This paper presents the results of mechanical and micro-structural propertiesof geopolymer mortars with obsidian, ground granulated blast furnace slag,and metakaolin. In this study, 13 different combinations were created using25% by weight of obsidian, blast furnace slag, and metakaolin. Geopolymermortars were poured at 0.45 water/binder ratio, 12M NaOH concentration,and exposed to thermal cure at 75C, 90C, 105C, and 120C for 72 h. Theflexural and compressive strengths of the mortars were measured and theirmicrostructural properties were analyzed. As a result of the study, the maxi-mum compressive strength was obtained as 93.7 MPa, and the maximum flex-ural strength was 21.4 MPa. From the results obtained when differentcombinations are used, it has been observed that the materials work in har-mony with each other and the optimum curing temperature is 90C. Also,scanning electron microscopy (SEM) and EDS analyses indicated that the com-pact structure of the geopolymer matrix and the Ca/Si ratio are directly propor-tional to the mechanical strength of the sample. The high wavelength valueformed in the main band as a result of Fourier-transformed infrared spectros-copy (FTIR) analysis shows that the mortars have high mechanical properties.