Research

Solved: the secret of how Rome's concrete conquered the sea

Solved: the secret of how Rome's concrete conquered the sea

UTAH-While you might expect 2,000-year-old piers to be on their last legs, ancient structures built by the Romans are now stronger than ever - thanks to seawater. However, Jackson and other scientists found an extremely unusual component inside the concrete, namely Tobermorite group minerals.

Portus Cosanus pier, Orbetello, Italy.

Basically, the Romans used a concrete made from lime and volcanic ash to build sea walls, and the volcanic material reacted with the sea water in a way that actually strengthened the construction.

Around A.D. 79, in his book "Naturalis Historia" Roman Author Pliny the Elder wrote that concrete structures in harbors, unprotected against the constant assault of the saltwater waves, become "a single stone mass, impregnable to the waves and every day stronger".

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. The Romans may have gotten the idea for this mixture from naturally cemented volcanic ash deposits called tuff that are common in the area.

Modern concrete is typically made with portland cement, a mixture of silica sand, limestone, clay, chalk and other ingredients melted together at blistering temperatures. Any reaction with the cement paste could form gels that expand and crack the concrete.

Prof Jackson said: "The Romans were concerned with this".

Jackson was particularly interested in the presence of aluminous tobermorite, a hardy silica-based mineral that's actually pretty rare and hard to make in the lab, yet is abundant in the ancient concrete. The new study, published on Monday in American Mineralogist, is helping researchers to piece together how and where this mineral formed during the long history of the concrete structures.

The researchers wanted to know what made the Roman cement so resilient. The mineral crystals developed in lime particles through pozzolanic reaction at slightly elevated temperatures.

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The new growth of aluminous tobermorite is often associated with crystals of phillipsite, another mineral. The interlocking plates boost the concrete's resistance to brittle fracture.

"Contrary to the principles of modern cement-based concrete", Jackson says, "The Romans created a rock-like concrete that thrives in open chemical exchange with seawater".

She has extensively studied ancient Roman texts but hasn't yet uncovered the precise methods for mixing the marine mortar, to fully recreate the concrete.

The researchers coupled these analyses with a technique at ALS known as X-ray microdiffraction, and a technique at the University of California, Berkeley, known as Raman spectroscopy, to learn more about the structure of crystals in the samples. The research has also inspired a hunt for the original recipe so that modern concrete manufacturers can do as the Romans did. The machine produces beams focused to about 1 micron or about a hundred times smaller than what can be found in a conventional laboratory. The X-ray technique measures an average signal from many tiny mineral grains, providing high resolution and fast data collection.

"It's been astounding what we've been able to find". Something else must have caused the minerals to grow at low temperature long after the concrete had hardened.

A reformulated recipe could also be tested for applications such as seawalls and other ocean-facing structures, and may be useful for safeguarding hazardous wastes. One factor, she says, is that the mineral intergrowths between the aggregate and the mortar prevent cracks from expanding, while the surfaces of nonreactive aggregates in Portland cement only help cracks spread further. She initially studied tuffs and then examined volcanic ash deposits, soon becoming captivated with their roles in producing the extraordinary durability of Roman concrete.

Now scientists are unlocking the secrets of how these ancient concretes have grown stronger over time, research that suggests that creating modern concretes the same way could not only build more durable structures but also generate much less of the global warming gas carbon dioxide.

"I think people don't really know how to think about a material that doesn't have steel reinforcement".