This article explores the utilisation potential of water treatment sludge (WTS) along with fly ash (FA) and ground granulated blast furnace slag (GGBS) as binding material for road subgrade application. The strength and durability of WTS-FA-GGBS geopolymers are examined through a succession of unconfined compressive strength (UCS), California bearing ratio (CBR) and alternate wetting–drying tests, respectively. The mechanical properties of WTS-FA-GGBS geopolymers were evaluated using a variety of factors, including binder proportions, curing period, curing conditions, (ambient curing and heat curing) and sodium hydroxide (NaOH) concentration. The results revealed that increasing the binder proportions, NaOH concentration and curing time improved the strength of the WTS-FA-GGBS geopolymer. Furthermore, heat curing was also conducted to examine the effect of heat on the mechanical properties of the geopolymer samples. The heat-cured geopolymer samples showed better unconfined compressive strength (UCS) values as compared to that of the ambient cured samples. The investigation indicated that all geopolymer samples satisfied the minimum strength standards for cement-stabilised mix, with a maximum mass loss of 8.17%. This is within the stipulated limitations (<14%) as per IRC:37-2018. The California bearing ratio (CBR) values of WTS-FA-GGBS geopolymers exceed the minimum requirements for cement-stabilised subgrade or subbase material given by IRC: SP:89-2010. Additionally, FESEM and XRD studies validated the alterations that occurred during geopolymerisation. Finally, a leaching study was conducted to investigate the geo-environmental impacts of different geopolymer specimens, and it showed that the heavy metal concentrations in the geopolymer leachates are well within the limits set by USEPA.