The amount of ash, and its hazardous impact on the environment, produced from the coal fired thermal power plants is continuously increasing. This poses a very challenging task of safe handling, proper disposal and utilisation of the ash. A very common method of disposal of the ash is the wet disposal, in a slurry form, in the lands nearby thermal power plants, known as lagoons. During the process of wet disposal, the ash interacts with water and looses its original physical as well as the chemical properties. The ash-water interaction may also lead to the formation of zeolites over a period of time, which may he used for various industrial applications. This paper deals with the effect of water interaction, and hence the formation of zeolite, on a typical Class 'F' lagoon ash, from India. (C) 2001 Elsevier Science Ltd. All rights reserved.