Within the scope of this study, while the performance of slag (S)-based geopolymer mortars with bottom ash (BA) reinforcement was examined, chloride and sulfate attack tests were also carried out to investigate their durability properties. For the durability tests of geopolymer composites, sodium chloride (NaCl) and sodium and magnesium sulfate (Na2SO4 and MgSO4) solutions were preferred for a period of 10 months and a 15% solution percentage. The performance of geopolymer composites after the effect of durability was determined by flexural and compressive strengths, SEM and XRD analyses, weight changes, and visual inspection. When the results obtained were evaluated, it was seen that 15% BA substitution provided the highest compressive strength. There was variation in durability tests. At the end of the 2-month period, there was an increase in the compressive strength, while a decrease was observed at the end of the 6-month period. The main factor that created these fluctuations was that alkali ions migrated from sample to solution while the solutions were diffusing into the matrix. Gypsum and ettringite formed in the pores were effective in the losses that occurred in the 6-month period. In addition, the alkali ions leaving the sample and passing into the solution effectively accelerated the formation of micro cracks. Thus, strength losses were observed.