The evolution of interfacial transition zone in alkali-activated fly ash-slag concrete

This paper investigates the mechanisms of microstructure evolution of interfacial transition zone (ITZ) in alkali-activated fly ash-slag (AAFS) concrete using scanning electron microscope. Results indicate that the formation of original ITZ depends on the so-called “wall effect”, leading to a deficit of large grains and a higher effective alkaline activator/precursor ratio compared to paste matrix. The alkaline reaction process is correspondingly accelerated, which promotes the formation of low Ca C-(N)-A-S-H gels and reduces the porosity in the ITZ. Afterwards, the high Ca C-(N)-A-S-H gels are generated due to the release of more Ca from slag, resulting in the continuous refinement of pores. The C-(N)-A-S-H gels with rich Si and Al are then produced at 7 d, attributing to the species diffusion from paste matrix to ITZ. Consequently, a compact and dense microstructure is formed in the ITZ at 28 d, which would be beneficial to the long-term performance of concrete.