Why Is Roman Concrete So Strong? Geologists Have Cracked The Case.

The Roman Empire fell more than a thousand years ago, but its concrete architecture still stands. That includes bridges and other structures that have lived out their lives in the briny environment of the sea. Compare that to our own modern bridges, which are generally designed to last about a century. How did the Romans pull that off? A 2017 study may finally have the answer.

Drilling at a marine structure in Portus Cosanus, Tuscany, 2003.

What Doesn't Kill Me Makes Me Stronger

Modern concrete is made from Portland cement, a mix of limestone, shells, and chalk that's combined with particles of shale, clay, slate, sand, iron ore, and other ingredients, then heated and ground into a fine powder. The Romans, by contrast, produced their concrete by mixing volcanic ash with lime and sea water to create a mortar, then adding chunks of volcanic rock.

In a study published in American Mineralogist, scientists say Romans can thank the sea water for their concrete's long life. Here's why: any chemical reaction with modern concrete (such as the alkali-silica reaction sometimes known as "concrete cancer") makes it expand and crack. When seawater flows through Roman concrete, it triggers chemical reactions too, but those reactions actually strengthen the material. Over the years, seawater dissolves parts of the concrete's volcanic ash and allows new minerals like Al-tobermorite and phillipsite to grow in their place. Those minerals grow crystals in an interlocking plate shape, which helps protect the concrete from cracking in the future.

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 cementing matrix.

A Lost Recipe

So if this millennia-old concrete is so strong, why aren't we using it today? For one thing, we don't know how to make it. "The recipe was completely lost," geologist and co-author Marie Jackson said in a press release. Ancient roman texts hint at the methods, but nobody has yet discovered the exact recipe. There's also the possibility that the precise volcanic rock the Romans used is the real secret to its success. "Romans were fortunate in the type of rock they had to work with," Jackson says. "We don't have those rocks in a lot of the world, so there would have to be substitutions made."

That doesn't mean it's impossible. Plus, for how much CO2 is emitted in the production of modern concrete — not to mention the costs involved — it's in our best interests to find our own version of a material that gets stronger over time. Jackson says she wants to continue delving into the work of Roman scholars to uncover the mysteries of their materials.

How Seawater Strengthens Roman Concrete

Written by Ashley Hamer July 19, 2017

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