Unlike modern concrete, which can rapidly deteriorate in marine environments, Roman concrete thrives in open chemical exchange with seawater. Most modern concrete is a mix of Portland cement and aggregates—materials such as sand or crushed stone that are not intended to chemically react over time. Roman concrete was made from volcanic ash, lime (calcium oxide), and seawater. Previous ALS studies showed that the lime, upon exposure to seawater, reacted with the volcanic ash to produce aluminous tobermorite (Al-tobermorite), a layered mineral that forms fine fibers and plates.
To better understand the longer-term chemical processes in the concrete, researchers studied samples from ancient pier and breakwater sites using a variety of techniques, including electron microscopy and Raman spectroscopy. At ALS Beamline 12.3.2, they used x-ray microdiffraction to trace the complex sequences of crystal growth at the micron scale. The results indicate that Al-tobermorite and a related zeolite mineral called phillipsite continue to form over millennia as seawater percolates through the massive structures, reinforcing the cementing matrix in a kind of regenerative process that strengthens the concrete.
The researchers suggest that a reformulated recipe for Roman concrete could be tested for applications such as seawalls and other ocean-facing structures, and may be useful for safeguarding hazardous wastes, since Al-tobermorite has special cation-exchange properties. The work ultimately could lead to a wider adoption of concrete manufacturing techniques with less environmental impact than modern cement manufacturing processes, which require high-temperature kilns. These are a significant contributor to industrial carbon dioxide emissions, which add to the buildup of greenhouse gases in Earth’s atmosphere.
Work performed at ALS Beamline 12.3.2.
M.D. Jackson, S.R. Mulcahy, H. Chen, Y. Li, Q. Li, P. Cappppelletti, and H.-R. Wenk, “Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete,” Am. Mineral. 102, 1435 (2017), doi:10.2138/am-2017-5993CCBY.
Adapted from the Berkeley Lab news release, “New Studies of Ancient Concrete Could Teach Us to Do as the Romans Did” and the University of Utah news release, “How Seawater Strengthens Ancient Roman Concrete.”