Effect of different water reducing agents on mechanical properties and micro-mechanism of modified hemihydrate phosphogypsum cementitious materials

The stockpiling of industrial by-product phosphogypsum causes pollution to the environment, and its effective utilization can be achieved by using cementitious materials from undisturbed hemihydrate phosphogypsum (UHPG) obtained by hemihydrate-wet process. This study investigated the creation of modified hemihydrate phosphogypsum mixture by proportionally blending quicklime with UHPG. Modified hemihydrate phosphogypsum cementitious material (MPGCM) was then prepared using melamine superplasticizer (SM) or naphthalene superplasticizer (FDN). The effects of these water reducing agents on the mechanical properties and micromechanisms of MPGCM were evaluated by single-factor experimental method. The results showed that both SM and FDN were effective in reducing water consumption and enhancing the fluidity of MPGCM. Additionally, both superplasticizers delayed the establishment of the MPGCM, with FDN showing a stronger retarding effect than SM. Compared with those mixed with FDN, the specimens mixed with SM exhibited markedly greater flexural and compressive strengths. The specimens with 2 % SM content achieved the highest 28 d flexural strength and compressive strength, measuring 16.61 MPa and 42.26 MPa. Microscopic analysis revealed that the water reducing agent can not only inhibit the disaggregation and dissolution of calcium sulfate hemihydrate and the crystallization of calcium sulfate dihydrate, but also promote the transformation of calcium sulfate dihydrate crystals from long rods to short columns or plates, so that the reticular structure is more compact and the mechanical strength is improved. This study suggests that the optimal composition, consisting of 97 % UHPG, 3 % quicklime, and 2 % SM, yields the optimum mechanical properties. These results provide a theoretical foundation for the application of phosphogypsum in practical engineering and support the sustainable development of phosphorus chemical industries.