Sulfate resistance of class C/class F fly ash geopolymers

Sulfate attack is one of the significant durability problems for the geopolymer applied for various severe conditions. In this study, the effects of the water-to-fly ash ratio (W/FA), class C fly ash (CF) content, and alkaline content on the setting times and compressive strength of geopolymer pastes were investigated. According to the results, geopolymer samples of three different strength grades (low, medium, and high) were exposed to a 10% magnesium sulfate (MgSO₄) solution at 80 °C. The weight changes, ultrasonic flight-times, and compressive strengths of the specimens were measured to evaluate their sulfate resistances. Control samples were exposed to water at the same temperature. Finally, microstructural analyses using Fourier-transform infrared spectroscopy, X-ray diffraction, and mercury intrusion porosimetry were performed to investigate the corrosion mechanism. The results indicated that higher CF contents led to shorter initial and final setting times and a higher compressive strength regardless of the W/FA and alkaline content. In addition, there was a small weight change for all the geopolymer with different strength grades after they were exposed to an MgSO4 solution. The low strength geopolymer exhibited a better sulfate resistance because of the sustainable increase in the compressive strength and reduction in the ultrasonic flight-time. Moreover, there was a close relationship between the corrosion resistance and the inner structure of the geopolymer mortars. This research provides a theoretical foundation for the application of geopolymers—particularly in coastal and saline areas, which are often subject to sulfate attack.