Investigation on the hydration mechanism and water resistance of anhydrous-hemihydrate phosphogypsum-slag compound material

Abstract

To promote the resource utilization of phosphogypsum solid waste, new compound materials (PGSCM) were developed using anhydrous phosphogypsum (APG), hemihydrate phosphogypsum (HPG) derived from phosphogypsum (PPG), ground granulated blast-furnace slag (GGBS) and Ca(OH)₂. This paper focused on investigating the hydration mechanism and water resistance of PGSCM by combining a series of experiments including compressive strength, water resistance, thermogravimetric analysis (TG-DTG) with molecular dynamics (MD) simulation. The results demonstrated that the setting process of PGSCM can be significantly influenced by HPG and APG. The addition of an appropriate dosage of HPG not only can be favors hydration and the setting process of PGSCM, but also enables maintaining a high compressive strength of 45 MPa, water absorption of less than 5 %, and a softening coefficient of approximately 0.85 at 28 days. The XRD and TG indicated that the addition of HPG can promote hydration and generate CaSO₄·2H₂O, contributing to compressive strength at an early age. The continuous hydration of GGBS forms substantial calcium silicate hydrate (C-S-H) gel, which enveloped both APG and CaSO₄·2H₂O. This physical encapsulation hindered their contact with water and OH⁻ ions, thereby resulting in only a small fraction of APG being hydrated at 28 days. Molecular dynamics simulations revealed that the interaction energy of APG with Ca(OH)₂ solution was significantly higher than that with water, while Ca²⁺ ions underwent rapid and substantial surface accumulation. Driven by the common ion effect, this synergistic mechanism markedly accelerated both the dissolution and crystallization processes of APG.