Invloed van de hydratatie condities op de microstructurele ontwikkeling van alkali-geactiveerde hoogovenslak

This thesis investigates the activation of blast furnace slag primarily with sodium hydroxide, and to a lesser extent with sodium sulphate. The aim is to clarify how the concentration and nature of the activating solution influence the hydration mechanisms, formation of products, and development of microstructure. Although extensively studied in recent decades, key aspects such as early-age structural evolution, formation of secondary hydration products, and how they are affected by the concentration of the activating solution remain insufficiently understood. This is partly due to limited interest in weak activating solutions for practical applications and partly due to the lack of well-established techniques capable of monitoring microstructural evolution in detail without interfering with the ongoing hydration. To address this gap, this thesis applies well-known analytical techniques in a novel combined approach. The early hydration of NaOH activated slag was studied by correlating microstructural features obtained from time-resolved small-angle X-ray scattering (SAXS) patterns with phase assemblage evolution probed simultaneously by wide-angle X-ray diffraction (WAXD). This approach revealed that the size of the nanometer-sized discs, used to model the CASH-related SAXS pattern, maintain their size regardless of NaOH concentration. However, their assembly varies with a closer packing observed at higher concentrations. Although this could suggest finer pores and lower porosity, an increase of the gel porosity was instead determined at higher NaOH concentration. These contrasting results can be reconciled by assuming that a more compact disc assembly incorporates a larger number of finer pores within the gel. The concentration of NaOH was also found to impact the timing of secondary products formation. At lower concentrations, these products formed later than CASH yet, they contributed to shielding slag particles from the pore solution and slowing hydration kinetics. In addition, NaOH concentration appeared to impact the structure of specific secondary products, namely hydrotalcite-like phases, possibly affecting the hydration kinetics as measured by isothermal calorimetry. In fact, the quantification of the hydration kinetics revealed a profound difference between the activation with concentrated and diluted NaOH solutions that may be linked with the formation of secondary products. Although Na2SO4 is known to be a weak activator, its combination with NaOH exhibits a synergistic effect, resulting in enhanced reactivity and improved microstructure. Both aspects are associated with a different hydration process and phase assemblage more closely resembling those of silicate activation. The outcome is a more homogenous microstructure, which is known to be beneficial for the mechanical performance. Finally, an additional aspect that emerged during this work concerns the carbonation susceptibility of slag; Due to its high calcium content, even anhydrous slag can carbonate. While negligible in highly reactive formulations, this carbonation process can significantly delay the hydration process in less reactive systems. The carbonation of anhydrous slag is expected to reduce the reactive surface available for dissolution, and to modify its composition by decreasing the Ca/Si. This creates a less reactive surface layer resulting in a slower dissolution with subsequent impact on the hydration kinetics.