Cocciopesto consists of fragments of bricks minutely crushed and is considered a metakaolin, which can be then mixed with lime plaster and was used in antiquity to make mortar or plaster for walls.
Mix cocciopesto with slaked lime and sand to make hydraulic plaster, which ancient Romans called opus signinum, for the underlayers of fresco wall painting. Cocciopesto reacts with lime to form plaster with cementious or hydraulic properties.
Plaster made with cocciopesto, in addition to possessing remarkable durability and resistance, has other characteristics that have favored its use, such as low water permeability. Cocciopesto, in partial or total replacement of sand, was used for the preparation of plasters or mortars based on slaked lime (calcium hydroxide; Ca(OH)2) which, in the absence of this reactive aggregate, could not harden if not in contact with the air, through the chemical process known as carbonation. The addition of this aggregate provides the function of hydraulicity, i.e., to obtain a plaster or mortar of lime with hydraulic properties, although the degree of hydraulicity obtainable is less than that which derives from the use of pozzolan.