Why Roman Concrete Endured Thousands of Years of Seawater Pounding, While Ours Can’t

When it comes to comparing our own modern architectural feats with those of past civilizations, we tend to think of ourselves as the proverbial king of the hill. It’s not hard to see why. The first skyscraper was built in 1885 — a 10-story building, now considered modest, supported by an internal and external fireproof steel frame. For 3,800 years, the tallest building was the Great Pyramid of Giza. But for all the fame of the Egyptian pyramids (and the Egyptians who built them), the vast majority of the pyramids ever built collapsed, were dismantled and quarried for their materials, or remain buried today. Score one for Team Modernity — at least, until Old New York is destroyed and New New York is built on top of it.

But there’s one group of people and one specific substance where we’re playing a distinctly out-of-tune second fiddle: roman concrete. You don’t have to be a material scientist to know water weathers rock. Our existing concrete structures don’t withstand this weathering very well; the lifespan of a well-built modern concrete structure is measured in decades (not counting major repair or rebuilding efforts). In contrast, there are Roman piers and breakwaters now more than 2,000 years old that aren’t just still holding up — in many cases, they’re stronger now than they were the day they were built. That’s according to this 2017 study, led by Marie Jackson of the University of Utah. It’s a follow-up to an earlier study in 2014, also led by Jackson at UC Berkeley in 2014 (both studies collaborated with the Advanced Light Source at the Lawrence Berkeley National Laboratory).

The Romans were themselves aware that this strengthening occurred. In the first century CE, Pliny the Elder wrote that the Roman method of making cement, which involved volcanic ash resulted in a creation “that as soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass (fierem unum lapidem), impregnable to the waves and every day stronger.”

The 2014 study had established the mixture of materials the Romans used to build their concrete and how their variant, which is made from volcanic ash, water, and lime, processed at much lower temperatures than the modern method of manufacturing Portland cement, differed from our own. We also knew that Roman concrete tends to be less susceptible to cracking, again due to the difference in materials composition and manufacture. But what wasn’t clear is how the Romans facilitated the chemical reaction that led to the formation of this super-strong, water-resistant concrete in the first place.

The University of Utah has an excellent explanation of what the researchers found. They already suspected that a material known as aluminous tobermorite, found widely in Roman concrete but not in modern recipes, was part of what gave Roman concrete some of its properties. The problem was, Al-tobermorite doesn’t form without high heat, and even then it doesn’t form in large batches. We know the pozzolanic reaction that the Romans used to make concrete wasn’t nearly as hot as modern Portland cement. So where’d the Al-tobermorite come from?

This microscopic image shows the lumpy calcium-aluminum-silicate-hydrate (C-A-S-H) binder material that forms when volcanic ash, lime and seawater mix. Platy crystals of Al-tobermorite have grown amongst the C-A-S-H in the cementing matrix. Image by the University of Utah

According to the minerologists, it came from reactions between the seawater and the concrete itself. Remember, Roman concrete is less susceptible to cracking than our own is today. As seawater pounded the concrete, the water saturated it. This dissolved components within the volcanic ash, releasing them to form new, interlocking crystalline compounds that further strengthen the concrete.

What’s particularly interesting is that, according to University of Utah geologist Marie Jackson, this kind of interlocking interaction is the opposite of what modern material engineers would seek to create. “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.”

Top image credit: University of Utah

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