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Finding the Key to an Immortal Mortar

Wednesday, December 24, 2014

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For 2,000 years, builders have sought the secret to the structural strength of The Colosseum, The Pantheon and other ancient treasures.

Now, scientists say, they have it.

Using advanced X-ray technology, an international team of researchers has examined the mortar used to bind the concrete structures in order to understand their longevity and endurance.

Roman concrete
Marie Jackson

The concrete walls of Trajan's Markets in Rome have stood the test of time and the elements for almost 2,000 years. They even survived a major earthquake in 1349.

Results of the research, conducted at Lawrence Berkeley National Laboratory’s Advanced Light Source, were announced Dec. 15. The findings were also published in the Proceedings of the National Academy of Sciences.

The study was led by Marie Jackson, a volcanologist and faculty scientist with the University of California Berkeley’s Department of Civil and Environmental Engineering.

Longevity Unlocked

The mortars used to bind the structures of ancient Rome consisted of a mixture of 85 percent (by volume) volcanic ash, fresh water and lime, which is calcined at a much lower temperature than modern cement.

Chunks of volcanic tuff and brick make up 45 to 55 percent (by volume) of the concrete.

Roman concrete
Roy Kaltschmidt, Berkeley Lab

Ancient Roman concrete consists of coarse chunks of volcanic tuff and brick bound together by a volcanic ash-lime mortar that resists microcracking.

Working with an ALS beamline, the scientists studied a reproduction of Roman volcanic ash-lime mortar that had been subjected to fracture testing experiments at Cornell University, according to Berkeley Lab.

By observing the mineralogical changes that took place in the curing of the mortar over a period of 180 days and comparing the results to 1,900-year-old samples, the team found that a “crystalline binding hydrate” prevented microcracks from propagating, Berkeley Lab reports.

Crack Prevention

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

In other words, the strätlingite crystals make the mix more crack resistant. The strätlingite crystals also show no corrosion, and their smooth surfaces suggest long-term stability—similar to geological strätlingite that persist for hundreds of thousands of years.

“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,” she said.

Good for the Planet, Too

In addition to its durability and resilience, the mortars have environmental advantages, the team says.

research team
Roy Kaltschmidt, Berkeley Lab

(From left) Marie Jackson, Qinfei Li, Martin Kunz and Paulo Monteiro at ALS beamline 12.3.2, where they conducted their study on ancient Roman concrete.

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). The process releases enough carbon—given the 19 billion tons of Portland cement used annually—to account for about seven percent of the total carbon emitted into the atmosphere each year, according to the team.

The Roman cement did not require heating at such temperatures, providing a large 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 [and] also improve their durability and mechanical resistance over time,” Jackson added.

   

Tagged categories: Architecture; Cement; Coating Materials; Concrete; Durability; EMEA (Europe, Middle East and Africa); Historic Structures; Lawrence Berkeley National Laboratory; Mortars; Research

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