The secrets of Roman architectural concrete

A group of scientists led by Dr Anthony Ingraffea of Cornell University has revealed clues to the longevity and endurance of such Imperial Roman monuments as the Colosseum and Pantheon.

The Romans developed a standard formula for making mortar about two millennia ago. This mortar binds cobble-sized fragments of tuff and brick, and it was used in the concrete walls of many monuments of Imperial Rome (from 27 BC, when Octavian became Emperor Augustus, through the 4th century CE).

As part of their study, Dr Ingraffea and his colleagues discovered that the long-term resilience of the Roman concrete is due to the mineralogical changes that occur as the volcanic ash-lime mortar cures.

The scientists studied a reproduction of the mortar as it cured over 180 days. They compared this material to samples dating back 1,900 years, and observed the formation of a durable calcium-alumino-silicate mineral that acts to bind and reinforce interfacial zones in the mortar, preventing obstacles to the growth of microcracks.

“We obtained X-ray diffractograms for many different points within a given cementitious microstructure. This enabled us to detect changes in mineral assemblages that gave precise indications of chemical processes active over very small areas,” said Dr Marie Jackson from the University of California, Berkeley, who is the first author of a paper published in the Proceedings of the National Academy of Sciences.

“The mortar resists microcracking through in situ crystallization of platystrtlingite, a durable calcium-alumino-silicate mineral that reinforces interfacial zones and the cementitious matrix.”

“The dense intergrowths of the platy crystals obstruct crack propagation and preserve cohesion at the micron scale, which in turn enables the concrete to maintain its chemical resilience and structural integrity in a seismically active environment at the millennial scale.”

Roman volcanic ash-lime mortar is of keen scientific interest not just because of its unmatched resilience and durability, but also for the environmental advantages it offer. 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 to account for about 7 % of the total amount of carbon emitted into the atmosphere each year.

Roman mortar, by contrast, is a mixture of about 85 % (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 % 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,” Dr Jackson said.
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Marie D. Jackson et al. Mechanical resilience and cementitious processes in Imperial Roman architectural mortar. PNAS, published online December 15, 2014; doi: 10.1073/pnas.1417456111

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