Influence of chloride ions on strength and microstructure of geopolymer concrete containing fly ash, and blend of fly ash-GGBS

Abstract

Chloride attack is an important aspect in long-term mechanical and durability performance of geopolymer concrete (GPC) in marine environment. Keeping this in view, in this study, the effect of chloride ions on compressive strength at different ages were investigated, as well as the microstructural changes in terms of molar ratio and morphology of GPC mixes. Two types of GPC mixes were prepared; the first one using 100% fly ash (FAGPC), developed under elevated curing temperature at 80 °C for 48 h, and the other one using a blend of 85% fly ash and 15% ground granulated blast furnace slag (GGBS) (FASGPC), developed under ambient condition. To investigate the effect of chloride ions, sodium chloride (NaCl) was added at 0%, 1.5%, and 3.5% by mass of geopolymer solids during the preparation of GPC mixes. From the test results, it is observed that the FASGPC exhibited lower strength as compared to FAGPC at the early age (7 days) due to lower extent of geopolymerization reaction under ambient condition. However, at later age (360 days), FASGPC showed higher strength as compared to FAGPC. The chloride added GPC mixes exhibited lower strength as compared to control GPC mixes. Furthermore, the FAGPC mixes with added chloride exhibited a higher decrease in strength when compared to FASGPC mixes at both ages. The percentage increase in strength with increase in age (i.e., from 7 to 360 days) was in the range of 106.5% to 110.5%, and 45% to 70.4% for FASGPC, and FAGPC mixes respectively, at all chloride concentrations. The obtained FESEM images indicated the formation of geopolymer gels in the GPC mixes. Furthermore, the variations in strength of GPC mixes with GGBS content and age are in line with the variations in atomic Al/Si and Ca/Si ratios obtained from the EDS analysis.