Developing Cast-in-Situ Phosphogypsum Walls

The raw materials of cast-in-situ phosphogypsum are mainly phosphogypsum, phosphorus slag powder, hydrated lime, cement, a retarder and a water reducer. Through preliminary investigations, it was found that the gypsum phase of phosphogypsum produced by different phosphoric acid plants is slightly different. Therefore, the gypsum phase of phosphogypsum raw materials should be tested before application. Figure 1 shows the construction photos of cast-in-situ phosphogypsum walls and treated phosphogypsum. The phosphogypsum and phosphorous slag micropowder were taken from Guizhou Wengfu Phosphorus Industry Group. The gypsum phase composition of phosphogypsum was determined and is shown in Table 1. The specific surface area of the phosphorus slag powder was 380~420 m²/kg and its chemical composition is shown in Table 2. The hydrated lime is commercially available, and the mass fraction of effective CaO was no less than 60%. The cement is P.O325 ordinary Portland cement and the water-reducing agent was polycarboxylic acid, with a concentration of no less than 10%. The retarder was sodium citrate. A JMS-6490LV scanning electron microscope was used to analyze the morphology of the phosphogypsum and phosphorus slag powder. The results are shown in Figure 2, where the scale bar in Figure 2a is $100 \mathsf{\mu }\mathrm{m}$ , and the scale bar in Figure 2b is $10 \mathsf{\mu }\mathrm{m}$ . The crystals of hemihydrate gypsum in phosphogypsum are generally parallelogram plates, and the gap between crystals is large. The phosphorus slag micro powder particles are in the shape of “gravel”, with clear edges and corners but no fixed cleavage surface. The results of the X-ray diffraction (SIEMENS D5000 XRD diffractometer, Munich, Germany ) analysis are shown in Figure 3. The phosphorus slag powder is mainly composed of glass, containing a small amount of pseudowollastonite, gunite and apatite. It is a volcanic material with potential activity and can be filled in the gap between hemihydrate gypsum crystals after casting. The acidity of phosphogypsum is neutralized by hydrated lime, the anhydrite is consumed by cement, the water reducer is used to reduce the water consumption, and the retarder is used to delay the setting time after being cast-in-situ.
Taking the total mass mix proportion of phosphogypsum, phosphorous slag micropowder and hydrated lime as 100%, the cement consumption was calculated according to the mass of the mixture of phosphogypsum, phosphorous slag micropowder and hydrated lime. The mixed dry materials were obtained after uniform mixing. The consumption of water reducer and retarder was calculated according to the mass of the mixed dry materials. Considering the possible construction deviation, the material mix proportion shown in Table 3 is proposed, and the water cement ratio is 0.43.