While modern marine concrete structures crumble within decades, 2000-year-old Roman piers and breakwaters endure to this day, and are stronger now than when they were first constructed.
University of Utah geologist Marie Jackson has found that this is because seawater filtering through the concrete leads to the growth of interlocking minerals that lend the concrete added cohesion.
The results have been published in American Mineralogist.
Romans made concrete by mixing volcanic ash with lime and seawater to make a mortar, and then incorporating into that mortar chunks of volcanic rock, the “aggregate” in the concrete. The combination of ash, water, and quicklime produces what is called a pozzolanic reaction, named after the city of Pozzuoli in the Bay of Naples.
Modern Portland cement concrete also uses rock aggregate, but with an important difference: the sand and gravel particles are intended 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.
Given the durability advantages of Roman concrete, why isn’t it used more often, particularly since manufacturing of Portland cement produces substantial carbon dioxide emissions?
“The recipe was completely lost,” Jackson explains. She has extensively studied ancient Roman texts, but hasn’t yet uncovered the precise methods for mixing the marine mortar, to fully recreate the concrete.
“Romans were fortunate in the type of rock they had to work with,” she says. “They observed that volcanic ash grew cements to produce the stuff. We don’t have those rocks in a lot of the world, so there would have to be substitutions made.”
She is now working with geological engineer Tom Adams to develop a replacement recipe, using materials from the western US.
Roman concrete takes time to develop strength from seawater, and features less compressive strength than typical Portland cement. For those reasons, it’s unlikely that Roman concrete could become widespread, but could be useful in particular contexts.
Jackson recently weighed in on a proposed tidal lagoon to be built in Swansea, United Kingdom, to harness tidal power. The lagoon, she says, would need to operate for 120 years to recoup the costs incurred to build it. “You can imagine that, with the way we build now, it would be a mass of corroding steel by that time.” A Roman concrete prototype, on the other hand, could remain intact for centuries.
Jackson says that while researchers have answered many questions about the mortar of the concrete, the long-term chemical reactions in the aggregate materials remain unexplored. She intends to continue the work of Pliny and other Roman scholars who worked assiduously to discover the secrets of their concrete. “The Romans were concerned with this,” Jackson says. “If we’re going to build in the sea, we should be concerned with it too.”Find the full study here.