Strength, durability, and toxicity characteristics of water treatment sludge based geopolymers for subgrade application

Abstract

This study examined the strength, durability and toxicity characteristics of geopolymer made from water treatment sludge (WTS) using fly ash (FA) and ground granulated blast furnace slag (GGBS) as binding materials for the subgrade application. The mechanical properties of WTS-FA-GGBS geopolymers were assessed considering various parameters such as binder proportions, curing period, curing condition, Sodium silicate (SS) to Sodium hydroxide (SH) ratio (i.e. SS/SH), and Sodium hydroxide (NaOH) concentration. The results showed that increasing binder proportions, NaOH concentration, and curing period positively impacted the strength of the WTS-FA-GGBS geopolymer. The unconfined compressive strength (UCS) of the geopolymer samples with an SS/SH ratio of 2.5 showed higher compressive strength than those at an SS/SH ratio of 1. WTS-FA-GGBS geopolymers cured at 100°C showed increased UCS values after 1 to 3 days, but strength decreased after 7 days due to a faster geopolymerisation reaction, resulting in excessive moisture loss and microcracks in the geopolymerized samples. The study also found that all the geopolymer samples met the minimum strength criteria for cement-stabilized mix with a maximum mass loss of 8.17% are well within the specified limits (<14%) as per IRC:37-2018. The California Bearing Ratio (CBR) values of WTS-FA-GGBS geopolymers, which range from 41.11% to 260.32%, surpass the minimum criteria for cement stabilized subgrade or subbase material, as specified by IRC: SP:89-2010. Additionally, the changes during geopolymerization were confirmed by FESEM, EDS, and XRD analysis. The toxicity characteristic leaching procedure (TCLP) test results showed that the amounts of heavy metals in the geopolymer leachate were within the USEPA-mandated limits.