Effects of Urea and Sodium Tripolyphosphate on the Hydration and Leaching of Cr(VI) in Solid Waste–Based Sulfoaluminate Cement

In this study, we evaluated the effect of the retarders urea and sodium phosphate on the hydration process and leaching of Cr(VI) in calcium sulfoaluminate cement from solid waste (WCSA). We investigated the mechanical properties and hydration processes of WCSA with varying urea and sodium tripolyphosphate contents using thermogravimetry coupled with differential scanning calorimeter (TG-DSC), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, and scanning electron microscopy (SEM); the leaching behavior of the heavy metal Cr(VI) in the cement was analyzed using inductive coupled plasma emission spectrometry (ICP). The experimental results showed that lower urea or sodium tripolyphosphate (STPP) contents (0.2%) improved the compressive strength of cement specimens by 18.31% or 3.125% at 28 days, respectively. After incorporating STPP and urea, the hydration heat flow and total hydration heat of WCSA decreased. The porosity first decreased and then increased with increased STPP and urea content. The involvement of urea in hydration produced carbonate, which destroyed the structure of ettringite and formed a new calcium carbonate phase, leading to the release of Cr(VI). In the presence of STPP, a three-dimensional network structure of hydroxyapatite was generated. Chromium ions entered the three-dimensional network structure of hydroxyapatite, increasing the solidification of Cr(VI) in WCSA.