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The Substrate Suitability of Phosphogypsum Composites for Marine Habitat Enhancement

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LSU researchers are studying the application of stabilized phosphogypsum (PG) blocks as artificial reefs and oyster substrate. The first study was focused on the mechanisms of preventing dissolution of PG and the mechanical properties of the stabilized PG blocks. It was found that 70%:30% PG:cement test blocks survived for more than one years while the 85%:15% PG:cement test blocks dissolved within two months of placement in the seawater. Optic imagery and microprobe analysis showed that a calcite (CaCO3) layer formed on 70% PG/30% cement blocks that did not exhibit softening when submersed in saltwater. This CaCO3 coating may act as a physical barrier to seawater intrusion, preventing block degradation. Ettringite formation was identified as the main reason for dissolution in 85% PG/15% cement blocks.

The mechanical properties studies were initiated to investigate the surface hardness and strength of PG block composites 15% and PG briquettes < 3% that were subjected to various curing and seawater submergence times. Surface hardness results for both the blocks and briquettes indicated a significant decrease (p>0.0001) after the 30 day submergence period followed by little to no change between the 30 and 60 day submergence periods. Strength results were more variable between the blocks and briquettes. The interactions between composition and treatment were more apparent for the blocks. Surface hardness and compressive strength were not good indicators of the integrity of composite PG in marine applications.

The second study was focus on reducing the binding agent content and cost of the stabilized PG blocks. Lime and fly ash were added as other binding agents to reduce the cement content. It was found that the same total amount of lime and cement contents in cement/lime PG composites that endured 12 weeks of field submergence were identified to leach the least calcium in the laboratory. A gradual reduction in block size despite biological growth was observed in the field test, suggesting the addition of lime is not fully adequate. The incorporation of fly ash as an ingredient seems to be a good alternative, as demonstrated in additional studies where combinations of PG (55-62%), cement (3-10%), and fly ash (35-42%) showed little signs of deterioration after two years of seawater field submergence. The PG (62%), cement (3%), fly ash (35%) blocks are currently the lowest cost. Calcium leaching evaluated in the laboratory through dynamic leach test provides a good indication of how composites will perform in the field.

The economic analysis of the PG, cement, fly ash blocks indicates that the cost of such a briquette, based on a 4,500,000 ton per year facility, would be approximately the same cost as limestone (@ $13.00/ton). Limestone and shell are available for similar purposes on the open market. Although there is some question about the density of the resulting phosphogypsum product, it does appear to be competitive with the only other material available on the market in the gulf region. It is possible that with further engineering work the cost of producing the phosphogypsum briquettes may be reduced to some amount less than that of commercially available limestone.