Stabilized phosphogypsum: class C fly ash: Portland type II cement composites for potential marine application

Area of Science:

• Environmental Science
• Materials Science
• Geochemistry

Background:

• Phosphogypsum (PG) is a byproduct of phosphoric acid production.
• PG contains low levels of Radium-226, posing potential environmental risks.
• Stabilization of PG is crucial for safe disposal or reuse.

Purpose of the Study:

• To evaluate the durability of phosphogypsum-stabilized blocks in marine environments.
• To assess the leaching behavior and long-term stability of these composites.
• To investigate the protective mechanisms against saltwater degradation.

Main Methods:

• Compositions of PG, portland type II cement, and class C fly ash were optimized using an augmented simplex centroid design.
• Blocks were subjected to a 1.5-year field submergence test and a 28-day saltwater dynamic leaching study.
• Analyses included scanning electron microscopy, wavelength dispersive microprobe, and X-ray diffraction.

Main Results:

• All phosphogypsum composite blocks survived the 1.5-year field test without degradation.
• Effective calcium diffusion coefficients ranged from 0.21 to 7.5 x 10(-14) m2 s(-1).
• A protective calcite layer (40-60 microm) formed on the surface of leached composites.

Conclusions:

• Stabilized phosphogypsum blocks demonstrate excellent durability in marine environments.
• The formation of a calcite layer effectively encapsulates the blocks, preventing dissolution.
• This stabilization method offers a viable solution for managing phosphogypsum waste.