9. Vitruvius On Architecture 2.6.1-2 (pre-eruption)
There is a naturally-occuring powder that produces remarkable results. It is found near Baiae, in the territories of the municipalities that surround Mt. Vesuvius. When mixed with lime and gravel it produces a strong building material, especially useful for piers built out into the sea, as the mixture hardens even under water. ... I should say, moreover, that the fires [sc. of Vesuvius] were stronger in the past and that the plentiful flames within the mountain had emerged and burned fields thereabouts. It is for this reason that the rock called "sponge" or "Pompeian pumice" seems to have been formed from some other sort of rock by the heat.