With ever-increasing demand for electricity, the production of ash, produced from the coal-fired thermal power plants, and its hazardous impact on the environment, is continuously increasing. This poses a very challenging task of safe handling, proper disposal and utilization of the ash. Several ash utilization schemes, developed and reported by earlier researchers, are being practiced frequently these days. However, these studies do not take into account the quality of ash produced and the changes it may undergo, before it can be used for different applications. A common method of disposing ash is its wet disposal, where the ash is mixed with water to make slurry and is disposed off in the ash ponds or lagoons. Such a disposal system causes ash, and the alkalis present in it, to interact with water over a period of time and may lead to the formation of ash zeolites. As such, it would be interesting to study the effect of this interaction (i.e. formation of zeolites also termed as zeolitization of the ash) on physical, chemical and mineralogical characteristics of the ash. As geotechnical properties of a material depend on these characteristics, the influence of zeolitization on these properties of the ash must also be investigated. Such investigations are essential for the bulk utilization of the lagoon ash, in particular as a fill material, where properties like compaction, consolidation, hydraulic conductivity and its shear strength are very important. In order to simulate such ash-water interaction, controlled laboratory experiments have been conducted on a typical Indian lagoon ash, The present study deals with the details of the effect of zeolitization on physical, chemical, mineralogical and geotechnical characteristics of this ash. Studies were also conducted to explore the possibility of utilization of the lagoon ash, and the zeolitized ash, for various environmental applications viz. retention and removal of heavy metals from the industrial sludge. (C) 2002 Elsevier Science Ltd. All rights reserved.