Impact of sodium chloride (NaCl) on fresh properties and microstructure of class F fly ash geopolymer mortar

Abstract

Even when industrial wastewater is treated, it usually still has a high concentration of impurities like neutral salts, such as sodium chloride (NaCl), posing a significant challenge for its safe and sustainable reuse. This study investigated the isolated effect of NaCl concentration (0.5-20 g/L) on Class F fly ash geopolymer mortar, alongside other key parameters including NaOH molarity (10M-14M) and alkaline liquid/ binder ratio (0.5-2.5), to identify an optimal formulation for saline conditions. The optimum blend, developed considering 15 g/L NaCl level, 12M NaOH level, and alkaline liquid/binder proportion of 0.5, returned a maximum compressive strength, of 33 MPa at 28 days. Although, initial and final setting times increased gradually as the concentration of NaCl increased, the compressive strength at 28 days was not affected significantly. Data of microstructural characterization, proved the presence of stable geopolymer gels and the lack of any harmful influences of the NaCl. The results indicate that it is possible to use treated saline wastewater in the production of geopolymer mortar and can offer a long-term sustainable approach to the reuse of industrial wastewater in construction.