The interfacial transition zone microstructure of ground ferronickel slag incorporated self-compacting concrete investigated by nanoindentation

The effect of using ground ferronickel slag (GFNS) as a supplementary binder on the interfacial transition zone (ITZ) and strength properties of self-compacting concrete (SCC) were investigated. Cement was replaced by 20-50% GFNS, and mechanical properties of the SCC were assessed. The ITZ was characterized by EDS line scanning. The elastic modulus and hardness properties of ITZ were determined by grid nanoindentation tests. Microstructural changes were followed up by XRD, SEM and EDS analysis, while the macro-level characterization was carried out by compressive and splitting tensile strengths. The strengths showed a decreasing trend with increasing GFNS content. The 28-day compressive strength of the mix with 35% GFNS was 66 MPa as compared to 79 MPa for the control mix. Moreover, up to 35% replacement level, this study finds a compatible ITZ, with a thickness of around 55 mu m. The elastic modulus and the hardness values from the nanoindentation test suggest a decrease of the calcium silicate hydrate with the increase of GFNS that decreased strength. XRD data confirmed this finding. The line scanning data from EDS reveals that the presence of Al, Fe and S in ITZ was not significant. Hence, the Ca/Si ratio plays a pivotal role in determining the ITZ characteristic, which increases with the increase of GFNS content in the binder. Strength diminishes gradually with increasing Ca/Si ratio, possibly due to the damaged silicate skeleton and the hydrolytic weakening effect. Therefore, microstructural observations and nanoindentation results well elucidate the strength results, and these experimental outcomes will provide a strong base for utilizing GFNS in SCC.