Why did Roman concrete last longer than many modern materials?
What do the mythical monster Hydra, human bones, and Roman concrete all have in common?
They are self-healing. When they break, they restore.
That is the secret to why Roman concrete lasts longer than many modern materials.
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Roman concrete, opus caementicium, is durable, versatile, and self-healing. These properties have enabled the likes of the Pantheon, ancient Roman aqueducts, and even parts of the Colosseum to survive two millennia, while some Brutalist buildings from the 1960s that were made from modern concrete are crumbling.
The recipe for making Roman concrete was believed to have been lost in ancient history. The Romans used a technique called hot mixing, in which quicklime was added directly to volcanic ash, water, sand, and aggregate. The hot mixing allowed for fast setting times and therefore quick production of concrete.
For years, researchers assumed volcanic ash from the area of Pozzuoli, Italy, called pozzolanic material, was responsible for the durability of Roman concrete. Although pozzolanic material does strengthen the structure, researchers now believe that the lime clasts unlock the concrete’s self-healing power. Lime clasts are tiny white chunks of limestone that were previously mistaken for evidence of sloppy mixing. Hot mixing left these lime clasts embedded throughout the mortar. Because limestone is rich in calcium, these clasts act as calcium reservoirs. When cracks form in Roman concrete, water seeps in and dissolves calcium from the lime clasts. The calcium then fills the cracks and hardens into new mineral material, effectively sealing the damage.
The ultimate test of lime clasts’ success is exposure to the elements. For example, when a bridge, tower, or seawall made from Roman concrete was affected by storms, earthquakes, or the ocean, cracks formed. These cracks were filled by the lime clasts that reacted with salt or rainwater to recrystallize. Voilà—self-healing concrete. Far less terrifying than regrowing multiple heads for every severed one, like the Hydra.
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The discovery of this technique was published in 2023 by an international research team from the U.S. universities Massachusetts Institute of Technology and Harvard University; the Archaeological Museum of Priverno, Italy; DMAT, a concrete and mortar manufacturer headquartered in Udine, Italy; and the Istituto Meccanica dei Materiali, a laboratory and consulting firm based in Montagnola, Switzerland. The researchers were cemented in history for their use of high-resolution multiscale imaging and chemical mapping techniques to identify the lime clasts.
This team of researchers sees potential for the application of Roman concrete in the 21st century. Modern concrete is the most produced material on Earth, and as the planet’s urban population grows, cities are constantly in need of more for the construction of buildings, roads, bridges, and other infrastructure. However, the production of cement, a key ingredient in modern concrete, accounts for about 8 percent of global greenhouse gas emissions. Roman concrete could in theory provide a more sustainable long-term alternative to modern concrete. The potential benefits of Roman-inspired concrete include:
- Longer service life of concrete constructions
- More durable 3D-printed concrete
- Stronger seawall structures with other marine use cases
- Better waste management barriers
Even with these benefits, however, Roman concrete is not a direct universal replacement for modern concrete. Roman concrete depended on particular materials, especially lime and reactive volcanic ash, and there is not currently enough identified, qualified, accessible, and economically transportable supplies of volcanic ash to recreate Roman concrete for the world’s entire concrete demand. Other challenges include protecting workers from the extreme heat and hazardous alkaline conditions involved in the production of Roman concrete; incorporating digital design tools into an ancient practice; updating existing building codes to accommodate new materials; coordinating between engineers, insurers, scientists, and regulators; and addressing the corrosion of steel reinforcement, a common cause of deterioration in modern concrete structures.
Ultimately, a blend of ancient knowledge and modern engineering will pave the path for long-lasting and sustainable construction. Roman concrete should not be taken for granite.