Fly ash is a by-product of industrial power facilities that contain spherical glassy particles with characteristics comparable to cement. It causes environmental pollution and can be used in concrete, mines, landfills, and dams. Using ordinary Portland cement results in high carbon dioxide emissions and alkali-activated fly ash-based mortar can provide the desired alternative. Here, an attempt has been made to use alkaline-activated fly ash in mortar as a substitute for cement. This study employs characterization techniques such as BET, XRD, FESEM, and UTM tests to verify the surface area and phase composition of the raw materials used and to investigate the strength properties of alkali-activated fly ash-based mortar. The primary goal of this research was to examine the impact of the SiO2/Na2O ratio on the strength properties, phase formation, and morphology of the mortar when the water/solid ratio was constant. Subsequently, the variations of properties were also studied by changing the water/solid ratio, keeping the SiO2/Na2O ratio constant. The compressive strength for each batch was tested at 7, 14, 21, and 28-day intervals. The mortar’s maximum compressive strength was 22.6 MPa for a batch of SiO2/Na2O ratio of 1.20 and a water-to-solid ratio of 0.3. This batch’s increased strength is attributed to the formation of an amorphous alkaline aluminosilicate gel, which is a cementitious material.