To save natural resources and protect the environment, the use of fly ash (FA) with a high loss on ignition (LOI) and silica fume (SF) in producing high-strength mortar (HSM) is the objective of this study. The effect of FA with high LOI on the durability and microstructure of HSM was investigated. All HSMs were designed with a water-to-binder ratio of 0.24, and the amount of SF is 20% total amount of binder materials. The reference mixture was designed without FA, while four others were created by using FA to replace 15%, 30%, 45%, and 60% cement. Test results indicated that the mortar quality was decreased with increasing high FA content. However, with the presence of SF, all mortars in this study still showed good quality with compressive strength of above 50 MPa and rapid chloride ion penetration value of below 1000 Coulombs. Many free-FA particles and impurities, which were observed in the scanning electron microscopy image of HSM samples with high FA content, are causes leading to a reduction in the mortar quality. In addition, the relationships between some properties of HSM were also established.