One of the fascinating mysteries of Ancient Rome is the impressive longevity of some of their concrete harbour structures. Battered by sea waves for 2,000 years, these things are still around while our modern concoctions erode over mere decades.
Now scientists have uncovered the incredible chemistry behind this phenomenon, getting closer to unlocking its long-lost recipe. As it turns out, not only is Roman concrete more durable than what we can make today, but it actually gets stronger over time.
Researchers led by geologist Marie Jackson from the University of Utah have been chipping away at the mysteries of Roman concrete for years, and now they have mapped its crystalline structure, figuring out precisely how this ancient material solidifies over time.
Modern concrete is typically made with portland cement, a mixture of silica sand, limestone, clay, chalk and other ingredients melted together at blistering temperatures. In concrete, this paste binds ‘aggregate’ – chunks of rock and sand.
This aggregate has to be inert, because any unwanted chemical reaction can cause cracks in the concrete, leading to erosion and crumbling of the structures. This is why concrete doesn’t have the longevity of natural rocks.
But that’s not how Roman concrete works.
Theirs was created with volcanic ash, lime and seawater, taking advantage of a chemical reaction Romans may have observed in naturally cemented volcanic ash deposits called tuff rocks.
Mixed in with the volcanic ash mortar was more volcanic rock as aggregate, which would then continue to react with the material, ultimately making Roman cement far more durable than you’d think it should be.
“We can go into the tiny natural laboratories in the concrete, map the minerals that are present, the succession of the crystals that occur, and their crystallographic properties,” says Jackson.
Aluminous tobermorite and a related mineral called phillipsite actually grows in the concrete thanks to the sea water sloshing around it, slowly dissolving the volcanic ash within and giving it space to develop a reinforced structure from these interlocking crystals.
“The Romans created a rock-like concrete that thrives in open chemical exchange with seawater,” says Jackson.
Making concrete the way Romans once did would be a boon to the modern building industry, but unfortunately the recipes have been lost to the tooth of time, so our only shot at recreating the ancient material is to reverse-engineer it based on what we know about its chemical properties.
“Romans were fortunate in the type of rock they had to work with,” says Jackson. “We don’t have those rocks in a lot of the world, so there would have to be substitutions made.”
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