Mixed in with the volcanic ash mortar was more volcanic rock as aggregate, which would then continue to react with the material, ultimately making Roman cement far more durable than you'd think it should be.
In a previous research project led by Jackson, the team had already gathered samples of Roman marine concrete from several ports along the Italian coast. Drilling for Roman concrete samples in Tuscany, Photo: J. Now the researchers mapped the samples using an electron microscope, before drilling down to an extremely high resolution with X-ray microdiffraction and Raman spectroscopy.
With these advanced techniques they could identify all the mineral grains produced in the ancient concrete over centuries. Jackson was particularly interested in the presence of aluminous tobermorite, a hardy silica-based mineral that's actually pretty rare and difficult to make in the lab, yet is abundant in the ancient concrete. As it turns out, aluminous tobermorite and a related mineral called phillipsite actually grows in the concrete thanks to the sea water sloshing around it, slowly dissolving the volcanic ash within and giving it space to develop a reinforced structure from these interlocking crystals.
Something else must have caused the minerals to grow at low temperature long after the concrete had hardened. So how does change influence the durability of Roman structures? 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 team concluded that when seawater percolated through the concrete in breakwaters and in piers, it dissolved components of the volcanic ash and allowed new minerals to grow from the highly alkaline leached fluids, particularly Al-tobermorite and phillipsite. This Al-tobermorite has silica-rich compositions, similar to crystals that form in volcanic rocks. The crystals have platy shapes that reinforce the cementing matrix.
Jackson says that this corrosion-like process would normally be a bad thing for modern materials. She is now working with geological engineer Tom Adams to develop a replacement recipe, however, using materials from the western U.
The seawater in her experiments comes from the Berkeley, California, marina, collected by Jackson herself. Roman concrete takes time to develop strength from seawater, and features less compressive strength than typical Portland cement. Jackson recently weighed in on a proposed tidal lagoon to be built in Swansea, United Kingdom, to harness tidal power.
The lagoon, she says, would need to operate for years to recoup the costs incurred to build it. Jackson says that while researchers have answered many questions about the mortar of the concrete, the long-term chemical reactions in the aggregate materials remain unexplored.
History contains many references to ancient concrete, including in the writings of the famous Roman scholar Pliny the Elder, who lived in the 1st century A. Vesuvius in A. Pliny wrote that the best maritime concrete was made from volcanic ash found in regions around the Gulf of Naples, especially from near the modern-day town of Pozzuoli. Its virtues became so well-known that ash with similar mineral characteristics—no matter where it was found in the world—has been dubbed pozzolan.
By analyzing the mineral components of the cement taken from the Pozzuoli Bay breakwater at the laboratory of U. They found that the Romans made concrete by mixing lime and volcanic rock to form a mortar.
To build underwater structures, this mortar and volcanic tuff were packed into wooden forms.
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