Effect of composite activator on hydration kinetics and micromechanical properties of alkali-activated slag

Alkali-activated slag (AAS) materials prepared from traditional activators such as NaOH exhibit low sustainability and workability while replacing NaOH with some weakly alkaline solutions (e.g., Na2SO4) is promising to mitigate the above issues. This paper performs a comprehensive study on the effect of different composite activator combinations i.e., NaOH + Phosphogypsum (PG), NaOH + Na2SO4 and Na2SO4 + Ca(OH)2, on the reaction kinetics, phase assemblage, micromechanical properties and microstructure of AAS. The reaction products and microstructure of AAS were characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), thermogravimetric (TG), backscattered electron microscopy (BSEM) and mercury intrusion porosimetry (MIP) techniques. Inductively coupled plasma optical emission spectrometry (ICP-OES) test was used to investigate the pore solution chemistry of AAS. The nanoindentation test was carried out to evaluate the micromechanical properties of AAS. Results indicate that the use of composite activators can significantly reduce the hydration heat of AAS compared to systems activated by a single activator type. Combining PG and NaOH as the composite activator results in more AFt phases, refining the pore structure of AAS and reducing the average critical pore size. Besides, the elastic modulus of the individual solid phases in AAS prepared by this combination significantly outperformed other AAS mixes.