Reaction Kinetics and Characterization of Slag-Based, High Strength, “Just-Add-Water” Type (One-Part) Alkali-Activated Binders

Abstract

The influence of different levels of alkalinity, expressed using M2O-to-binder ratio (n) and activator SiO2-to-M2O ratio (Ms), (M being the Na+ or K+ cation) on the reaction kinetics, compressive strength development and the reaction product formation in slag-based systems activated using alkali powders are discussed. The fundamental idea is to better understand one-part, “just-add-water” type alkali activated binders that are easy-to-use than the systems that rely on liquid activators. The difference in the behavior of the systems with changes in the cationic species (Na+ or K+) and the overall levels of alkalinity is elucidated. Heat release and its rate, and thermal analysis and Fourier Transform Infrared Spectroscopy (FTIR) are used for the characterization of the reaction and the products formed. The influence of activator alkalinity on the initial dissolution and acceleration phases is examined using isothermal calorimetry. An increase in Ms is found to result in reduced early-age and slightly increased later-age compressive strengths. The activator cationic species influences later-age strengths, with K-silicate activated mortars showing a higher strength. The strength data is related to the C-(A)-S-H gel formation. The study shows that slag-based binders can be proportioned to obtained compressive strengths in excess of 80 MPa at later ages (56 days), and up to 30 MPa within 72 hours. The optimal alkali levels based on n and Ms for both the Na- and K-based activator systems are determined.