 The Roman theatre at Aquileia was constructed using mortars containing Pulvis Petyalana, a type of poxelonic aggregate outcropping near Pozzolini, which was used to strengthen masonry and produce hydraulic concrete for harbour piers. Pulvis Petyalana had been previously used in Campania but this is the earliest known example of its use outside of the region. The presence of Pulvis Petyalana in the mortar suggests that the builders of the theatre were aware of the material's ability to resist water infiltration in the low-lying deltaic plane where it was constructed. This article was authored by Simone Dallaria, Michelle Seco, Andrea Arguioto and others.