Clay formation and metal fixation during weathering of coal fly ash

The enormous and worldwide production of coal fly ash cannot be durably isolated from the weathering cycle, and the weathering characteristics of fly ash must be known to understand the long-term environmental impact. We studied the weathering of two coal fly ashes and compared them with published data from weathered volcanic ash, it's closest natural analogue. Both types of ash contain abundant aluminosilicate glass, which alters to noncrystalline clay. However, this study reveals that the kinetics of coal fly ash weathering are more rapid than those of volcanic ash because the higher pH of fresh coal fly ash promotes rapid dissolution of the glass. After about 10 years of weathering, the noncrystalline clay content of coal fly ash is higher than that of 250-year-old volcanic ash. The observed rapid clay formation together with heavy metal fixation imply that the long-term environmental impact of coal fly ash disposal may be less severe and the benefits more pronounced than predicted from previous studies on unweathered ash. Our findings suggest that isolating coal fly ash from the weathering cycle may be counterproductive because, in the long-term under conditions of free drainage, fly ash is converted into fertile soil capable of supporting agriculture.