Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are mainly used for this purpose. Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.Cement paste must coat all particles and fill small voids. The use of fine sands make for a thirsty mix- high surface area. The use of coarse aggregates makes a boney mix with a lot of voids.