What is "opus signinum"

Opus signinum ('cocciopesto' in modern Italian) is a building material used in ancient Rome. It is made of tiles broken up into very small pieces, mixed with mortar, and then beaten down with a rammer. Pliny in his Natural History describes its manufacture: "Even broken pottery has been utilized; it being found that, beaten to powder, and tempered with lime, it becomes more solid and durable than other substances of a similar nature; forming the cement known as the "Signine" composition, so extensively employed for even making the pavements of houses."