Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part I: Effect of MgO

This study investigates the hydration and microstructure of three alkali-activated slags (AAS) with varying MgO contents (8 to 13 wt.%) using two alkaline activators, NaOH and Na2SiO3·5H2O. The primary objective was to understand the effects of MgO on the hydration process and subsequent mechanical properties. Through a series of experiments involving X-ray diffraction (XRD), thermal analysis (TGA), and scanning electron microscopy (SEM), the research reveals that higher MgO content accelerates the reaction rate and enhances compressive strengths during the initial hydration period. Specifically, it was observed that extending the MgO content from 8% to 13% led to a 9% increase in the volume of hydrates and a significant 50% to 80% rise in compressive strength after 28 days for the WG-activated slag pastes. Conversely, only a marginal increase in compressive strength was noted in NaOH-activated slags. The findings highlight the importance of MgO in the formation of hydration products, particularly C(−A)–S–H and hydrotalcite-like phases, ultimately informing the design of more effective alkali-activated binders for construction applications.