Geotechnical properties and application of lime modified phosphogypsum waste

Residual deposits of Phosphogypsum are readily available in large quantities in South Africa and currently are landfilled or pumped into the sea, as there are limitations to engineering applications due to the radionuclides content. In this study, mild acid treated residual phosphogypsum was stabilized with Lime Fly Ash (LFA) binder, and the geotechnical properties and changes in phosphogypsum radioactivity were evaluated. Part of the LFA binder were replaced with basic oxygen slag (BOG) and the properties of the LFA-BOG composite were also evaluated. Citric acid as a reducing reagent was effective in reducing the radionuclides in the residual phosphogypsum and stabilization of acid treated phosphogypsum with lime fly ash (LFA) improved the Unconfined Compression Strength (UCS). In particular composites containing 50 % PG mobilized UCS of 1.08 MPa and optimum replacement of 30 % of the LFA with BOF slag further improved the strength to 3.2 MPa. The final composites met the requirements of South African Roads (TRH4, 1996) specification and have equivalent strength values as C1-C4 materials and fit criteria for subbbase in terms of strength durability, and are also applicable for masonry bricks production in accordance with SANS 1215.