The poor properties of river-sand soil (RS) (uniform and poorly particle size distribution, less cohesion, and its unsuitability for compaction) are prone to limiting its applications in engineering projects. Water treatment sludge (WTS)- based geopolymer was adopted as a sustainable alternative to conventional methods to improve this soil type. The study exhibited the potential use of WTS as a geopolymer for RS soil stabilization. Different WTS to RS ratios were chosen by weight (0, 20, and 40%) with an alkaline activator (AA) solution of varied sodium hydroxide (SH) (8M) to sodium silicate (SS). The study utilized different AA (SH: SS) ratios, which were (0, 0.75, and 1.5%). AS SH: SS were at various polymerization temperatures of 30, 45, and 60 °C for 48 h. The study investigated the effectiveness of using the unconfined compressive strength (UCS) test to evaluate soil strength improvement. The findings revealed that the optimal mixing ratio for WTS is 20%. When mixed with RS soil, this ratio increases the unconfined compressive strength two times. For WTS: RS mixture ratios of 0%, the best AA0% ratio (SH: SS 0:100) and the UCS value increased by 3% and 9% compared to AA ratios of 0.75 and 1.5%, respectively. The UCS value at WTS: RS 20% and AA0% was enhanced by 1.9% and 9% compared to AA ratios of 0.75 and 1.5%, respectively. However, for mixture ratios of 40% and AA0%, the UCS decreased to 1.6% and 2%, respectively, due to an excess of Si/Al ratio.