How have Roman structures lasted so long? New research finds concrete answers
Ancient Israeli coastal city of Caesarea composed of surprising mortar recipe, Utah University geologist discovers
Deputy Editor Amanda Borschel-Dan is the host of The Times of Israel's Daily Briefing and What Matters Now podcasts and heads up The Times of Israel's Jewish World and Archaeology coverage.
A team of Utah University geologists has discovered the secret behind the staying power of structures built from Roman cement. Up to 2,000 years old, these man-made landmarks, such as the harbor at Caesarea Maritima in Israel, endure where modern cement buildings crumble.
To find the key, a team led by University of Utah geologist Prof. Marie Jackson looked to its elders. Pliny the Elder, that is.
In his first century CE “Naturalis Historia,” Pliny the Elder wrote about rock-like processes involving volcanic ash “that 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 and every day stronger,” as quoted by Jackson in a new study released this week in American Mineralogist.
After testing samples of Roman concrete from several Roman ports in Italy, Jackson’s team concluded, “Long-term chemical resilience of the concrete evidently relied on water-rock interactions, as Pliny the Elder inferred.”
In studying the drill cores of Roman harbor concrete, Jackson and her team noted the rare mineral aluminous tobermorite (Al-tobermorite) in the makeup of the mortar. According to a Phys.org article on the newly released study, this type of mineral crystals “formed in lime particles through pozzolanic reaction at somewhat elevated temperatures.” Since Al-tobermorite is difficult to make even in the best modern laboratory, its presence surprised Jackson.
“As geologists, we know that rocks change,” Jackson told Phys.org. “Change is a constant for earth materials. So how does change influence the durability of Roman structures?”
Perhaps equally remarkable to the Roman builders’ intuition of the chemical processes involved in creating the concrete-stabilizing mineral Al-tobermorite is the standardization of their building materials for projects far-flung throughout the Roman Empire.
The Israeli harbor of Caesarea Maritima, named in honor of Caesar Augustus, the founder of the Roman Empire, was established by Herod the Great, who ruled in 37–4 BCE, according to the Biblical Archaeological Society. “Herod no doubt built this great harbor to satisfy a practical need, for there was no other sheltered anchorage along the route from Alexandria, in Egypt, to the ports of Syria and Asia Minor,” according to BAS.
After Herod, the maritime city and its massive harbor — the largest of its time — became the seat of the local Roman governor.
The construction of the city’s breakwaters was an ingenious use of dense “hydraulic concrete,” a mixture of mortar and volcanic sand imported from Italy called pulvis puteolanus (mined near the Bay of Naples), pumice and lime.
“The Romans shipped thousands and thousands of tons of that volcanic ash around the Mediterranean to build harbors from the coast of Italy to Israel to Alexandria in Egypt to Pompeiopolis in Turkey,” Jackson told Smithsonian Magazine.
According to archaeologist Kenneth G. Holum, “Herod’s men built the breakwaters by laying a series of immense concrete blocks on the sea bottom, forming a chain of artificial islands that were then joined by more conventional masonry.”
The mixture of the seawater with the concrete formula made for an extremely stable structure. Whereas seawater damages modern concrete, in Roman concrete, the Pulvis Puteolanus “actually plays a role in mitigating deterioration when water percolates through it,” Jackson told Smithsonian Magazine.
Jackson’s team found that the percolation of seawater through the concrete “dissolved components of the volcanic ash and allowed new minerals to grow from the highly alkaline leached fluids, particularly Al-tobermorite and phillipsite,” according to Phys.org.
‘We’re looking at a system that’s contrary to everything one would not want in cement-based concrete’
“We’re looking at a system that’s contrary to everything one would not want in cement-based concrete. We’re looking at a system that thrives in open chemical exchange with seawater,” Jackson told Phys.org.
Although Jackson has spent years poring over Roman texts in search of the secret recipe, it was “completely lost,” she said.
However, as modern building engineers contemplate upcoming projects, Jackson told The Guardian, she hopes her work will open up their minds to potential alt-neu processes.
“I think [the research] opens up a completely new perspective for how concrete can be made – that what we consider corrosion processes can actually produce extremely beneficial mineral cement and lead to continued resilience, in fact, enhanced perhaps resilience over time,” said Jackson.
If the Caesarea Maritima harbor is any indication, it would be wise to listen to our Roman elders.
