Study Reveals Corrosive Seawater Strengthened Ancient Roman Structures

Roman Author Pliny the Elder in his book ‘Naturalis Historia’ written around A.D. 79, stated that concrete structures in harbors, exposed against the constant battering of the saltwater waves, become “a single stone mass, impregnable to the waves and every day stronger.”

This was not a misrepresentation. While present-day oceanic concrete structures crumble in a matter of decades, 2,000 year old Roman breakwaters and piers are still intact to this day, and are stronger today than when they were first built. Geologist Marie Jackson from University of Utah studies the microscale structures and minerals of Roman concrete in the same manner as she would a volcanic rock. She and her co-workers have discovered that seawater filtering through the concrete results in the growth of interlocking minerals that provide the concrete extra cohesion. The results of the research are published in American Mineralogist.

Roman concrete vs. Portland cement

Romans created concrete by blending volcanic ash with seawater and lime to make a mortar, and then adding pieces of volcanic rock into that mortar to act as the “aggregate” in the concrete. The combination of quicklime, water and ash produces a pozzolanic reaction, named after the city of Pozzuoli in the Bay of Naples. The Romans may have gotten the idea for this mixture from naturally cemented volcanic ash deposits known as tuff that are generally found in the area, as Pliny depicted.

The conglomerate-like concrete was used in multiple architectural structures, including the Trajan’s Markets and Pantheon in Rome. Enormous marine structures protected harbors from the ocean and acted as wide anchorages for warehouses and ships.

Modern Portland cement concrete also uses rock aggregate, but with a crucial variation: the gravel particles and sand are meant to be inert. Any reaction with the cement paste could result in the formation of gels that expand and break the concrete.

This alkali-silica reaction occurs throughout the world and it’s one of the main causes of destruction of Portland cement concrete structures.

Marie Jackson, Geologist, University of Utah

Rediscovering Roman concrete

During Jackson’s sabbatical year in Rome, her interest in Roman concrete began. She initially examined tuffs and then explored volcanic ash deposits, soon becoming enthralled with their roles in producing the extraordinary sturdiness of Roman concrete.

Jackson along with her co-workers started examining the factors that made architectural concrete in Rome so strong. One factor, she says, is that the mineral intergrowths between the aggregate and the mortar prevent fractures from expanding, while the surfaces of non-reactive aggregates in Portland cement only aid cracks to spread further.

In another research of drill cores of Roman harbor concrete performed by the ROMACONS project in 2002-2009, Jackson and her co-workers discovered a highly rare mineral, aluminous tobermorite (Al-tobermorite) in the marine mortar. The mineral crystals formed in lime particles through pozzolanic reaction at moderately elevated temperatures. The presence of Al-tobermorite amazed Jackson. “It’s very difficult to make,” she says of the mineral. Synthesizing it in the laboratory demands elevated temperatures and results in only small amounts.

Seawater corrosion

For the new research, Jackson and other Researchers looked back at the ROMACONS drill cores, studying them using a range of techniques, including microfluorescence and microdiffraction analyzes at the Advanced Light Source beamline 12.3.2 at Lawrence Berkeley National Laboratory. They learnt that Al-tobermorite and an associated zeolite mineral, phillipsite, formed in pumice particles and pores in the cementing matrix. From earlier research, the team realized that the pozzolanic curing process of Roman concrete was brief. Some other factor must have caused the minerals to form at low temperatures long after the concrete had hardened.

No one has produced tobermorite at 20 degrees Celsius. Oh — except the Romans! As Geologists, we know that rocks change. Change is a constant for earth materials. So how does change influence the durability of Roman structures?

Marie Jackson, Geologist, University of Utah

The team concluded that when seawater permeated through the concrete in piers and in breakwaters, it dissolved parts of the volcanic ash and allowed new minerals to develop from the very alkaline leached fluids, primarily phillipsite and Al-tobermorite. This Al-tobermorite contains silica-rich compositions, akin to crystals that develop in volcanic rocks. The crystals are platy-shaped and strengthen the cementing matrix. The interlocking plates boost the concrete’s resistance to brittle breakage.

Jackson explains that this corrosion-like process would generally be a negative thing for advanced materials, “We’re looking at a system that’s contrary to everything one would not want in cement-based concrete,” she says. Jackson also added, “We’re looking at a system that thrives in open chemical exchange with seawater.”

Modern Roman concrete

Given the hardiness benefits of Roman concrete, why is it not used more frequently, especially since manufacturing of Portland cement creates considerable carbon dioxide emissions?

“The recipe was completely lost,” Jackson says. She has comprehensively researched ancient Roman texts, but has not yet discovered the precise techniques for mixing the marine mortar, to totally recreate the concrete.

Romans were fortunate in the type of rock they had to work with. They observed that volcanic ash grew cements to produce the tuff. We don’t have those rocks in a lot of the world, so there would have to be substitutions made.

Marie Jackson, Geologist, University of Utah

She is currently working with Geological Engineer Tom Adams to make a replacement recipe, but, using materials from Western U.S.A. The seawater used in her experiments was collected by Jackson from the Berkeley, California, marina.

Roman concrete needs time to form strength from seawater, and features less compressive strength than regular Portland cement. For those reasons, it is doubtful that Roman concrete could become widespread, but could be beneficial in specific contexts.

Jackson lately weighed in on a planned tidal lagoon to be constructed in Swansea, UK, to harness tidal power. She says that the lagoon would have to operate for over a century to recover the costs incurred to construct it, “You can imagine that, with the way we build now, it would be a mass of corroding steel by that time.” A Roman concrete prototype, in comparison, could stay undamaged for centuries.

Jackson states that while Researchers have found answers to a number of questions regarding the mortar of the concrete, the long-term chemical reactions in the aggregate materials are yet to be studied. She plans to pursue the research work of Pliny and other Roman Scholars who worked diligently to discover the mysteries of their concrete, “The Romans were concerned with this,” Jackson says. She added that, “If we’re going to build in the sea, we should be concerned with it too.”