Bottom ash is exposed to the atmosphere to allow metal oxides and hydrates to react with water and carbon dioxide to form carbonate. These reactions reduce the leaching ability of the metals and reduce the potential impact on the environment. Reactions with water can cause swelling of the material, therefore weathering is essential. Weathering (maturation) is normally achieved by leaving the ash in a stockpile to allow rainfall and time to complete the reactions. For the new generation of incinerators a weathering time of at least 3-4 months is required. The leachate produced during storing/weathering requires appropriate disposal as it may contain high concentrations of highly soluble salts and minor amounts of metals, especially copper, chlorides and sulfates.
Weathering is sometimes followed by cement stabilization, which is typically carried out on the construction site by mixing the bottom ash with cement or other pozzolanic materials to form a monolithic material that effectively excludes moisture (physical encapsulation). In addition, the cement environment provides a highly buffered framework that limits the solubility of most trace metals by maintaining a high pH. For some trace metals, however, the high pH provides higher solubility.