Microstructural and ²⁹Si MAS NMR spectroscopic evaluations of alkali cationic effects on fly ash activation

The influence of alkali cation (M = Na or K) on the geopolymerization of fly ash is evaluated. NaOH, KOH (4 M and 8 M), or Na/K silicates (SiO₂/M₂O = 2.0) are used as the activators. Na-activated systems demonstrate higher strengths than the K-based ones after thermal activation, with the 8 M NaOH activation being the most efficient. The total pore volume fractions are comparable between the pastes activated using similar Na- or K-based activators, but the volume of pores of size >0.2 μm is higher for the K-based systems, attributed to the reduced dissociation efficiency, faster condensation and reduced time for rearrangement. Compressive strength is shown to be closely related to the volume pores of size >0.2 μm. The presence of soluble silicates from the activator significantly influences the pore size distribution. ²⁹Si MAS NMR spectroscopy reveals an increase in gel structural order and higher amounts of Si-rich units as the activator alkalinity increases, and a higher degree of disorder for the K-based systems.