# A Sustainable Composite Cementitious Material Manufactured by Phosphogypsum Waste
## 3 Results and Discussion
### Mechanical Properties of Composite Cementitious Material  
Figure 9 shows the flexural strength of a composite cementitious material mortar containing 10% to 90% phosphogypsum after 3, 7, and 28 days. The changing trend of composite cementitious material flexural strength is presented, with the flexural strength increasing first and then decreasing with increasing phosphogypsum from 0% to 90%, indicating that the flexural strength reached its maximum value with the incorporation of 10% phosphogypsum among all mixtures. For example, at 28 days, the flexural strength of the mortar with 10% phosphogypsum was 9.02 MPa, while the cement mortar was 8.56 MPa, indicating that the flexural strength of the mortar with 10% phosphogypsum rose by a factor of 1.05 times that of the cement mortar. It is clear from this that integrating 10% phosphogypsum into mortar might improve flexural strength, but that incorporating more than 10% has a detrimental effect, which was consistent with reaction degree and non-evaporable water content. Figure 10 shows the compressive strength of a composite cementitious material mortar containing 10% phosphogypsum and 90% gypsum after 3, 7, and 28 days. The experimental results clearly demonstrated that as phosphogypsum concentration grew from 0% to 90%, compressive strength initially increased and then progressively decreased; this is comparable to flexural strength. The compressive strength should be up to the greatest value with 10% phosphogypsum inclusion, which was 49.8 MPa after 28 days of curing, compared with pure cement mortar (44.5 MPa), which was increased 1.12 times and was  
also the strongest of all mixes. More than 10% phosphogypsum inclusion exhibits an undeniable negative influence on compressive strength. Figure 7: Microstructure of composite cementitious material at 3 days: (**a**) 0% phosphogypsum; (**b**) 10% phosphogypsum; (**c**) 30% phosphogypsum; (**d**) 50% phosphogypsum; (**e**) 70% phosphogypsum; sum; (**f**) 90% phosphogypsum. Figure 8: Microstructure of composite cementitious material at 28 days: (**a**) 0% phosphogypsum; (**b**) 10% phosphogypsum; (**c**) 30% phosphogypsum; (**d**) 50% phosphogypsum; (**e**) 70% phosphogypsum; (**f**) 90% phosphogypsum. According to the results, adding phosphogypsum to mortar can increase mechanical characteristics, pore structure, and micromorphology. The non-evaporable water and reaction degree experiments also revealed that the activity of phosphogypsum was relatively low; as a result, the higher the phosphogypsum incorporation, the worse the mechanical characteristics of mortar, especially in the early stages. In Figure 9, the compressive strength of mortar containing 90% phosphogypsum after 3 days was less than 10% of that of mortar containing only cement. This problem was primarily caused by the nature of the raw materials, which delayed the setting time and hindered paste hydration. Impurities in phosphogypsum included H\({}_{3}\)PO\({}_{4}\), Ca(H\({}_{2}\)PO\({}_{4}\))\({}_{2}\):H\({}_{2}\)O, Ca\({}_{3}\)(PO\({}_{4}\))\({}_{2}\), and CaHPO\({}_{4}\):2H\({}_{2}\)O [13]. It was discovered that the phosphorus in phosphogypsum had a detrimental effect on hydration, delaying the setting time and diminishing the mechanical properties. According to [PERSON] [5], the use of phosphogypsum would prolong the induction period and acceleration period of paste, and manage the setting time without impeding the evolution of the paste's mechanical properties. In this experiment, phosphogypsum contained a  
Figure 10: Compressive strength of composite cementitious material. Figure 9: Flexural strength of composite cementitious material. certain number of sulfate compounds [21], which prevented the hydration of gypsum and decreased the reaction degree of phosphogypsum, the non-evaporable water content of composite cementitious material, and the mechanical properties of mortar specimens with phosphogypsum.