Built to Last
Roman concrete is a marvel of the ancient world. Unfortunately, the recipe for it was lost in history during the invasion of Rome. The Pantheon is still the world’s largest unreinforced concrete dome in the world – some 2,000 years after it was built. A recent discovery has helped scientists understand more about the methods and ingredients behind this almost indestructible material.
The Roman author Pliny the Elder (23 AD – 79 AD) wrote in his Naturalis Historia that concrete structures in harbours, which were exposed to the constant barrage of saltwater, become “a single stone mass, impregnable to the waves and every day stronger”. Pliny’s observation stands true to this day: While most modern marine concrete structures crumble within a few decades, centuriesold Roman piers and breakwaters remain standing, and are, in fact, stronger today than when they were first built centuries ago.
Romans used their highly robust concrete in several architectural builds, including the Pantheon and Trajan’s Market in Rome. It was used to create massive marine structures which protected harbours from the open sea, also acting as anchorages for ships and warehouses.
Marie Jackson, a geologist at the University of Utah in the United States, has spent years studying the minerals and
microscale structures of Roman concrete as she would a sample of volcanic rock. Jackson first took an interest in Roman concrete during a sabbatical year in the Italian city. She studied tuffs (naturally cemented volcanic ash deposits) and was fascinated by the role such materials played in producing the remarkable and durable Roman concrete.
Together with her colleagues, Jackson began studying what it was that made architectural concrete in Rome so resilient. Jackson and her team soon discovered that seawater filtering through the concrete resulted in the growth of interlocking minerals, and it was decided that these were responsible for the increased cohesion of the material. Once they’d figured this out, it remained to be uncovered exactly what the Romans put into their cement in the first place.
According to historians, the Romans made concrete by mixing volcanic ash with lime and seawater to make a mortar, before adding chunks of volcanic rock – the “aggregate” in the concrete. The combination of ash, water, and quicklime produces something called a pozzolanic reaction – named after the city of Pozzuoli in the Bay of Naples. It is thought that the Romans may have obtained the idea for this mixture from tuff, which was quite commonly found in the area. To put the Romans’ recipe into perspective, modern Portland cement concrete also uses rock aggregate but with one important difference: The sand and gravel particles are intended to be inert, as any reaction with the cement paste could form gels that expand and crack the concrete.
A project conducted by Jackson between 2002 and 2009 called the ROMACONS project, uncovered a remarkably rare mineral, aluminous tobermorite (Al-tobermorite), in the marine mortar. Jackson was surprised by the find seeing as this mineral was particularly difficult to make. She noted that synthesising it in a laboratory requires high temperatures and yields only small quantities. Jackson’s study of the drill cores began to get quite technical, the examinations revealing that Al-tobermorite and a related zeolite mineral, phillipsite, formed in pumice particles and pores in the cementing matrix. However, it was decided that something else had caused the minerals to continue growing, and for so long after the concrete had hardened.
After further extensive study, Jackson and her team concluded that when seawater permeated the concrete, it dissolved components of the volcanic ash, allowing new minerals – particularly Al-tobermorite and phillipsite – to grow from the highly alkaline leached fluids, and in so doing, increasing the concrete’s resistance and strength. Jackson said of this finding: “We’re looking at a system that’s contrary to everything one would not want in cement-based concrete. We’re looking at a system that thrives in open chemical exchange with seawater.”
With the secret to this material’s superstrength now revealed, the question arose as to why this concrete is not used more often in modern times. The answer is simple: because the recipe has been completely lost.
Despite Jackson’s extensive scrutiny of ancient Roman texts, the precise methods for mixing the marine mortar were nowhere to be found. However, even if the recipe was located, the type of rock the Romans worked with is uncommon in most of the world, and substitutions would thus have to be made. Jackson is determined to find a workable solution, however, and is currently collaborating with geological engineer Tom Adams to develop a replacement recipe, albeit one that uses materials from the United States and seawater from the Berkeley Marina in California.