Preparing calcium sulfoaluminate cement (CAS) from solid waste phosphogypsum (PG) instead of natural gypsum is an effective way to utilize solid waste. In this paper, CAS clinker was successfully prepared from PG and the mineral content of calcium sulfoaluminate (${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$) in the sample was above 65%. The effects of raw material ratio, calcination temperature, and time on clinker composition were investigated. The mechanical properties of different samples were tested. The optimum ratio for preparing CAS using PG was 42.23% limestone, 17.43% PG, and 40.34% bauxite. The optimal calcination conditions are a high temperature of 1250 °C for 45 min. The 3-day compressive strengths of the laboratory-prepared CAS were all above 50 MPa. It was found that as the calcination temperature increased, the amount of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ produced gradually increased. Temperatures above 1300 °C resulted in the decomposition of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$. The calcination time did not significantly affect the mineral composition of the clinker or the strength of the cement. ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ was observed to be rounded and hexagonal platelets with crystal sizes of 1 to 2 μm, a relatively small size that is favorable to the hydration of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, as observed by SEM images. In addition, the high calcination temperature affected the particle morphology of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, changing it from a well-defined polygonal structure to a molten state. The test results provide helpful information for improving PG utilization and applying PG in CAS production.

Using phosphogypsum (PG) as a raw material to prepare calcium sulphoaluminate cement (CAS) is an effective way of treating phosphogypsum. In order to meet the different application requirements of CSA cement and promote the application of CAS cement, it is necessary to add mineral admixtures to adjust the performance of cement. This paper incorporated two minerals, gypsum dihydrate (C$H₂) and calcium hydroxide (CH), into cement clinker prepared from phosphogypsum. The compressive strength and hydration process of the mixtures with different blending levels were investigated around the C₄A₃$-C$H₂ system (SC) and the C₄A₃$-C$H₂-CH system (SCC). The optimum dosing level was determined on the basis of the strength and hydration properties. In the SC system, adding C$H₂ promoted the hydration of C₄A₃$. The compressive strength of the cement was highest at a C$H₂/C₄A₃$ molar ratio of 1.5, with a 7-day compressive strength of 56.5 MPa. AFt was mostly needle-rod and columnar and was tightly cemented to the gel phase, improving the denseness of the matrix. When the molar ratio was 2, the strength of the cement was inverted, and the shape of the AFt changed from needle and rod to columnar, the size of the grains increased, and it could not be filled with the AH₃ phase in an excellent staggered manner. At the same time, C$H₂ was not fully reacted, increasing matrix porosity and inversion of strength. In the SCC system, adding CH reduced the cement’s compressive strength, and the compressive strength reduction increased with the increase in admixture. According to the experimental results, CH inhibited the formation of AFt, resulting in the appearance of new hydration products, AFm. As the amount of CH increased, the amount of hydration products, AFm, increased, while the amount of AFt and AH₃ decreased. However, adding CH raised the paste’s pH and later facilitated the development of strength. The optimum admixture of CH/C₄A₃$ was 0.5 mol.