Modern concrete—used in everything from roads to buildings to bridges—can break down in as few as 50 years while two thousand year old structures built by the Romans are still standing. In fact Roman concrete actually grows stronger over time, not weaker.
An ancient recipe for mortar, written down by Roman engineer in 30 B.C.E. talks of a “concoction of volcanic ash, lime, and seawater, mixed together with volcanic rocks and spread into wooden molds that were then immersed in more sea water”.
Romans knew that this concrete was effective when exposed to seawater as: “a single stone mass, impregnable to the waves and everyday stronger.”
Researchers at the University of Utah have recreated Roman concrete and found that the seawater had dissolved components of the volcanic ash, allowing new binding minerals to grow. Thus the seawater was reacting with the volcanic material in the cement and creating new minerals that reinforced the concrete.
The researchers found that when seawater percolates through a cement matrix, it reacts with the volcanic ash and crystals to forma very rare hydrothermal mineral called aluminum tobermorite and a porous mineral called phillipsite.
The Romans 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. They knew it was “impregnable to the waves and every day grew stronger”.
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.
This research opens up a completely new perspective for how concrete can be made, Jackson says. “What we consider corrosion processes can actually produce extremely beneficial mineral cement and lead to continued resilience, in fact, enhanced perhaps resilience over time.”
Jackson argues that Roman concrete should be used to build the seawall for the Swansea lagoon in South Wales.
The researchers and now working to develop methods that use common volcanic products and can be usable globally.