The Romans developed a standard formula for making mortar about two millennia ago.

It reveals key discoveries in understanding the longevity of ancient Roman architectural concrete used in many concrete monuments that has withstood the elements for nearly 2,000 years.

The crucial ingredient, volcanic sand, has long been thought to be the main reason for the longevity of Rome's ancient monuments, including Hadrian's Temple, Trajan's Market and the Baths of Diocletian.

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Marie Jackson of the University of California Berkeley's Department of Civil and Environmental Engineering, lead author of the study, and her colleagues used x-ray beams at the Advanced Light Source (ALS) of the U.S. Department of Energy's Lawrence Berkeley National Laboratory.

"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," explained Dr. Jackson.

Dr. Jackson stated that mortar resists microcracking because of in situ crystallization of platy stratlingite, which is a durable calcium-alumino-silicate minerals that strengthens the inter-facial 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," said Dr. Jackson.

Aside from its unmatched resilience and durability, Roman volcanic ash-lime mortar also offers environmental advantages.

Most modern concrete is bound by limestone-based Portland cement, which is a combination of limestone and clay.

On the other hand, Roman mortar is a mixture of about 85 percent by volume volcanic ash, fresh water, and lime, which is calcined at much lower temperatures compared to Portland cement.

Dr. Jackson and her team hopes to find ways to include a substantial volumetric component of volcanic rock in the production of specialty concretes, so it can reduce carbon emissions associated with its production.

The details of the study were published in the journal Proceedings of the National Academy of Sciences.

TagsRoman Architecture, Ancient Rome, Roman Monuments, Baths of Diocletian, mortar, volcanic ash-lime mortar, modern concretes, volcanic sand