The Pantheon, a temple to the Gods, was completed in 128 CE and the temple’s dome is still the largest unreinforced concrete dome in the world. Why does Roman concrete (opus caementicium) still stand strong while the modern version decays? Roman concrete is an artificial building material composed of an aggregate, a binding agent, hydrated lime (calcium oxide) and seawater. An aggregate, the filler, was material such as gravel or broken bricks; the primary binding agent, the mortar, was pozzolana (or pozzolanic ash), which is chocolate-red volcanic earth from deposits near the Bay of Naples. It’s the ash’s unique mix of minerals which appears to have helped the concrete withstand chemical decay and damage and, although salt seawater is very damaging to modern concrete, the water also played an important role in making Roman concrete. It appears that chemical reactions among the lime paste, volcanic ash and seawater created microscopic structures within the concrete that trapped molecules like chlorides and sulphates that harm concrete today, actually reinforced the Roman concrete.

Gaius Plinius Secundus, or Pliny the Elder, a Roman historian, naturalist and natural philosopher wrote about Roman concrete in his Naturalis Historia ‘that as soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass, impregnable to the waves’.

University of Utah geologist Marie Jackson and her team have made another discovery by finding abundant aluminous tobermorite growing through the fabric of the concrete. Tobermorite, a mineral, has silica-rich compositions, similar to crystals that form in volcanic rocks. The crystals have small, flat and flaky shapes that reinforce the cementing matrix and prevent cracks from growing, with structures becoming stronger over time as the minerals grew. The finding provides a blueprint for durable construction material for use in marine environments and opens a completely new perspective for how concrete can be made.