Roman concrete has a formidable reputation, with coastal structures made from the material remaining standing even after more than 1500 years of history. In contrast, modern concrete breaks down within decades when exposed to saltwater. Now researchers have found out why.
Roman concrete was made by mixing volcanic ash, quicklime, and volcanic rock. While the components of the material have long been known, the exact recipe and the secrets behind its durability had been lost to time.
The research team, led by Marie Jackson, a geologist from the University of Utah, examined Roman concrete cores from Roman piers, breakwater and other marine structures on a microscopic level, using spectroscopic tests and imaging techniques. They found that the long lasting properties of the concrete was thanks to the addition of seawater, and the fact that the material was made to interact with the environment it is in.
Modern concrete, on the other hand, is designed to stay inert, and any interaction with the environment over time causes it to be damaged.
In Johnson’s tests, the team found that when seawater percolates through small cracks in the concrete, it reacts with the volcanic material, forming aluminous tobermorite and phillipsite crystals. Over time, the crystals form throughout the fabric of the concrete, reinforcing it and preventing cracks from growing.
Thus, rather than being corroded by seawater, Roman concrete is reinforced by the processes usually considered to be damaging to modern concrete.
This confirmed historical accounts by Pliny the Elder, who wrote in “Natural History” that Roman concrete “as soon as it comes into contact with the waves of the sea and is submerged, becomes a single stone mass, impregnable to the waves.”
According to Jackson, the research opens up a new perspective for how concrete can be made, and how materials can be engineered to work together with their environment in order to develop resilience.
Jackson is now working to re-create Roman concrete using seawater in San Francisco. Possible applications include more durable construction material for marine structures like seawalls, more environmentally friendly concrete creation processes, etc.
The biggest challenge for the researchers is fine-tuning the mixes of material, and developing methods that can utilise common volcanic products.