Study on hydration kinetics and characteristics of high-content modified phosphogypsum cement-based materials

Phosphogypsum (PG), a by-product of the phosphate fertilizer industry, is compromised by acidic impurities such as phosphorus and fluorine. These impurities degrade the quality of PG and limit its application in construction. Lime neutralization offers a viable method for purifying PG. In this study, PG was modified with different lime contents (0, 1%, 2%, 3% and 4%) at 200℃ for different times (2 h, 4 h and 6 h) to prepare high-temperature modified phosphogypsum (HPG). The impact of HPG on the hydration process of cement and the mechanical properties of cementitious materials is assessed through hydration heat testing. XRD and FESEM are used to analyze the composition of hydration products and the microstructure of cementitious materials, respectively. Based on the results of cement hydration heat, a hydration mechanism model for PG-based cementitious materials is established. The results show that the acidic impurities in PG can be removed by neutralization reaction of quicklime (QCL), thus increasing the pH value of PG, while the modification time has little effect on the pH value of gelling material. The strength of 2% quick lime-modified cementing material reached the maximum at all ages. Compared with the unmodified cementing material, the 3d flexural strength and compressive strength increased by 1.8 MPa and 5.5 MPa, respectively, and the 28d flexural strength and compressive strength increased by 1.9 MPa and 8.4 MPa, respectively. The addition of 2% QCL promoted the hydration degree of the cementing material in the nucleation growth stage, thus improving the cement performance and the overall strength and durability of the cement. The microscopic analysis shows that the addition of 2% QCL can promote the growth of hydration products of the cementitious materials and promote the development of strength of the cementitious materials. Considering all factors, 2% QCL is identified as the optimal content for neutralizing PG. Combining all factors, 2% QCL was determined to be the best content of neutralizing PG.