No visit to Rome is complete without a visit to the Pantheon, Trajan’s Markets, the Colosseum, or the other spectacular examples of ancient Roman concrete monuments that have stood the test of time and the elements for nearly two thousand years.
All of these building have weathered earthquakes, barbarian invasions and the persistent onslaught of Mother Nature. For years researchers have figured there must be something special about the concrete used to build the Roman monuments that lend them to such longevity. The key ingredient is a specific blend of limestone and volcanic ash used in the mortar.
In a new study by an international and interdisciplinary collaboration of researchers a key discovery has been made about the composition of Roman concrete using beams of X-rays.
Mixing mortar according to the recipe of 1st century Roman architect Vitruvius and then letting it harden for 180 days proves that the mortar includes dense clusters of a durable mineral called strätlingite. The crystals formed because of a reaction that took place over time between the lime and the volcanic matter in the mortar. This helped prevent microscopic cracks by reinforcing interfacial zones. The strätlingite crystals provide superior reinforcement and are resistant to corrosion.
By comparison, Portland cement (the most common modern concrete blend) lacks the lime-volcanic ash combination, and doesn’t bind well compared with Roman concrete. Portland cement, in use for almost two centuries, tends to wear particularly quickly in seawater, with a service life of less than 50 years.
Another advantage of the Roman recipe is that it’s more environmentally friendly than Portland cement. 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.