Chemical Mechanisms of Cement Stabilization and Durability Enhancement in High-Content Modified Phosphogypsum

Phosphogypsum, a byproduct of phosphate fertilizer production, accumulates in large quantities annually, posing significant environmental challenges due to harmful components such as fluorine, heavy metals, and acidic salts. To mitigate these issues, phosphogypsum is often combined with cement and single modifiers such as sodium silicate, hydrated lime, and defluorinating agents for use in pavement applications. However, concerns about the durability of unmodified or singly modified high-content phosphogypsum have hindered its widespread adoption. To address this issue, this study explored the use of sodium silicate, hydrated lime, and defluorinating agents as composite modifiers to enhance the durability of cement-stabilized phosphogypsum. The mechanisms of modification by individual and composite additives were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Durability was evaluated through three-point bending fatigue, freeze–thaw, and drying shrinkage tests on both unmodified and modified cement-stabilized phosphogypsum. The results demonstrated that the composite modification of sodium silicate, hydrated lime, and defluorinating agents significantly improved the material’s density, strength, and stability by enhancing hydration products and stabilizing fluoride ions. The modified material exhibited superior fatigue and freeze–thaw resistance, with reduced mass loss and slower strength decline compared to unmodified phosphogypsum. Additionally, the modified material showed improved drying shrinkage performance due to enhanced hydration. However, caution is recommended when using these materials in regions with high moisture content and significant temperature fluctuations.

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