Phase changes under efflorescence in alkali activated materials with mixed activators

Phase changes under efflorescence in alkali activated materials with mixed activators

Efflorescence in alkali-activated materials is a strong function of precursor and activator composition, which dictates their engineering properties and durability. In this study, the efflorescence of naturally cured NaOH/Na2SiO3 alkali-activated fly ash and alkali-activated fly ash-slag blended binder mixes was assessed with alkali concentration of 9 wt% Na2O, and 10 to 30 wt% of slag, and compared with binder mixes with 9 wt% Na2O, and 10 to 30 wt% of slag along with 2 wt% Na2CO3. The effects of efflorescence were assessed using visual and leaching inspections, and the compressive and split tensile strengths were determined post activation. Atomic absorption spectrometry was used to quantify free alkalis in the leachate, and X-ray diffraction, and Fourier transform infrared spectroscopy, magic-angle-spinning nuclear magnetic resonance and thermo-gravimetric analysis were performed to analyse the phase changes in binder pastes after efflorescence. The increased slag content facilitated the formation of C-A-S-H gel and enhanced both chemical and mechanical properties of binder pastes. Furthermore, the inclusion of slag content also led to the reduction of the open porosity and efflorescence formation. Subsequent exposure of binder specimens to efflorescence conditions aided the formation of carbonate products, degradation of N-A-S-H and N-(C)-A-S-H gel, and a decrease in split tensile strength in the binder paste specimens.