This study examines the effect of fly ash properties on the chloride penetration resistance of mortar. Seven mortars containing 20 wt.-% fly ash were prepared and compared to a reference mortar with 100% CEM I 42,5N. Fresh and hardened mortar properties and their resistance to chloride ingress were determined. No consistent correlation between fly ash properties and mortar properties could be identified. Fresh mortar properties do not necessarily improve when cement is replaced with fly ash. Of the seven fly ashes tested, four improved workability. One sample even decreased it. The flexural strength of all samples after 90 days was slightly higher than the reference. The compressive strength of all samples up to 28 days was lower than that of the reference. After 90 days, four samples had almost the same strength as the reference. The rapid chloride migration test showed that compressive strength does not correlate with chloride resistance. After 28 days, all fly ash samples had significantly higher (worse) DRCM values than the reference. After 90 days, most samples approached the reference, and only one sample exceeded the reference. Further improvement in strength and chloride resistance is possible with increasing age due to the pozzolanic reaction of the fly ash. This variability highlights the complexity of fly ash behavior in concrete, suggesting that standard compliance alone is insufficient for ensuring optimal durability against chloride penetration. This is particularly critical in infrastructure construction, which demands high durability standards.