More than 2000 years ago, the Roman Empire created a concrete that allowed them build enormous structures that have been able to withstand the test of time, including being exposed to water. For years, scientist have been left baffled by the ancient material’s ability to remain intact, but it seems that they may have finally made a breakthrough and discovered its secret.
Romans made their concrete by mixing volcanic ash with lime and seawater to make a mortar, and then incorporated mortar chunks of volcanic rock (the ‘aggregate’ in the concrete) into the mixture. This concrete was then used to build many architectural structures, including the Pantheon and Trajan’s Markets in Rome. Fast-forward to today sees modern Portland cement concrete also incorporating rock aggregate, but there’s a slight difference; the sand and gravel particles are inactive components in the mixture.
According to the findings in the journal American Mineralogist, the secret lies in the chemical properties of two of the ancient mixture’s components: lime and volcanic ash, which contained a rare mineral known as aluminium tobermorite. When exposed to seawater, the substance would crystallise in the lime while curing. Rather than be eroded by the water, its presence actually gave the material additional strength.
Using an electron microscope, X-ray micro-diffraction and Raman spectroscopy, the researchers found significant amounts of tobermorite growing through the composition of the concrete, and a related, porous mineral called phillipsite. Through continued exposure to seawater, the scientists predict, the crystals were able to keep growing over time, reinforcing the concrete and preventing cracks from forming.
“Contrary to the principles of modern cement-based concrete,” said lead author Marie Jackson, “the Romans created a rock-like concrete that thrives in open chemical exchange with seawater. It's a very rare occurrence in the Earth,” she added.
“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.