Chloride resistance of class C/class F fly ash-based geopolymer mortars with different strength grades

Chloride resistance is important for the durability of geopolymers used in construction applications, particularly in coastal and saline areas. In this study, the workability, mechanical performance, and chloride resistance of class C/class F fly ash-based geopolymer mortars were investigated. First, the effects of the water-to-fly ash ratio, class C fly ash content, and sand-to-ash ratio, on the fluidity, and mechanical properties of the geopolymers were examined. Second, based on the results, geopolymer mortars with three different strength grades were chosen for chloride resistance tests. After these were subjected to a NaCl solution, their weight change, ultrasonic flight-time, and compressive strength were determined. Moreover, corrosion products were evaluated by conducting microstructural analysis. The results indicated that a high class C fly ash content, small water-to-fly ash ratio, and small sand-to-ash ratio led to low fluidity and a high compressive strength of the specimens. The ultrasonic flight-time and the pore structures of the geopolymers were strongly related. Furthermore, after corrosion, the hydration products appeared as new crystalline zeolite phases in the low-strength samples. Compared with the high- and medium- strength mortars, the low-strength samples showed excellent chloride resistance.