The use of industrial waste as a potential stabilizer of marginal construction materials is cost effective. Phosphogypsum and fly ash are industrial wastes generated in very large quantities and readily available in South Africa. In order to explore the potential stabilization of vastly abundant expansive soil using larger quantity phosphogypsum waste as a potential modifier, composites with a mixture of lime-fly ash-phosphogypsum-basic oxygen furnace slag were developed. However because of the presence of radionuclide, it was necessary to treat the phosphogypsum waste with mild citric acid. The effect of the acid treatment on the geotechnical properties and microstructure of expansive soil stabilized with phosphogypsum-lime-fly ash-basic oxygen furnace slag (PG-LFA-BOF) paste was evaluated, in comparison with the untreated phosphogypsum. Expansive soil stabilized with acid-treated PG-LFA-BOF paste exhibited better geotechnical properties; in particular, the high strength mobilized was associated primarily with the formation of various calcium magnesium silicide and coating by calcium silicate hydrate and calcium aluminate hydrate. The soil microstructure was improved due to the formation of hydration products. The stabilized expansive soil met the specification for road subgrades and subbase. Stabilization of expansive soils with phosphogypsum, fly ash, and basic oxygen fly ash does not only improve engineering properties of soil but also provides a solution in relation to disposal and environmental pollution challenges.
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.
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