Old and new uses for volcanic ash

Those unfortunate travellers who had their plans affected by the recent eruption of the Eyjafjallajökull volcano in Iceland could be forgiven for thinking that volcanic ash has no useful purpose whatsoever.

However, mixing finely ground volcanic pumice or ash with quicklime makes pozzolanic cement. When combined with sand and aggregate in the usual manner, this makes a concrete that, just like the product made with Portland cement, will even set underwater.

The term “pozzolanic” appeared in Roman times and is derived from the Italian port of Pozzuoli, a few miles west of Naples, where it was first used extensively. The raw material probably came from the nearby volcano of Vesuvius.

Pozzolanic cement was used to line aqueducts and in the construction of the Colosseum and the Pantheon in Rome. The Greeks also used cement made with volcanic ash and some examples dating from the 6th or 7th century BC still exist on Rhodes.

A similar concrete was used by the ancient Egyptians when building the pyramids and the sphinx at Giza, and the Minoans also made cement using the ample supplies of volcanic ash and limestone found on Crete. These structures provide ample evidence of the effectiveness and durability of pozzolanic cement.

It still makes economic sense to use volcanic ash where available. For example, apart from the top 20cm of tarmac, the airport runway on the Greek island of Santorini consists of compacted volcanic ash, as does part of the runway on Kos.

This knowledge may be of some comfort to you while you wait for your flight after the next volcanic eruption.