulation or manufacturing) doesn't mean they were stupid.
Scarcity (or an other kind of pressure) tends to focus the mind.
Friend of mine went on to study industrial design. The entrance exam had, among other things, a test who would build the best shovel and the best bridge just using a sheet of paper. Some of the designs these guys/girls came up with were simply amazing in terms of usability and load tolerance.
This sounds like the kind of thing that should be put on an emergency priority because of the benefits (longer lasting, environmentally better, saves lives, etc). Too bad we don't have the kind of social system or government that would do that like an intelligent species would.
"its manufacture was less environmentally damaging"
I would like to know how the 'environmental damage' occasioned by the Roman manufacture of cement was quantified, especially with respect to CO2 emissions, which seem to have achieved paramount importance.
They used higher quality raw materials in their process. They used quick lime CO instead of currently used lime stone which is CaCO3 to create their cement. Both can be called lime along with calcium hydroxide. Their kiln temperatures were lower because of that. You can still do this today but it is not economical.
When making clinker in well insulated kilns today, heat is recycled in the gas and the quality of the cement is controlled with greater precision. The temperature is definitely higher, but I am not sure if the Romans were able to use less CO2 in their cement process overall. Definitely less CO2 in concrete as they didn't use fossil fuels for transportation, crushing, grinding ect.
Using Pozzolan in concrete is old technologies. They have been using Pozzolan & Fly ash in concrete for a long time. But it was use as a substitute for Portland Cement in the concrete mixtures, as Portland Cement is more expensive. And many time they preferred Cement over Pozzolan & Fly ash as it give the Concrete more strength.
However taking durability in Sea Water into consideration, using Pozzolan in concrete is a much better option in the long run, in Sea Water condition, since Sea Water will deteriorate the concrete over time. But in a none Sea Water condition, Pozzolan is not needed.
This information is good for the residences that can afford to live by the sea front in those expensive houses.
They used quick lime CO instead of currently used lime stone which is CaCO3 to create their cement. Both can be called lime along with calcium hydroxide. Their kiln temperatures were lower because of that. You can still do this today but it is not economical. .
There's no naturally occurring quick lime. They started out with limestone just like everybody else does.
Hmmm, I was under the impression that you can obtain some quicklime from volcanic ejecta and presumed that was the reason why the Romans went for that instead of the more readily available sources of calcite such as marble. I haven't been following the field the last couple years though, so I might be wrong here.
"Today's concrete fails almost always due to rusting of steel reinforcing bars. Stainless & composite rebars are available but expensive. We need to use more Roman technology: compression-based structures that don't need rebar."
Rebar does not rust in concrete if you have proper protection. Per Chapter 19, section 7 'Concrete Protection for Reinforcement' of the Uniform Building Code. Rebars needs the following amount of concrete covering it.
1. Cast against earth .......3"
2. Exposed to earth or weather .................... 1-1/2" or 2"
3. Not exposed to earth or weather.................3/4" or 1-1/2"
The above give you the general idea about protection of steel reinforcement in concrete from rust. And in situation where reinforcement are expose to sea water, it will rust regardless. Unless you use epoxy coated steel reinforcement, which will protect it even against the sea water.
But using Pozzolan will require larger concrete members to compensation for lower strength.
Stony Brook University did a fairly long term study of the use of concrete, a catalyst and coal ash which did not effect the soil it was in. Proposed was seawalls and other mediating structures.
I read a woman in a private company had invented a mixing machine that incorporated small pieces of metal, based on stress and need, in the concrete as it was transported to the forms. The shapes were researched and appeared shaped like carabineers, different size and diameters, allowed concrete in new forms, now stronger than with rebar. Perhaps a stronger "slurry wall" would also be built, and the interface at bedrock, more secure than the flat-ends of a rebar cage.
Law requires rebar to be cleaned, labor intensive, when reused for example in a bridge. There was a multiple machine that uses high pressure water to break, remove old concrete and clean the rebar using water pressure. Faster than by hand, the concrete is poured as part of this "train", which however is very loud.
" I read a woman in a private company had invented a mixing machine that incorporated small pieces of metal, based on stress and need, in the concrete as it was transported to the forms. The shapes were researched and appeared shaped like carabineers, different size and diameters, allowed concrete in new forms, now stronger than with rebar."
Old technology that has been around for decades. It is called fiber mesh. It looks like as if there are pieces of Brillo pads in the concrete.
Even the epoxy coated rebars I mentioned above is now being replaced by MMFX rebars for use in sea water and corrosive environment.
You sound a layman peaking into the world of Concrete & Construction.
reading this article I got the idea that this material formed under great pressure and heat. As in only periodically presented on the surface of the Earth great heat and pressure.
perhaps due to Sol's release of stored energy from a delayed magnetic pole reversal?
@ Solid recovery : "They used quick lime CO" [sic] You meant CaO.
They used the materials they had close at hand, and in time discovered how well it worked for them.
Researchers have been trying to discover the secret(s) of Roman concrete for a long time. Hopefully, this is it.
Besides reducing the CO2 production (about 1 ton of CO2 per ton of concrete made and placed) this discovery could also reduce the cost of concrete a fair bit, since it will require less energy to make.
I think we should be trying to build 500 year buildings, like some of the 400,000+ cob buildings in the UK are.
Not quite everybody: some people use(d) sea shells. Easy to do when you're near the sea.
It seems that this is old news and not a secret.
Per The 2000 Concrete Manual on section 9.4, sub-section 'Use of Pozzolans' stated "it reduces water permeability, especially for lean mixes, and improves resistance to aggressive solutions, such as seawater and sulfate or acid waters."
for concrete exposed to seawater."
And it seems that Concrete with Pozzolans added have low strength in the beginning but will exceed regular concrete strength months and years down the road.
This scientific report is not a secret. But the chemical reaction described here might be a something that the concrete construction industry does not know of, or write about.