Fabrication and Performance of Phosphogypsum-Based Composite Cementitious Materials

Phosphogypsum (PG) is an industrial byproduct generated during the wet process production of phosphoric acid. The global stockpile exceeds 6 billion tons, but the utilization rate is only 15%. A large amount of PG is stored in open-air piles, causing serious pollution to soil, water, and air. To overcome this limitation, green and high-strength phosphogypsum-based composite cementitious materials (PGGQ) were prepared using PG and slag as the main raw materials, modified with quicklime, water reducer, and retarder. The effects of different materials on the mechanical properties and water resistance of PGGQ were studied. Finally, X-ray diffraction, scanning electron microscopy, and thermogravimetric (TG) and differential scanning calorimetry (DSC) microscopic tests were used to analyze the microstructure and hydration mechanism of PGGQ. The results showed that the optimal mass ratio of PG to slag is 8∶2. The optimal content of quicklime, water reducer, and retarder are 2%, 0.15%, and 0.1%, respectively. With this proportion, the 28-day compressive strength, flexural strength, and softening coefficient reached 27.81 MPa, 6.58 MPa, and 0.65, respectively, representing improvements of 107.5%, 82.8%, and 75.7%. The microscopic results showed that the quicklime activated the pozzolanic reaction of slag, promoting the formation of AFt and C─ S─ H gels. The generated AFt and C─ S─ H intertwined to form a dense skeleton, making the cementitious materials denser and more waterproof.