Have you ever wondered how concrete sea walls built by ancient Romans are able to withstand being battered by ocean waves for thousands of years? Modern versions, meanwhile, easily disintegrate over a matter of decades.
A new study may have the answer: a rare mineral forms from chemical reactions between the concrete and seawater, strengthening the age-old material.
Engineers today may borrow these ideas from ancient Roman builders to make stronger and more sustainable concrete, according to the researchers.
The Difference Between Roman Concrete And Portland Cement
Ancient Roman civilization made concrete through mixing volcanic ash with lime and seawater to bind rock fragments. This concrete was incorporated in many architectural sites, including the Pantheon and Trajan’s Markets in Rome.
Modern concrete, on the other hand, uses a paste of water and Portland cement, which is a fine powder largely made up of limestone and clay for holding small rocks together. Any reaction with the cement paste could create gels that expand and crack the concrete — an alkali-silica reaction that usually leads to the destruction of structures made from the material.
In their earlier research, University of Utah geologist Marie Jackson and her team reported the unique chemistry of Roman concrete, specifically the presence of rare mineral aluminum tobermorite.
The presence of the hard-to-make mineral, surprised Jackson, who explained that high temperatures are needed to synthesize it in the lab in only small quantities.
Taking Cues From Ancient Concrete
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.
“As geologists, we know that rocks change. Change is a constant for earth materials. So how does change influence the durability of Roman structures?” said Jackson in a statement.
They found the silicate mineral phillipsite, a common feature of volcanic rocks, with aluminum tobermorite crystals growing from it. Al-tobermorite appeared to grow from the phillipsite whenever seawater bathed the concrete, making it more alkaline.
“It’s a very rare occurrence in the Earth,” Jackson emphasized in a Nature report, adding that phenomenon has only been witnessed in places like Iceland’s Surtsey volcano.
The growth of tobermorite strengthens the concrete, its long crystals allow the material to flex instead of break in the face of stress.
If Roman concrete thrives in open chemical exchange with seawater, the opposite takes place in modern concrete: it erodes as saltwater rusts the steel reinforcements and then washes away the compounds that glue the material together.
But while this ancient Roman technique of making concrete is better and friendlier to the environment, the recipes have been lost through time. Scientists are now seeking to develop a replacement recipe, reverse-engineering based on information on its chemical workings.
Successfully cracking the recipe could lead to a material that can also last for centuries and entail fewer carbon emissions.
The study was detailed in the journal American Mineralogist.