Enhancement of phosphogypsum-based solid waste cementitious materials via seawater and metakaolin synergy: Strength, microstructure, and environmental benefits

Phosphogypsum-based cementitious materials (PGCM) possess the potential to solidify corrosive ions in seawater and may serve as a viable alternative to Ordinary Portland Cement (OPC). However, despite this potential, limited research has explored the use of seawater as mixing water in PGCM, and the hydration mechanism underlying their interaction remains unclear. This study aimed to examine the impact of seawater on the macroscopic and microscopic characteristics of PGCM, as well as the underlying mechanisms and the evolution of PGCM properties in the presence of a synergistic effect between seawater and supplementary cementitious material, metakaolin (MK). The results demonstrate that using seawater as mixing water for PGCM mortars reduces workability. Conversely, sulphate ions in seawater shortened the induction period of PGCM, accelerated ettringite formation, shortened the setting time of PGCM, and enhanced the early strength of PGCM. However, the enhancement of the late strength of PGCM by seawater was limited. The synergistic effect of seawater and MK significantly increased the compressive strength of PGCM, with an enhancement of 45.31% and 20.48% at 28 and 90 days, respectively. This enhancement was linked to the hydration reaction of Na⁺ ions in seawater and MK, forming N-A-S-H gel network structure that influenced the microstructure of PGCM. Moreover, the incorporation of seawater and MK in PGCM offers both economic and environmental sustainability benefits.