Abstract: A method for processing incinerator bottom ash (IBA) comprises the steps of carbonating IBA aggregate material by CO2 sequestration and providing a stabilizing additive for mixing with the carbonated IBA aggregate material, wherein the additive comprises one or more components from group (b1) and one or more components from group (b2), wherein group (b1) consists of aluminium chloride and at least one other metal chloride, and wherein group (b2) consists of silica, zeolite and apatite. When the carbonated IBA and additive is mixed a stabilized IBA composition is formed, the stabilized IBA composition being suitable for use as a substitute for traditional aggregates in the manufacture of concrete and concrete products.