Distribution of Pozzolana Across the Mediterranean via Ship Ballast

The Mediterranean’s greatest concrete supply chain hid below decks. Between 10 BCE and 50 CE, ships leaving Puteoli and Baiae took on pozzolana as ballast—cheap, dense, and disposable. At destinations, captains sold or offloaded the gray dust. What steadied a hull mid‑voyage hardened a harbor after it [17].

Archaeologists and geologists later traced this current. Roman‑style concretes appeared far from Campania—at Caesarea, along the Nile delta near Alexandria—and the mineral fingerprints matched Campanian ash. The simplest route isn’t a mystery convoy but a habit: fill the hold, sail, dump, build. The ballast economy explains the spread with the quiet efficiency of commerce [17].

Ancient texts flicker with awareness of substitutes. Pliny, cataloging earths, mentions cementitious dusts near Cyzicus and Cassandrea in Asia Minor. Engineers took note; where local earth worked, they saved coin. Where it didn’t, they paid to bring pulvis Puteolanus across the sea. In both cases, De Architectura’s 1:2 lime:pozzolana ratio and cofferdam method traveled as a set of instructions that could be followed the same day cargo hit the quay [2–3, 17].

On the pier, the transaction had a color and a sound. Stevedores slit sack mouths; the ash poured like smoke onto wooden planks, turning sandals gray. The creak of gangways mixed with the rasp of trowels as builders sampled a quick test mix. If the gray turned to stone underwater, the price felt fair.

By 50 CE, the practice had built a feedback loop. More harbors meant more traffic; more traffic meant more ballast; more ballast meant more ash seeded across coasts. The empire’s maritime arteries thus carried not only grain and wine, but the mineral logic of its harbors [17, 3].