GLOBAL SPOTLIGHT
Why Roman concrete still stands
strong while modern version decays
Source : The Guardian
Scientists have cracked the secret to Roman water-based structures ’ strength – and findings could help today ’ s builders
Their structures are still standing more than 1 500 years after the last centurion snuffed it : now the Romans ’ secret of durable marine concrete has finally been cracked .
The Roman recipe – a mix of volcanic ash , lime ( calcium oxide ), seawater and lumps of volcanic rock – held together piers , breakwaters and harbours . Moreover , in contrast to modern materials , the ancient water-based structures became stronger over time .
Scientists say this is the result of seawater reacting with the volcanic material in the cement and creating new minerals that reinforced the concrete . “ They spent a tremendous amount of work [ on developing ] this – they were very , very intelligent people ,” said Marie Jackson , a geologist at the University of Utah and co-author of a study into Roman structures .
As the authors note , the Romans were aware of the virtues of their concrete , with Pliny the Elder waxing lyrical in his Natural History that it is “ impregnable to the waves and every day stronger ”. Now , they say , they ’ ve worked out why . Writing in the journal American Mineralogist , Jackson and colleagues describe how they analysed concrete cores from Roman piers , breakwaters and harbours .
Previous work had revealed lime particles within the cores that surprisingly contained the mineral aluminous tobermorite – a rare substance that is hard to make . The mineral , said Jackson , formed early in the history of the concrete , as the lime , seawater and volcanic ash of the mortar reacted together in a way that generated heat .
But now Jackson and the team have made another discovery . “ I went back to the concrete and found abundant tobermorite growing through the fabric of the concrete , often in association with phillipsite [ another
14 mineral ],” she said . She said this revealed another process that was also at play . Over time , seawater that seeped through the concrete dissolved the volcanic crystals and glasses , with aluminous tobermorite and phillipsite crystallising in their place .
These minerals , say the authors , helped to reinforce the concrete , preventing cracks from growing , with structures becoming stronger over time as the minerals grew . By contrast , modern concrete , based on Portland cement , is not supposed to change after it hardens – meaning any reactions with the material cause damage .
Jackson said : “ 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 .”
The findings offer clues for a concrete recipe that does not rely on the high temperatures and carbon dioxide production of modern cement , but also providing a blueprint for a durable construction material for use in marine environments . Jackson has previously argued Roman concrete should be used to build the seawall for the Swansea lagoon .
“ There ’ s many applications but further work is needed to create those mixes . We ’ ve started but there is a lot of fine-tuning that needs to happen ,” said Jackson . “ The challenge is to develop methods that use common volcanic products – and that is actually what we are doing right now .”
Article available at : https :// www . theguardian . com / science / 2017 / jul / 04 / why-roman-concrete-still-stands-strong-whilemodern-version-decays
ISSUE 34 - JUNE 2018