Microstructure development of slag activated with sodium silicate solution: Experimental characterization and thermodynamic modeling

Slag is a by-product of the steel industry that can be used as a binder with alkali solutions. In this study, the microstructural development is discussed for two alkali-activated slag mixes with promising properties for structural applications. The reaction products for both mixes were characterized by X-ray diffraction, thermogravimetry, Fourier-transform infrared spectroscopy and nuclear magnetic resonance. The Ca/Si ratio of C-A-S-H gel and the secondary products such as hydrotalcite, M-S-H and zeolites depend on the activator solution. The dissolution kinetics is followed by isothermal calorimetry and scanning electron microscopy. Calorimetry data can be adjusted with the maximum heat release obtained from thermodynamic modeling to predict the degree of dissolution. Thermodynamic modeling using the Pitzer ion activity model was applied for alkali-activated slag. This model is relevant for the first two days of reaction. For later age, the extended Debye-Huckel ion activity model and the Pitzer ion activity model give identical results. Phase assemblage predicted using thermodynamic modeling with the correct CASH model and consistent M-S-H and zeolite datasets agreed well with experiments.