It turns out that the Roman era concrete not only is more durable than modern concrete, it actually gets stronger with age. Almost 2000 years ago, round A.D. 79, Roman author Pliny the Elder wrote in his Naturalis Historia that concrete structures in harbors, exposed to the constant assault of the saltwater waves, become “a single stone mass, impregnable to the waves and every day stronger.” He may well have been right.
Concrete is something that we all see but very few think about. Many use the terms concrete and cement interchangeably, but that's not correct. Concrete is a mix of sand and rock called aggregate that's bonded together today with Portland cement, a mixture of silica sand, limestone, clay, chalk and other ingredients melted together at blistering temperatures.
The Romans, though, didn't have the techniques for producing Portland cement that we have today.
Romans made concrete by mixing volcanic ash with lime and seawater to make a mortar, and then incorporating into that mortar chunks of volcanic rock, the “aggregate” in the concrete. The combination of ash, water, and quicklime produces what is called a pozzolanic reaction, named after the city of Pozzuoli in the Bay of Naples. The Romans may have gotten the idea for this mixture from naturally cemented volcanic ash deposits called tuff that are common in the area, as Pliny described.Modern concrete depends on the aggregate not interacting chemically with the Portland cement. The Roman concrete doesn't utilize that chemistry. It turns out, this might be the reason their concrete gets stronger with time and ours erodes away.
The story of this research is from University of Utah geologist Marie Jackson, who became interested in the Roman concrete formations while on a sabbatical trip. This led her to study Roman concrete with the tools a physical geologist might use to analyze the microstructures formed in it. She and her colleagues have found that seawater filtering through the concrete leads to the growth of interlocking minerals that hold the concrete together with increasing cohesion.
In another study of drill cores of Roman harbor concrete collected by the ROMACONS project in 2002-2009, Jackson and colleagues found an exceptionally rare mineral, aluminous tobermorite (Al-tobermorite) in the marine mortar. The mineral crystals formed in lime particles through pozzolanic reaction at somewhat elevated temperatures. The presence of Al-tobermorite surprised Jackson. “It’s very difficult to make,” she says of the mineral. Synthesizing it in the laboratory requires high temperatures and results in only small quantities.
For the new study, Jackson and other researchers returned to the ROMACONS drill cores, examining them with a variety of methods, including microdiffraction and microfluorescence analyses at the Advanced Light Source beamline 12.3.2 at Lawrence Berkeley National Laboratory. They found that Al-tobermorite and a related zeolite mineral, phillipsite, formed in pumice particles and pores in the cementing matrix. From previous work, the team knew that the pozzolanic curing process of Roman concrete was short-lived. Something else must have caused the minerals to grow at low temperature long after the concrete had hardened. “No one has produced tobermorite at 20 degrees Celsius,” she says. “Oh — except the Romans!”
The team concluded that when seawater percolated through the concrete in breakwaters and in piers, it dissolved components of the volcanic ash and allowed new minerals to grow from the highly alkaline leached fluids, particularly Al-tobermorite and phillipsite. This Al-tobermorite has silica-rich compositions, similar to crystals that form in volcanic rocks. The crystals have platy shapes that reinforce the cementing matrix. The interlocking plates increase the concrete’s resistance to brittle fracture.Unfortunately, the Roman recipes for concrete have been lost to the ages. Dr. Jackson and her colleagues continue to research possible ways of recreating the recipes. She is now working with geological engineer Tom Adams to develop a replacement recipe, using materials from the western U.S. The seawater in her experiments comes from the Berkeley, California, marina, collected by Jackson herself.
Jackson says that this corrosion-like process would normally be a bad thing for modern materials. “We’re looking at a system that’s contrary to everything one would not want in cement-based concrete,” she says. “We’re looking at a system that thrives in open chemical exchange with seawater.”
So why is 2000 year-old Roman concrete better than modern concrete? It seems pure luck. They were in an area where the chemistry of their volcanic ash with their lime and their rocks would work properly. It might not be replicable with those ingredients from everywhere in the world, but they observed, took notes and experimented, eventually making the most of what they had.
Roman concrete wouldn't be applicable everywhere. It's weaker in compression than modern concrete, for example, but would shine in areas exposed to saltwater. It's conceivable that a return to the Roman recipes, or something researchers could come up with based on the Romans' work, could make docks, piers or all sorts of structures in salt water or lagoons much more durable than modern concrete would. It's also possible this research will lead to ways to make something that is better than both our modern concrete and the Roman version.