The characteristics and formation mechanism of the dark rim in alkali-activated slag

This paper presents comprehensive investigation on the microstructure and micromechanical properties of the hydration products in alkali-activated slag by adopting the coupled techniques of nanoindentation, optical microscopy, backscattering electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDS), aiming to provide insight into the formation mechanism of the dark rim in AAS. It is found that the dark rim is primarily comprised of a mixture of Mg-Al layered double hydroxide (LDH) and C-A-S-H gel, and the existence of this LDH phase contributes to the relatively high hardness and elastic modulus of the dark rim. The dark rim is in-situ formed in the outer layer of GGBFS. The dissolution and diffusion of Ca and Si in the outer layer of GGBFS results in a lower average atomic number in the dark rim region, contributing to its relatively lower grey level. Based on the experimental results, the formation mechanism of the dark rim is discussed in this investigation.