Compressive strength and mineralogical and microstructural characterization of the pastes prepared from alkali-activation of low calcium fly ash were investigated. XRD and FTIR were used to observe the mineralogical characterization; SEM, MIP, and NMR were used to observe the microstructural characterization of the pastes. The higher strength of paste was attributed to a more compact and continuous gelatinous matrix due to restructuring of reaction product. The lower values of both porosity (31.0%) and mean pore diameter (9.7 nm) of fly ash-pastes alkali-activated at 85 ℃ for 24 hr indicate higher reactivity of these pastes compared with other pastes, and result in the higher compressive strength.