Why Roman concrete still stands strong while modern version decays
Why Roman concrete still stands strong while modern version decays
A researcher drills at a marine structure in Portus Cosanus Tuscany
05 July, 2017, 07:08
Modern Portland cementconcrete also uses rock aggregate, but with a vital difference: the gravel particles and sand are meant to be inert.
To understand the long-term chemical processes that occurred in the Roman structures, researchers used thin, polished slices of the concrete with an electron microscope in Germany to map the distribution of elements in the mineral microstructures. In 2013, she examined mineral and microscale properties of Roman concrete and found that unlike modern cement concrete, Roman concrete's filler is chemically reactive, making it stronger and more resilient. Scientists began their search with an ancient recipe for mortar, laid down by Roman engineer Marcus Vitruvius in 30 B.C.E.
Jackson is testing new materials using seawater and volcanic rock from the western US, and has said that the planned Swansea tidal lagoon should be constructed using this ancient Roman technique. The Roman scholar Pliny the Elder described underwater concrete structures that become "a single stone mass, impregnable to the waves and every day stronger".
The team concluded that when seawater permeated through the concrete in breakwaters and in piers, it dissolved parts of the volcanic ash and allowed new minerals to form from the extremely alkaline leached fluids, mainly Al-tobermorite and phillipsite.
Previous samples from the ancient sea walls and harbors showed that the concrete contained a rare mineral named aluminum tobermorite.
"Lime (also known as calcium oxide) - exposed to seawater in the Roman concrete mixture - probably thoroughly reacted with volcanic ash early in the history of the massive harbor structures".
"Contrary to the principles of modern cement-based concrete", Jackson says, "The Romans created a rock-like concrete that thrives in open chemical exchange with seawater".
She explained that the recipe was completely lost, and although she has extensively studied ancient Roman texts, she hasn't found the precise method for mixing the marine mortar, to recreate the concrete.
"This rocklike concrete is behaving, in many ways, like volcanic deposits in submarine environments", said Jackson, according to The Washington Post.
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Marie Jackson, who led the study, said: 'This alkali-silica reaction occurs throughout the world and it's one of the main causes of destruction of Portland cement concrete structures'. She first studied tuffs and then investigated volcanic ash deposits, soon becoming fascinated with their roles in producing the remarkable durability of Roman concrete.
The ancient Romans used concrete everywhere, particularly in their mega-structures like the Pantheon and Trajan's Markets in Rome.
The combination of ash, water and lime leads to pozzolanic reaction, approximately the same processes contribute to the formation of volcanic tuffs.
"Romans were fortunate in the type of rock they had to work with", says Jackson.
In their new study, the researchers took samples of Roman concrete to the Advanced Light Source, an X-ray synchrotron located at the Lawrence Berkeley National Laboratory in California, to map out where the minerals can be found in the samples. If we were able to create concrete today in the same way the Romans did, it would survive much longer if it was a structure near the sea. The process for making Portland cement, a key ingredient of modern concrete, requires temperatures of roughly 2,640 degrees F, and the fuels burned to generate this heat are responsible for 7 percent of global carbon dioxide emissions.
An analysis of 2,000 year old cement made from volcanic ash and limestone suggests seawater was the secret ingredient. Jackson is also leading a scientific drilling project to study the production of tobermorite and other related minerals at the Surtsey volcano in Iceland.
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.
Jackson is trying to develop a similar concrete using materials readily available in the U.S. For those reasons, it is doubtful that Roman concrete could become widespread, but could be beneficial in specific contexts.
Jackson recently weighed in on a proposed tidal lagoon to be built in Swansea, United Kingdom, to harness tidal power. "You can imagine that, with the way we build now, it would be a mass of corroding steel by that time".
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