Study Reveals Key To Longevity Of Imperial Roman Monuments

Researchers have made a key discovery in helping to understand the longevity and endurance of Roman architectural concrete that was made during international and interdisciplinary collaboration with help from beams of X-rays at the Advanced Light Source (ALS) of the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab).

Researchers worked with an ALS beamline, in which the team studied a reproduction of Roman volcanic ash-lime mortar that had been subjected to fracture via testing experiments at Cornell University. By observing certain mineralogical changes, researchers found that the curing of the mortar over a period of 180 days when compared with 1,900 year old samples of the original showed crystalline binding hydrate prevents microcracks from propagating.

"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," said lead study author Marie Jackson, a faculty scientist with the University of California (UC) Berkeley's Department of Civil and Environmental Engineering, in a news release. "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."

For the study, researchers used ALS beamline 12.3.2. to make X-ray micro-diffraction measurements of slices of the Roman mortar that were just about 0.3 millimeters thick.

"We obtained X-ray diffractograms for many different points within a given cementitious microstructure," Jackson added. "This enabled us to detect changes in mineral assemblages that gave precise indications of chemical processes active over very small areas."

Minenrlogical changes revealed that mortar reproduction gained strength and toughness over 180 days as calcium-aluminum-sillicate-hydrate (C-A-S-H) cementing binder coalesced and strätlingite crystals grew in interfacial zones between volcanic scoria and the mortar matrix. The toughening of these interfacial zones is reflected in the bridging crack morphology, which was measured by co-author Landis at the University of Maine, using computed tomography scans of the fractured mortar specimens.

Yet researchers noted that the strätlingite crystals showec no corrosion and their smooth surfaces suggest long-term stability, similar to geological strätlingite that persists for hundreds of thousands of years.

"The in situ crystallization of the strätlingite crystals produces interfacial zones that are very different from any interfacial microstructure observed in Portland cement concretes," Jackson concluded. "High porosity along the interfacial zones of inert aggregates in Portland cement concrete creates the sites where crack paths first nucleate and propagate."

In the future, researchers said they hope to activate aggregates as slag or volcanic ash to in innovative concretes so as to develop strätlingite reinforcements in interfacial zones like the Roman architectural mortars.

More information regarding the findings can be seen via the journal Proceedings of the National Academy of Sciences (PNAS) or via this article. 

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