Comparative Analysis of the Influence of Sodium and Potassium Silicate Solutions on the Kinetics and Products of Slag Activation

This paper primarily explores the influence of the alkali cation (Na or K) on the reaction kinetics, product formation, gel structure, and mechanical properties of alkali activated slag systems. For the same activator M_s, i.e., molar SiO₂–M₂O ratio (M = Na or K), a shorter induction period, a larger acceleration peak, and consequently, a higher amount of total heat release under isothermal conditions is observed for the K-silicate activated slag pastes. The early-age compressive strengths in these systems roughly relate to the heat release response. The later-age (7 days and beyond) compressive strengths are observed to be higher for the Na-silicate activated systems, which is corroborated by: (1) higher amounts of C–(A)–S–H gel in this system indicated by a thermal analysis-based approximate quantification method, and (2) higher combined intensities of Q¹ and Q² structures that point to increased degrees of reaction, and lower amounts of unreacted slag obtained from ²⁹Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The NMR spectra also show evidences of Al-substituted C–S–H gel, with a higher amount of substitution when Na-silicates are used.