It is well-known that some treatments of fly ash may improve its reactivity and consequently the strength of mortars containing these processed ashes. In this paper the effects of treatments on fly ashes are compared: sieving, grinding, and magnetic extractions from aqueous suspensions. The influence of these processed ashes on compressive strength development of mortars containing processed fly ashes for long curing periods was studied. The gain or loss of compressive strength with respect to mortar that contains original ash has been calculated; most mortars present positive values, although for the longest curing times (more than 90 weeks), these values tend towards unity except for ground fly ash mortars. The grinding treatment has proved to be an effective treatment for improving the compressive strength of mortars containing fly ashes over long curing times. The high activity index values versus curing time for mortars containing 60% replacement demonstrate that for high fly ash volume mortars the processing of original ash has very favourable effect on the strength. Moreover, the morphology of the cement/fly ash hydrated matrix of old mortars was analysed by SEM and also thermogravimetric methods have been applied for evaluation of lime content and hydrates. For old mortars containing a high volume of ground fly ashes (60% replacement) residual lime due to hydration of Portland cement has not been detected, suggesting that the optimum replacement in order to produce the maximum quantity of pozzolanic reaction products would be lower than 60%.