Fly ash of some thermal power plants is rejected when their chemical compositions do not satisfy the limit values in related standards and consequentially conveyed to landfills. In this study, a rejected fly ash, which has high sulphite, free lime, and magnesium oxide contents and does not comply with the classification standards of fly ashes, having cementing properties, has been examined. This rejected fly ash is moistened with water in the thermal power plant and then transported to landfills. This humidified fly ash that has been in contact with snow and rain for a long time turns to hydrated fly ash. The analysis of this hydrated fly ash showed that the sulphite, free lime, and magnesium oxide content levels of the hydrated fly ash had decreased compared to the original rejected fly ash. After the hydrated fly ash in the landfill was ground and sieved, self-compacting mortars (SCMs) were produced by replacing PC with 10, 20, and 30% hydrated fly ash. At various pH levels, leaching of Ca2+ in SCMs was investigated. Hydrated fly ash is found to be excellent powder material to increase the leaching resistance of Ca2+ in an SCM when used to replace PC with 20% hydrated fly ash. (C) 2019 Elsevier Ltd. All rights reserved.