This paper investigates the effectiveness of NaOH concentration and fly ash dosage on the mechanical properties, pore structure, water resistance, and water permeability of a new mortar manufactured by using a special kind of sandstone and fly ash. Mechanical properties were evaluated by means of compressive strength. Mercury intrusion porosimetry (MIP) was employed to investigate the pore structure and pore size distribution. A water permeability test was carried out to find out the permeation characteristics of Pisha sandstone-based mortar. Thermogravity and differential scanning calorimetry (TG-DSC), Fourier transform infrared (FTIR) analysis, and scanning electron microscopy (SEM) were used to analyze the reaction products, indentify the phases of the reaction products, and observe... the morphology, respectively. Test results revealed that NaOH concentration and fly ash dosage had significant effects on the mechanical properties, pore structure, and water permeability of Pisha sandstone-based mortar. The specimen (NaOH concentration 3.1 mol/L, fly ash wt.15%) exhibited the highest compressive strength (19.32 MPa [2801 psi]), water resistence (0.76) and lowest water permeability value (5.1 × 10–8 m/s [16.73 × 10–8 ft/s]) at 90 days.