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".
The same salty seawater spray that strengthened ancient Roman piers millennia ago makes modern coastal concrete constructions crumble - here's why. University of Utah geologist Marie Jackson studies the minerals and microscale structures of Roman concrete as she would a volcanic rock. This insight is surprisingly spot on, according to new research which found seawater is a secret ingredient that makes Roman concrete particularly durable by encouraging the growth of rare minerals. 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.
Modern Portland cement concrete also uses rock aggregate, but with an important difference: the sand and gravel particles are meant to be inert. Any reaction with the cement paste could form gels that expand and crack the concrete.
"This alkali-silica reaction occurs throughout the world and it's one of the main causes of destruction of Portland cement concrete structures", Jackson says.
Now, she's at least figured out how Roman concrete holds up so well in water.
Geoscientist Marie Jackson at the University of Utah in Salt Lake City began exploring Roman concrete during a sabbatical year spent in Rome in the 1990s.
One factor they found was that minerals tended to grow between the mortar and rock, preventing cracking. Previous studies showed how the aluminous tobermorite crystallized in the lime remnants during a period of elevated temperature. The presence of Al-tobermorite surprised Jackson.
Seeing how Jackon is a geologist, though, she immediately realized that the mineral must have appeared later.
Aluminous-tobermorite is a rare mineral in nature that has proven very hard to synthesize in the lab and also requires high temperatures above 176 degrees Fahrenheit to create, which makes it impractical for modern concrete to form that mineral once it has set.
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.
Following on from this, the researchers have now studied the drill cores again, and found several other minerals, including zeolite and phillipsite, had formed in the cement.
The work ultimately could lead to a wider adoption of concrete manufacturing techniques with less environmental impact than modern Portland cement manufacturing processes, which require high-temperature kilns. Something else must have caused the minerals to grow at low temperature long after the concrete had hardened. "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. Platy crystals of Al-tobermorite have grown amongst the C-A-S-H cementing matrix.
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'.
Besides its durability, Roman concrete has also been found to have a smaller carbon footprint than its modern counterpart.
The ancient Roman recipe is very different than the modern one for concrete, though.
Jackson is working with a geological engineer to rediscover the Romans' complex recipe for concrete.
'Romans were fortunate in the type of rock they had to work with. She and her colleagues have used materials such as fly ash, produced during the burning of coal, to give concrete "self-healing' properties, whereby the material closes up cracks after they form".
One concern regarding future applications of volcanic ash in concrete is that "we can not take apart the Bay of Naples to build sustainable concretes around the world", Jackson said. In modern concrete, seawater would make the structure crack.
Nobumichi Tamura, an ALS staff scientist, said the X-ray beamline where the Roman concrete samples were studied can produce beams focused to about 1 micron, or 1 thousandth of an inch, "which is useful for identifying each mineral species and mapping their distribution". For instance, the Roman cement could be very useful for a tidal lagoon project to be built in Swansea, United Kingdom, meant to harness tidal power. To recuperate the cost incurred from building it, the lagoon would have to operate for 120 years. A Roman concrete prototype, on the other hand, could remain intact for centuries. Both are a combination of mortar and aggregate.
It's not that easy at all, says Jackson.
Jackson added, "We can go into the tiny natural laboratories in the concrete, map the minerals that are present, the succession of the crystals that occur, and their crystallographic properties".
Once the paper publishes, find the full study here.