Researchers led by volcanologist Marie D. Jackson of the University of California at Berkeley have unlocked the secret of how the ancient blend of limestone and volcanic sand helped Roman monuments survive 2,000 years of earthquakes and floods.
They said the blend also left a much smaller carbon footprint than modern mortar, which has to be heated at high temperatures.
The researchers reproduced Roman mortar using Vitruvius’s recipe. They let it harden for 180 days, then examined it with X-ray equipment. They found dense clusters of a durable mineral called strätlingite had formed. The strätlingite crystals helped prevent the spread of microscopic cracks by reinforcing interfacial zones, which researchers called “the weakest link of modern cement-based concrete.”
The crystals formed because of a reaction that took place over time between the lime and volcanic matter in the mortar.
Rome lies between two volcanic districts, Alban Hills to the south and Monti Sabatini to the north. Roman builders experimented with different mortar recipes before settling on one that used red and black volcanic sand from the Pozzolane Rosse ash flow from an ancient Alban Hills volcano.
Emperor Augustus is credited with making pozzolonic mortar the standard in ancient Rome. The perfect mortar was a priority for Augustus, who initiated a monument building and repair campaign after he became the first emperor of Rome in 27 A.D.
Roman builders found it “substantially improved the margin of safety associated with increasingly daring structural designs,” the researchers wrote.
“The pozzolonic mortar perfected by Roman builders during first century A.D. is key to the durability of concrete components in structurally sound monuments well maintained over two millennia of use,” the researchers wrote.
Like modern cement, the Roman’s recipe starts with limestone. The builders heated it to produce quicklime then added water and volcanic ash at a ratio of three parts ash to one part lime.
They mixed the mortar with four-inch volcanic fragments to make concrete.
The strätlingite crystals observed by the researchers formed in tiny spaces called “interfacial zones” around sand and gravel-sized bits of volcanic material, binding them together more tightly and making the mixture more crack-resistant.
Strätlingite crystals are similar to microfibers added to modern cement to reinforce the interfacial zone where it is prone to crack. However, the strätlingite crystals provide superior reinforcement and are resistant to corrosion.
Another advantage of the Roman recipe is that its more environmentally friendly than Portland cement, the most popular modern variety. Portland cement is made by heating limestone at high temperatures, which burns enough fossil fuel to account for 7 percent of the total carbon emitted into the atmosphere each year, according to a press release from Berkeley.
If modern cement were tweaked to resemble the original Roman recipe, it could cut carbon emissions and make modern buildings more durable, the researchers claimed.