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The key ingredient was pozzolana, a type of volcanic ash that reacts with water to make cement — thus functioning as natural clinker.

Ceratech is exploiting an industrial version of pozzolana: fly ash, the fine particles filtered out of the combustion gases from coal-burning electricity plants.

The matter transported consisted of soil of various kinds—sand, ashes, fragments of lava, pozzolana and whitish pumice, enclosing grains of uncalcined lime, similar in every respect to those of Pompeii.

There are sulphur and manganese mines in the island of Melos, and the volcanic island of Santorin produces pozzolana, a kind of cement, which is exported in considerable quantities.

The chocolate-brown earth imported from Pozzuoli or dug from beds in the campagna, is known as pozzolana, and early in the history of Rome her builders discovered that when mixed with lime it made a remarkably strong cement.