This study explored the use of untreated high-carbon biomass fly ash (BFA) and less alkaline Na2CO3 as an activator to create alkali-activated materials (AAM). This article investigated how the Na2CO3/Na2SiO3 (SC/SS) ratio of the alkaline activator solution (AAS) influenced the setting time, structural development and physical-mechanical properties of BFA-based AAM pastes that were cured at room temperature. With an increase in the SC/SS ratio, AAM pastes' initial and final setting times shorten, respectively. The AAM sample with the lowest density and ultrasonic pulse velocity (UPV) value also has the maximum water absorption. The compressive strength of AAM samples dropped after 7 days of curing while the SC/SS ratio increased. Nevertheless, after 28 days of curing, the compressive strength of AAM samples rose with a rising SC/SS ratio. The results for density, water absorption, and UPV were corroborated by the scanning electron microscope (SEM) analyses, which demonstrated that when the SC/SS ratio rises, the structure of the samples gets denser and more homogeneous, in line with the increased compressive strength of the samples.