Scientist cracks ancient Rome’s concrete puzzle
ITALY: In the first century BC, the Roman writer Vitruvius wrote of ‘‘a kind of powder which from natural causes produces astonishing results’’. This substance could be used, he said, even underwater, where the sea hardened it to a strength ‘‘which neither the waves nor the force of the water can dissolve’’.
Two thousand years on, the harbours made from that Roman concrete have still not dissolved. If only the same could be said for modern concrete, which degrades in the sea within decades.
Now, scientists appear to have worked out why the Romans were able to build such long-lasting maritime architecture. The same sea that destroys modern concrete really does harden the Roman kind.
It has long been a mystery precisely what made Roman concrete different. A century after Vitruvius, this wonder material was praised by Pliny the Elder for its ability to harden in salt water, becoming a ‘‘single stone mass, impregnable to the waves and every day stronger’’. But its constituent parts were largely just a cement made from volcanic ash, lime and seawater, mixed with an aggregate of volcanic rock.
Portland cement, used today in its stead, is made from calcium, silicon, aluminium and iron. It is designed to be inert once set, requires steel reinforcement, uses a lot more carbon dioxide in construction - and, if it does react, it crumbles.
By examining cores from piers still standing in what was the Roman empire, Marie Jackson from the University of Utah found that the opposite happened with the Roman recipe.
When seawater infiltrates tiny cracks in the concrete, it dissolves some of the ash. Rather than undermining the structure, the alkali fluid that is left allows minerals to grow that strengthen it. These include crystals of a substance called aluminous tobermorite, which help to bond the material.
‘‘The corrosion of the volcanic components is actually a beneficial process,’’ Jackson said. ‘‘Sea water is percolating through, and as it percolates through it changes the composition and increases the mechanical resilience of the structure.’’
Her study, published in the journal American Mineralogy ,is the latest to try to unpick the mechanisms behind the success of Roman concrete.
Jackson said there was now enough understanding to recreate it, and marine constructions should consider using Roman methods.
She also said the research had vindicated the observations of Pliny and Vitruvius.
‘‘Vitruvius wrote his incredibly valuable text around 30BC, when a lot of these structures were being constructed on the western coast of Italy. He was there when this tremendous advance took place.’’
Almost more significant were the later observations, which found that the structures were still strengthening. Pliny marvelled that ‘‘on the hills of Puteoli there exists a dust, so named because it is the most insignificant part of the Earth’’, that could form such a strong structure.
Jackson does not think it was chance that the Romans found a substance that was hardened rather than weakened by age.
‘‘These are incredibly intelligent people who experimented over several decades to get this mix right,’’ she said. – The Times