# A Sustainable Composite Cementitious Material Manufactured by Phosphogypsum Waste
## 3 Results and Discussion
### Mechanical Properties of Composite Cementitious Material
This effect was primarily caused by an increase in hydrate deposition, calcium ion concentration in the paste and phosphate ion concentration on the cement particle surface. Based on the aforementioned circumstances, the incorporation of phosphogypsum lengthened the induction, duration and acceleration period of paste; these results were consistent with the effect of CaSO\({}_{4}\) 2H\({}_{2}\)O. During the hydration process, numerous hydration products, such as clitheriform and layered calcium hydroxide, fibrous calcium silicate hydrate, needle-like ettringite, and a specific number of pores and unhydrated particles in the mortar could occupy the interior space of the mortar. In Figures 9 and 10, it was also revealed that an increase in phosphogypsum inclusion led to a continual decrease in cement mortar's mechanical properties; however, incorporation of 10 percent phosphogypsum improved cement mortar's mechanical properties. Although [PERSON] [22] suggested that the introduction of 20% phosphogypsum might improve the mechanical properties of magnesium oxychloride cement, in this case, combined with the research situation of this paper, the incorporation of ultrasonic washed phosphogypsum should not exceed 30%, and its mechanical properties should be essentially comparable to those of cement mortar. This effect was primarily due to the presence of soluble sulfate and phosphate in phosphogypsum, which facilitated the development of hydration products. Additionally, the filler action of phosphogypsum could collaborate with the impact of the soluble sulfate and phosphates to compact the microstructure of cement mortar containing phosphogypsum, thereby enhancing its mechanical properties. [PERSON] et al. [23; 24] proposed that the compressive strength of concrete used in the military field should be improved to leave the objects or individuals within them unharmed to the largest extent possible. This study shows that adding appropriate phosphogypsum can ameliorate the performance of cement mortar and then enhance the strength of concrete, which is of great significance to broadening the application field of phosphogypsum. Figures 4 and 5 demonstrated that the porosity and pore size distributions of composite cementitious material were significantly lower than those of pure cement mortar. This demonstrated that phosphogypsum might maximize the advantage of macroaggregates to improve the compactness of mortar, then minimize porosity and refine pore size distribution. On the contrary, the formation of hydration products such as needle-like ettringite in Figure 7 demonstrated the participation of phosphogypsum incorporated in the secondary hydration reaction, which wrapped around the surface of cement particles and prevented them from participating in the hydration reaction. Thus, despite the high gypsum content in phosphogypsum, a low reaction degree was observed (as shown in Figure 6). However, as the cement hydration process continued, the mechanical qualities of the mortar steadily increased. Taken together, sulfates in phosphogypsum promote the dehydration period for the formation of ettringite minimally or not at all, and greatly consume the calcium hydroxide concentration. Phosphogypsum can reduce porosity, refine pore size distribution, and improve the mechanical characteristics of mortar due to the presence of water-soluble phosphorus and fluoride. Consequently, phosphogypsum can be used to produce sustainable composite cementitious materials with high performance, which encourages phosphogypsum recycling and protects the environment.