²⁹Si NMR investigations on alkali activated fly ash or slag: The role of the alkali cation

The influence of alkali cation (M = Na or K) on the alkaline activation of fly ash and slag are evaluated. NaOH, KOH (4M and 8M), or Na or K silicates (SiO₂/M₂O = 2.0) are used as the activators for fly ash systems which are then heat-cured, while Na or K silicates (SiO₂/M₂O = 1.5 and 2.5) are used to activate slag systems, which are then cured in ambient conditions. For alkali activated systems proportioned using Na-based activators, the compressive strengths were found to be higher for both the source materials. For the fly ash based systems, ²⁹Si MAS NMR spectroscopy identifies abundance of Q⁴ structures with Al incorporation (N-A-S-H gel). An increase in gel structural order and the presence of higher amounts of Si-rich units as the activator alkalinity increases are noted, along with a higher degree of disorder for the K-based systems. The reaction product signature for the activated slag systems, dominated by the Q² species are consistent with Al-substituted C-S-H (C-A-S-H) gel.