|Modern concrete, used in everything from roads to buildings to bridges,
can break down in as few as 50 years. But more than 1000 years after the
western Roman Empire crumbled to dust, its concrete structures are still
standing. Now, scientists have finally figured out why: a special
ingredient that makes the cement grow stronger, not weaker, over time.
Scientists began their search with an ancient recipe for mortar, laid
down by Roman engineer Marcus Vitruvius in 30 BCE. It called for a
concoction of volcanic ash, lime, and seawater, mixed together with
volcanic rocks and spread into wooden moulds that were then immersed in
more sea water. History contains many references to the durability of
Roman concrete, including this cryptic note written in 79 BCE,
describing concrete exposed to seawater as: 'a single stone mass,
impregnable to the waves and everyday stronger.'
What did it mean? To find out, the researchers studied drilled cores of
a Roman harbour from Pozzuoli Bay near Naples, Italy. When they analysed
it, they found that the seawater had dissolved components of the
volcanic ash, allowing new binding minerals to grow. Within a decade, a
very rare hydrothermal mineral called aluminium tobermorite
(Al-tobermorite) had formed in the concrete. Al-tobermorite, long known
to give Roman concrete its strength, can be made in the lab, but it's
very difficult to incorporate it in concrete.
But the researchers found that when seawater percolates through a cement
matrix, it reacts with volcanic ash and crystals to form Al-tobermorite
and a porous mineral called phillipsite. So will you be seeing stronger
piers and breakwaters anytime soon? Because both minerals take centuries
to strengthen concrete, modern scientists are still working on
recreating a modern version of Roman cement.