Geopolymers are synthesized by activating aluminosilicate materials with alkaline solutions. This study investigates the performance of geopolymer mortar produced using a high volume of coal bottom ash (70% CBA) with ground granulated blast-furnace slag (30% GGBS), activated by derived sodium silicate (DSS) from rice husk ash (RHA) using the hydrothermal method, and commercially available sodium silicate (COM-SS) as a control activator. Workability and fresh density were used to assess the effect of the precursor type and activator on fresh properties. Moreover, the performance of the suggested geopolymer mortar was examined by testing the hardened density and mechanical properties, including the compressive and flexural strengths. In addition, X-ray diffraction (XRD) was used to unravel the underlying geopolymer matrix composition that governs the behavior of these geopolymer systems. Specimens activated with COM-SS consistently exhibited lower workability compared with those activated using RHA-9M-SS. The lowest fresh density is in the 70% CBA-R mix at 2179 kg/m³. The mechanical performance was enhanced using RHA as an activator, with a compressive strength value of 40 MPa achieved. The XRD results reveal the presence of significant minerals, including quartz, calcite, cristobalite, albite, mullite, and zeolite.