What Roman concrete made with crushed pottery was used for waterproof linings?

The Roman concrete made with crushed pottery and used for waterproof linings was opus signinum.

Opus signinum was a lime-based Roman building material containing small pieces or powder from broken pottery, tiles, bricks, or amphorae. The ceramic aggregate gave the mortar useful hydraulic and waterproofing properties. Romans applied it to floors, tanks, cisterns, baths, aqueducts, swimming pools, and other structures exposed to water.

Its name is linked to Signia, modern Segni in central Italy, a town known in antiquity for its tiles. The technique was not exclusively Roman: related pottery-and-lime mortars appeared earlier in Phoenician and Punic settings, then spread through the Mediterranean before becoming widespread in Roman construction.

A common mistake is to treat opus signinum as identical to opus caementicium, the broader Roman concrete tradition. Opus signinum is a specialized form distinguished by its crushed ceramic content and its frequent use as a waterproof or damp-proof finish. It could also serve as a plain, durable floor surface, sometimes polished and sometimes used beneath mosaics.