Effects of phosphogypsum substitution on the performance of ground granulated blast furnace slag/fly ash-based alkali-activated binders

Phosphogypsum (PG) is a byproduct of the phosphate fertilizer industry. Using PG waste to produce environmentally friendly building materials is becoming a desirable waste recovery method. However, unprocessed PG contains impurities, which often impedes its application in the construction industry. Considering that PG can provide Ca2+ and SO42 to a reaction system, this study develops an economic and environmental approach for optimizing the performance of PG-based alkali-activated binders (PGAABs) based on the combined use of PG, ground granulated blast furnace slag (GGBFS) and fly ash as precursors. An experimental investigation was carried out to understand the effects of PG content, Na2O content (M+) and alkali modulus (AM) on the properties of fresh and hardened PGAABs. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) results showed that using PG as a partial substitute for GGBFS in the precursors facilitated the alkali-activated reaction. The compressive strength of PGAABs cured at room temperature increased with increasing PG content, while the setting time decreased with increasing PG content. Compared with the specimen without PG, the PGAABs required lower activator doses to obtain high compressive strengths. Leaching results also confirmed that using PGAAB with 10% PG content can result in cost-related and environ-mental benefits.