Cocciopesto (Opus Signinum) is one of the most brilliant engineering inventions of Roman antiquity. By mixing crushed brick with air lime, the Romans discovered how to create a waterproof yet breathable mortar, able to harden even underwater, long before the invention of modern cement.
Pure air lime, on its own, hardens very slowly by carbonation and is soluble in water. Adding crushed brick fragments changes everything: clay fired at low temperatures (600-800 degrees C) contains silica and alumina in amorphous, reactive form. These minerals react at ambient temperature with calcium hydroxide, forming hydrated calcium silicates and aluminates (C-S-H): the pozzolanic reaction, which confers hydraulic properties and a set that occurs even in the presence of water.
In historic architecture and contemporary bio-construction, cocciopesto is considered the dehumidifying render par excellence. Its macroporous structure works like an intelligent sponge: it absorbs rising capillary damp and allows it to evaporate freely. Unlike cement, mineral salts crystallise harmlessly within the large pores, without causing detachment of the render.
Standards
European and international references applicable.
Physical properties
Usage environment
Not all crushed bricks produce the pozzolanic reaction. Modern industrial bricks fired above 1000 degrees C have vitrified crystalline silica that does not react with lime. Only traditional or artisanally fired brick at low temperatures is effective.