Improving mechanical properties of hemihydrate phosphogypsum via alkali-activated mineral admixtures

The low strength of hemihydrate phosphogypsum (HPG) limits its wider applications in construction, and the use of alkali-activated mineral admixtures to improve HPG's mechanical properties has enormous environmental benefits. In this paper, slag and fly ash were added to HPG, which was then activated by NaOH. The effects of alkali activation on the mechanical properties of HPG were investigated, and the hydration mechanism of modified and enhanced HPG was investigated by X-ray diffractometer (XRD), Thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR), and Low-field nuclear magnetic resonance (LNMR). The results showed that alkali-activated mineral admixtures can improve the compressive strength of HPG up to 40 MPa with the optimal mix ratio: HPG:(GGBS+FA) = 60:40 by weight, where the ratio of GGBS to FA is 4:1 by weight, and NaOH accounts for 1% by mass of the binder. When NaOH was added at 1wt% of binder, it could effectively activate slag and fly ash to enhance the mechanical properties of HPG. With the increase of the dosage of NaOH, settings of HPG were prolonged and its strength was reduced. The hydration products such as ettringite (AFt) and C-S-H gel generated by NaOH-activated slag and fly ash can effectively fill the pores of HPG paste and thus improve its mechanical properties. When hydrated for 28d, the decrease in strength due to the increase in NaOH dosage can be attributed to the decrease in AFt generated from hydration. The findings of this study pave the theoretical and technical foundations for greater and wider utilization of phosphogypsum for a sustainable future.