Siliceous fly ash can be applied as a cement replacement material for mortars used in cement-based floor overlays. At present, there is no knowledge of the appropriate curing conditions that need to be applied on concrete floors. Moreover, previous research related to curing conditions has left research gaps related to the conditions in which such floors are cured. This study focuses on understanding the importance of the curing conditions that are typical for concrete overlays on the physical, chemical, mechanical, and microstructural properties of siliceous fly ash-modified mortars. The results indicate that curing conditions have a significant impact on these properties (particularly long-term effects). Differences were observed between siliceous fly ash-modified mortars cured in different conditions. In this research, we investigated the volume porosity, water absorption, microstructure, hydration process, pore segmentation, and strength of composites that have had cement partially replaced with the addition of fly ash. It should be noted that the use of siliceous fly ash in properly cured mortars that are applied in floor overlays could lead to a better use of green cementitious composites.