Turkey's plants are fired by lignite, producing class C fly ash containing a high percentage of lime. Sulphate and alkali levels are also higher in class C fly ashes. Therefore, fly ash is, commonly, unsuitable as an additive in cement or concrete in Turkey. In this study, highly alkaline fly ash obtained from the Yenikoy thermal power plants is combined with soil samples in different proportions (5%, 15%, and 25%) and changes in the geomechanical properties of Ankara clay were investigated. The effect of curing time on the physical-mechanical properties of the fly ash mixed soil samples was also analyzed. Free swelling index values showed a decrease of 92.6%. The California bearing ratio has seen a more drastic increase in value (68.7 times for 25% fly ash mix). (C) 2016 The Authors. Published by Elsevier Ltd.