The by-product produced by the coal-fired power plant in Nagan Raya, Aceh, Indonesia is referred to as Fly Ash Nagan Raya (FANR). Based on available characterization data, the particles of FANR exhibited a misty surface texture and contained several key chemical constituents, including Si, Fe, Al, and Ca. Owing to this composition, which closely resembles that of ordinary Portland cement, FANR was a pozzolanic material capable of participating in secondary cementitious reactions that contribute to strength development when combined with a calcium-rich environment. However, FANR absorbed mixing water, reducing the consistency and workability of fresh mortar. The misty surface of the material increases water demand, which in turn influences the mortar’s physical and mechanical properties. As a result, the unique surface characteristics of FANR significantly altered the physical and mechanical properties. The purpose of this study was to assess the qualities of FANR as a cement substitute in mortar. This study examined water-to-binder ratios of 0.55 and 0.50 with FANR incorporated at levels of 5%, 10%, 15%, 20%, and 25%. The study analyzed chemical composition, surface features, setting time, workability, and compressive strength. It found that FANR has a misty surface, spherical particles, and contains Si, Fe, Al, and Ca. In fresh mortar, these properties shortened the setting time and reduced workability. The highest 28-day strength occurred at a 0.50 water-to-cement ratio with 5% FANR, with continued strength gains beyond 28 days as FANR content increased, showing its influence on mortar properties.