Effect of low-calcium and high-magnesium ferronickel slag on the microstructure and micromechanical properties of alkali-activated blended ground granulated blast furnace slag

Abstract

In recent years, low-calcium and high-magnesium ferronickel slag (FNS) has been gradually used in the preparation of alkali-activated cements (AACs). An in-depth exploration of the microstructural characteristics is crucial for a thorough understanding and enhancement of the macroscopic behaviors of AACs. In this study, the microstructural composition and micromechanical properties of the alkali-activated ground granulated blast furnace slag (GGBS)-FNS cements (AASF) prepared with sodium silicate and sodium hydroxide as alkali activators were quantitatively studied. Backscattered electron image analysis (BSE-IA) and nanoindentation were selected to reveal the micromechanical properties of different phases in FNS particles and AASF pastes. The results show that the elastic modulus of sodium aluminosilicate hydrate (N-A-S-H) in alkali-activated FNS cement is about 17 GPa, which is similar to that of the gel phase in alkali-activated fly ash cement (AAFA) and alkali-activated metakaolin cement (AAMK), confirming that the micromechanical property of N-A-S-H gel is independent of the kind of precursor materials. The Ca/Si and Al/Si of the hydration products in AASF gradually decline as FNS is incorporated, whereas the Mg/Al steadily rises, increasing the elastic modulus of the inner hydration product (IP) phase in AASF. The reaction degree of GGBS is higher in the sodium silicate-activated AASF system, while more N-A-S-H gels are formed in the sodium hydroxide-activated AASF system. The incorporation of FNS is conducive to improving the reaction degree of GGBS, resulting in the increase of calcium silicoaluminate hydrate (C-A-S-H) content and a significant decrease in IP content in AASF.