Effect of slag on strength, microstructure and rebar corrosion of chloride-rich geopolymer concrete

The effects of slag substitution (0, 30 and 60%) and chloride concentration (1.5 and 3.5% sodium chloride) on microstructural changes during strength development between 28 and 360 days, rebar corrosion up to 600 days and chloride binding capacity (CBC) in chloride-rich geopolymer concrete (GC) were investigated. The microstructural changes of the GC were evaluated through field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy analyses. The strength enhancement was higher for the GC mixes based on fly ash (FA) (F-GC mixes) than for the GC mixes based on FA and slag (F/S-GC mixes). The presence of chloride in the GC mixes caused a strength reduction at all ages, but the F/S-GC mixes with the higher slag content mostly showed a smaller strength reduction than the other mixes. Furthermore, the F/S-GC mixes made with the higher slag content exhibited less negative corrosion potential and lower corrosion current density than the other mixes, indicating better resistance against rebar corrosion. The CBC was mostly higher for the GC mixes with a higher slag content. A greater amount of calcium-bearing gels and a higher atomic calcium/silicon ratio in the F/S-GC mixes were responsible for reducing the influence of chloride on strength reduction and rebar corrosion, when compared with the F-GC mixes. Shifting of the Si–O–Si(Al) bond to a lower wavenumber indicated the formation of more binding gel, and thereby a denser microstructure, in the F/S-GC mixes.