If you've ever wondered why the ancient structures of Rome have endured for millennia, when our own modern concrete is susceptible to cracks and crumbles, well, now you have your answer. Researchers recreated the Roman recipe and discovered that the formation of a certain kind of crystal in the concrete is the reason for the durability.
In "Mechanical resilience and cementitious processes in Imperial Roman architectural mortar" published in PNAS, researchers, led by Marie D. Jackson of the University of California at Berkeley, detail their work and the results. The team reproduced the Roman concrete recipe, allowed it to harden for 180 days, and then examined it using X-Rays.
The Roman recipe used by the team involves adding volcanic rocks to a liquid mortar. To make the mortar, ancient Romans — and the modern research team — started by heating limestone into quicklime, and then added water and volcanic ash. The key ratio for this mixture is three parts ash to one part lime. Rome had no shortage of volcanic ash to use, since volcanoes lay to north and south of Rome. But the ancient Romans settled on the Pozzolane Rosse ash from the Alban Hills volcano to the south. This "pozzolonic mortar," say the researchers, "is key to the durability of concrete components in structurally sound monuments well maintained over two millennia of use."
It's the reaction that occurs between the lime and the volcanic material that produces the stronger concrete, the researchers found. As the concrete hardened, strätlingite crystals formed in spaces around the sand and the volcanic gravel, making the structure stronger. The crystals do the same work that microfibers do in our modern concrete, but are resistant to corrosion and are generally better at reinforcing those spaces.
The press release from Berkeley names another benefit to the Roman formula:
Most modern concretes are bound by limestone-based Portland cement. Manufacturing Portland cement requires heating a mix of limestone and clay to 1,450 degrees Celsius (2,642 degrees Fahrenheit), a process that releases enough carbon – given the 19 billion tons of Portland cement used annually – to account for about seven-percent of the total amount of carbon emitted into the atmosphere each year.
Roman architectural mortar, by contrast, is a mixture of about 85-percent (by volume) volcanic ash, fresh water, and lime, which is calcined at much lower temperature than Portland cement. Coarse chunks of volcanic tuff and brick compose about 45-to-55-percent (by volume) of the concrete. The result is a significant reduction in carbon emissions.
"If we can find ways to incorporate a substantial volumetric component of volcanic rock in the production of specialty concretes, we could greatly reduce the carbon emissions associated with their production also improve their durability and mechanical resistance over time," Jackson says.
Stronger and more environmentally sound. On the concrete front, the Romans have us beat.
[via The Washington Post]