Optimizing phosphogypsum neutralization for enhanced setting regulation in Portland cement matrices

Abstract

This study investigates the application of chemical, thermal, and combined treatments on phosphogypsum (PG) to assess its potential as a calcium sulfate source in cement production, replacing natural gypsum. Treatments included washing PG with water (WS), adding 5 % solid hydrated lime (SL), washing with a lime solution (LS), calcination at 200 °C (C200) and 800 °C (C800), and combinations of calcination followed by lime washing (C200-LS and C800-LS). The physical, chemical, and mineralogical properties of neutralized PG (NPGs) were analyzed using laser granulometry, ICP-OES, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectroscopy. Furthermore, the effects of these treated PGs on cement hydration were assessed by examining the mineralogical, thermal, and mechanical properties of cement pastes, including isothermal calorimetry, XRD, TGA, and compressive strength tests. Soluble phosphorus levels significantly increased in the treated PG, especially in the C200 and SL treatments, which exhibited levels 8 and 5 times higher than untreated PG. pH levels were also raised, with the SL treatment showing the highest increase, tripling that of untreated PG. Raman spectroscopy and TGA confirmed phase transitions from gypsum to hemihydrate and anhydrite depending on the temperature. Cement pastes incorporating treated PG exhibited accelerated hydration, particularly with the C200-LS and C800-LS treatments, which reduced the induction period by 38 % and 29 %, respectively, compared to the REF group. In contrast, the difference between the SL and REF groups was minimal. The C200-LS treatment also enhanced the formation of hydration products like portlandite, C-S-H and ettringite. The highest 28-day compressive strengths were observed for the SL (42.82 MPa) and C200 (40.79 MPa) treatments, indicating improved cement performance with these methods.