The use of fly ash as a supplementary cementing material in concrete and mortar has dramatically increased in recent years due to its improvement of concrete and mortar properties and its environmentally friendly impact. In this thesis, the sulfate resistance and alkali-silica reaction in mortars containing 0, 20, 40, 60, and 80% fly ash replacement levels were investigated to determine the effect of fly ash content on durability. Four types of cement and two types of fly ash were used in the tests, the cements used were of different alkali contents and with and without blended silica fume. The fly ashes consisted of medium and high calcium contents.  The results showed that the expansion resulting from sulfate attack as well as the alkali-silica reaction considerably decreased with the increase of fly ash content, but the strength of the mortar samples greatly decreased as well. The effectiveness of fly ash in both tests was highly dependent on their CaO content and on the chemistry of cements utilized. 20% Sundance fly ash replacement was sufficient to maintain the sulfate expansion after six months of exposure below CSA allowable expansion limit of 0.05% for high sulfate resistance cement and also below the Canadian limit of 0.1% in the ASR test. For Rockport fly ash, replacement levels at 40% were needed to maintain the expansion below the limit of 0.1% in ASR and sulfate test. The presence of silica fume blends significantly reduced the expansion due to alkali-silica reaction and sulfate attack; addition of fly ash to silica fume blended cements did not significantly improve performance.