Around A.D. 79, Roman author Pliny the Elder wrote in his Naturalis Historia that concrete structures in harbours, exposed to the constant assault of the saltwater waves, become "a single stone mass, impregnable to the waves and every day stronger." He wasn't exaggerating. While modern marine concrete structures crumble within decades, 2,000-year-old Roman piers and breakwaters endure to this day, and are stronger now than when they were first built.
Now scientists have determined the mystery of why these ancient coastal structures are still standing. Researchers, led by University of Utah geologist Marie Jackson, looked at the microscopic structures of Roman concrete samples by subjecting them to numerous spectroscopic tests and imaging techniques. The tests showed a rare reaction took place that triggered the growth of aluminous tobermorite crystals. Further geology detective work proved that the crystals were formed when seawater seeped through the little cracks in the Roman concrete, reacting with the mineral phillipsite, found in volcanic rock.
The concrete used by Roman builders in piers and harbours was made in such a way that it grew ever stronger over time. Modern concrete, by comparison, tends to decay in just decades when exposed to saltwater. These findings could have an important role to play as many communities worldwide brace for rising sea levels.
Romans created concrete by mixing volcanic ash, quicklime and chunks of volcanic rock. Even through we've figured out the ingredients, scientists still don't know the recipe. How did the Romans manage to make the concrete so long-lasting? The key has turned out to be in the chemical reaction caused by the addition of seawater. However "the recipe was completely lost," Jackson says. She has extensively studied ancient Roman texts, but hasn't yet uncovered the precise methods for mixing the marine mortar, to fully re-create the concrete.
The Roman concrete was made to interact with its environment, as opposed to modern concrete which stays inert and gets damaged over time. Seawater is the reason why the Roman mixture gets stronger. As seawater reacts with volcanic material, new minerals are created that reinforce the concrete.
"Romans were fortunate in the type of rock they had to work with," she says. "They observed that effects of volcanic ash. We don't have those rocks in a lot of the world, so there would have to be substitutions made," Jackson added.
She is now working with geological engineer Tom Adams to develop a replacement recipe that hopefully will be just as strong and long-lasting as the Roman mix.
