Efflorescence, the deposition of crystalline salts on surfaces of materials like brick and concrete, when exposed to moisture, poses significant challenges for alkali-activated fly ash-slag based geopolymer concrete, particularly in humid conditions. This not only causes aesthetic issues but also weakens the material’s structural integrity. Hydrophobic modification of geopolymer materials offers an excellent solution to mitigate the issue of efflorescence. The present study conducts an experimental investigation on ways to inhibit efflorescence in alkali activated geopolymer systems based on the surface hydrophobic modification of geopolymer concrete using an organosilane based compound. The organosilane compound is applied on the surface of hardened geopolymer concrete samples by means of brushing. The study successfully modified the geopolymer surface, which shifted its characteristics from hydrophilic to hydrophobic. This transformation was evident by a notable increase in the Water Contact Angle (WCA) value with the highest WCA recorded as 119.96˚. As a result, the material exhibited significantly lower capillary absorption and water diffusion, leading to a marked reduction in the leaching of soluble alkali ions that cause efflorescence. Different mechanical and durability tests such as compressive strength test, split tensile strength test, water absorption and sorptivity were conducted to study the efficacy of the surface treatment. In addition, SEM analysis as well as XRD analysis was also performed to understand the changes in the geopolymer microstructure. A possible proposed reaction mechanism suggests that silane molecules chemically bond to the geopolymer surface. This bonding occurs through a reaction between the hydroxyl groups on the geopolymer surface and the reactive groups of the silane, creating a stable hydrophobic layer. This chemical bonding provides effective resistance to moisture-induced efflorescence as is evident from the visual inspection of the samples. In conclusion, this method provides a solution to the efflorescence problem in addition to enhancing the durability and visual appeal of geopolymer concrete especially in damp environments.