In De Architectura, Vitruvius described with amazement a building material mastered by the Romans: “There is also a kind of powder which from natural causes produces astonishing results. … This substance, when mixed with lime and rubble, not only lends strength to buildings of other kinds, but even when piers of it are constructed in the sea, they set hard under water.”
Today, concrete continues to enjoy unprecedented popularity in building construction. It is the most common manmade material on earth, the second most consumed substance after water, and the veritable foundation of contemporary society. Although modern concrete may be considered an advanced building material, it pales in comparison to the original Roman formulation. Simply compare today’s version—which often shows degradation after 50 years—with the Roman monuments that still stand after two millennia and the underwater Roman structures that show little decay despite their harsh marine environments.
Scientists at the University of California, Berkeley, recently studied Roman marine concrete to understand the ancient material’s secrets. Using X-ray spectroscopy on samples excavated from a harbor near Tuscany, Italy, they found evidence of the stable compound calcium-aluminum-silicate-hydrate (C-A-S-H). By contrast, modern Portland cement contains calcium-silicate-hydrate (C-S-H). The researchers allege that the addition of aluminum and the reduced amount of silicon in the Roman version result in its superior longevity. They also found that Roman concrete contains crystal lattices made of aluminum tobermorite, a hydration product that improves stiffness—and which modern Portland cement lacks.
Roman concrete is also less carbon intensive. In Portland cement, the limestone and clay mixture must be heated to 1,450 C (2,642 F). The fuel required to reach this temperature—coupled with the carbon released from the resulting calcium carbonate—emits significant amounts of greenhouse gas. Meanwhile, Roman cement used less lime, which was made from limestone heated at 900 C (1,652 F). This reduction in processing temperature and lime content may be the key to reducing concrete’s high carbon footprint.
By incorporating pozzolan or volcanic ash materials from regions with large natural deposits, neo-Roman concrete could offset 40 percent of the Portland cement used today, the Berkeley researchers estimate. However, mining more pozzolan is not the only answer. The industrial waste products flyash, slag, and silica fume, which are used to offset Portland cement, perform similarly to natural pozzolan. The research suggests that a more thorough study of the C-A-S-H compounds in these materials would determine which most closely approximate the binding characteristics of Roman concrete. Such research could lead to improvements in the longevity and environmental footprint of concrete without the need for additional mining.