Ancient secrets of the sea

Scientists studying Roman seawalls find long-lost ingredients for modern structural materials.
29 August, 2017
Much of what humans know about the Roman Empire that once spanned the ancient world is in the architecture.
From France and Britain, to Israel and Turkey, Rome left a rich historical and cultural record. Some geologists see something different, though, beyond the tourist sites of Rome, and they've made a discovery about seawall construction in antiquity they think will influence our modern ideas.

A team at the University of Utah, led by Marie Jackson, studies what Pliny the Elder observed in CE 79: Romans built seawalls that with constant exposure to the waves become "a single stone mass, impregnable to the waves and every day stronger." It's one of the reasons we still see them today, and in a world of crowded urban centers and rising seas, Jackson went looking for clues about the materials. The research, published in American Mineralogist, explains why reviving an ancient practice may work.
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What she first observed was that, on land, the Roman concrete mix likely replicated what ancients knew about volcanic rock. The mortar mix used to build the Pantheon or Trajan's Market was made from volcanic ash, seawater and lime to cause a pozzolanic reaction, named after the Italian city of Pozzuoli. So Roman concrete was understood to be "alive" in ways that defy the brief longevity of our modern Portland concrete, with a material composition that considers sand and gravel particles to be inert filler.

When the Roman mix was used in seawalls, the chemical reactions over time yielded a rare aluminium-based mineral called aluminium tobermorite, that with porous phillipsite, strengthened rather than fatigued the concrete over time. By studying core samples from the piers and breakwall ruins, the Utah team learned that the ingredients specified in 30 BCE by Marcus Vitruvius, a Roman engineer, created the marine mix to allow the aluminium tobermorite to flourish in ways rarely seen by modern material science. The mineral also is difficult to make in the laboratory, requiring high heats that result in low yields.
"The recipe was completely lost," explains Jackson, who has extensively studied ancient Roman texts in an effort to recreate the precise mix. Scientists at the Advanced Light Source beamline at Lawrence Berkeley National Laboratory helped with analysis to understand how the aluminium tobermorite formed in marine environments led to a different concrete curing process, leading to the next research step.

Jackson is working with Berkeley geological engineer Tom Adams to develop a replacement recipe with potential for specific applications where the durability and longevity of a concrete barrier are at a premium. For example, British experts are planning a tidal lagoon near Swansea Bay with plans to generate clean energy. The Tidal Lagoon Power firm says it harkens back to a tradition of British tidal energy that began in the 7th century, but Jackson has urged them to consider ancient building practices too.
The corrosion caused by seawater usually stresses modern Portland concrete mixes in about 50 years, and estimates range from 60 to 70 years on how long steel reinforcements in the concrete can be protected. Jackson argues that the tidal lagoon facility will need to operate for 120 years in order to break even on the investment – making the ancient Roman marine mix a better choice for Swansea.

Paulo Monteiro, a colleague at the Berkeley laboratory, adds that making Portland concrete is carbon-intensive too and accounts for about 7 percent of emissions that industry releases. Making a better marine mix, and using the more natural methods, may help to reduce the carbon costs as well.
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