This paper deals with the study of corrosion level of reinforcing steel bars embedded in Portland cement mortars containing different types of fly ash. Fly ashes used were obtained by physico-chemical treatments of an original F class fly ash to modify their magnetic properties and reduce their particle size. An original fly ash (T0) and three types of modified ashes were tested according to treatment duration and magnetic properties (T60, ground fly ash; TNM, non-magnetic fraction; TM, magnetic fraction). Corrosion tests on reinforced mortar specimens with and without different types of fly ashes, cured at 40 °C, and under accelerated carbonation conditions and seawater immersion, have been performed in order to obtain conclusions on durability. From the corrosion point of view the addition of TNM in mortars showed to be much more effective than addition of the original T0 fly ash.