Sodium sulphate resistance of geopolymer mortar produced using ground ferronickel slag with fly ash

This study investigated the durability of geopolymer mortar made from ground ferronickel slag (GFNS) with fly ash (FA) after immersion in 5% sodium sulphate solution for 1.5 years. An increase of compressive strength was found after immersion of the samples in sodium sulphate solution. The mortar containing 75% GFNS was found to have the highest compressive strength after sulphate exposure. The high strength of FA-GFNS geopolymer is ascribed to the presence of stable sodium magnesium alumino-silicate hydrate (NMASH) product. The expansions of geopolymer mortars after immersion in sulphate solution were well below the value recommended by the American Concrete Institute (ACI). Microstructural analysis confirmed the absence of expansive gypsum or ettringite in geopolymer mortar after sodium sulphate solution exposure. Though all the samples showed good stability against sulphate exposure, the NMASH gel and the lower calcium content of GFNS made the FA-GFNS blended geopolymer less susceptible to sulphate attack compared to the 100% FA geopolymer.