Mystery of Ultrastrong Roman Concrete Solved

Italy’s shores are dotted with Roman sea walls that have withstood 2,000 years of relentless beating by salty waves, conditions that would reduce our concrete structures to rubble within years. What’s more, the Roman variant strengthened under the exposure. How, until now, has remained a mystery.

Scientists analyzed the chemistry of the ruins from four sites along Italy’s Mediterranean coast, discovering that the Roman concrete was made of rare volcanic ash, minerals, lime, and chunks of volcanic rock.

Roman sea wall and pier remnants like this one in Portus Cosanus, Tuscany, were tested up and down the coast.

The secret to its strength was a combination of a rare crystal and a porous mineral. When exposed to seawater, a chemical reaction within the concrete generated heat, which allowed crystals of aluminum tobermorite to grow from a mineral known as phillipsite, found in volcanic ash. As seawater collided with the cement over the years, the crystals kept growing, reinforcing the concrete. The tobermorite grew inside of the mixture, binding to the rocks that bulk it up and preventing cracks that typically cause concrete to fail.

Aric Jenkins, reporting for Time:

“Contrary to the principles of modern cement-based concrete, the Romans created a rock-like concrete that thrives in open chemical exchange with seawater,” lead author Marie Jackson from the University of Utah said in the journal.

“It’s a very rare occurrence in the Earth,” she added.

This microscopic image shows the lumpy calcium-aluminum-silicate-hydrate (C-A-S-H) binder material that forms when volcanic ash, lime, and seawater mix. Platy crystals of Al-tobermorite have grown amongst the C-A-S-H in the cementing matrix.

Modern buildings are made using Portland cement, comprised of limestone, sandstone, ash, chalk, iron, and clay. Rocks and sand are then used as aggregates to bulk up the mixture in place of the volcanic rock that the Romans could use. While our cement today is very strong, it can erode when exposed to harsh environments like seawater. To provide strength and resist structural failure, Portland cement relies on steel reinforcement, but sea water causes the cement to expand and the steel to corrode.

Producing Portland cement is also environmentally costly—its production is responsible for 5% of global emissions of CO2. The Roman alternative could be used for more sustainable buildings that could withstand the elements for longer.

For now, this new discovery will be hard to apply to today’s concrete. Scientists still need to conduct tests to fully understand the exact combination of minerals and compounds used in the Italian sea walls Plus, the volcanic rock used by ancient Romans is extremely rare, so a modern version would have to rely on different materials.