Stone powder sludge is a by-product of the manufacturing process of crushed sand. Most of it is dumped with soil in landfills, and the disposal of stone powder sludge causes a major environmental problem. This paper investigates the applicability of stone powder sludge in fly ashbased geopolymer. For this, stone powder sludge was used to replace fly ash at a replacement ratio of 50% and 100% by weight. The compressive strength of the samples was measured and scanning electron microscopy/ energy dispersive spectroscopy (SEM/EDS) analysis and X-ray diffraction (XRD) were performed. The test results indicated that the optimum level of the alkali activator ratio ($Na_2SiO_3$/NaOH) for fly ash-based geopolymer using stone powder sludge was 1.5. The strength development is closely related to the NaOH solution concentration. In addition, the compressive strength of the sample cured at $25^{circ}C$ was significantly improved between 7 days and 28 days, even though the strength of the sample showed the lowest value at 7 days. Microscopy results indicated that a higher proportion of unreacted fly ash spheres remained in the sample with 5M NaOH, and some pores on the surface of the sample were observed.