A Sustainable Composite Cementitious Material Manufactured by Phosphogypsum Waste

Abstract

The phosphogypsum is a by-product of the phosphate fertilizer industry. It has accumulated over several decades, and not only takes up a large amount of land, but also poses a significant risk to the environment and resource waste. In order to promote the use of phosphogypsum, its hydration hardening characteristics are studied using a scanning electron microscope, X-ray diffractometer and mercury intrusion porosimeter. When the amount of phosphogypsum is increased, a decreasing trend in the reaction degree, non-evaporable water and portlandite is observed. Among them, the reaction degree and non-evaporable water, flexural strength and compressive strength reached their maximum when the content of phosphogypsum was 10%, which were as follows: 22.34 %, 21.13 %, 9.02 MPa and 49.8 MPa, respectively. Meanwhile, phosphogypsum can participate in the secondary hydration reaction in the system and act as a microaggregate. The addition of no more than 30% phosphogypsum can enhance mechanical characteristics, reduce porosity, refine pore size distributions and densify the microstructure. The findings of this study may aid in the production of phosphogypsum-based composite cementitious materials with superior performance, thereby promoting phosphogypsum recycling and protecting the environment.

1. Introduction

Phosphogypsum is a waste gypsum that is produced as a by-product of phosphoric acid manufacture. Every year, the world produces between 100 and 280 million tons of phosphogypsum [1]. Despite this, only 15% of phosphogypsum is properly exploited, and a substantial proportion of phosphogypsum remains in places near rivers or seas, as well as in open-air piles, with no specific treatment [2]. CaSO₄·2H₂O and phosphorus, fluorine, heavy metals, organic materials and radioactive elements are all present in phosphogypsum [3]. The accumulation of phosphogypsum will create major environmental contamination due to the effect of toxic contaminants such as land waste, pollution of the atmosphere and water bodies, and so on [4]. Although phosphogypsum is inexpensive, it requires water washing, chemical neutralization with basic solutions, calcination and other pre-treatment operations before it can be used, all of which raise the cost while also producing high levels of environmental pollutants [5,6]. As a result, the use of phosphogypsum in the building sector on a wide scale is restricted. Thus, enhancing the recycling rate of phosphogypsum is the most important objective for reducing the impact of phosphogypsum on the environment, and promoting long-term environmental development.

Phosphogypsum is being recycled as a raw material for building products, marine applications, agricultural fertilizers, road construction, and backfilling underground mining stopes [7]. The use of phosphogypsum as a filler material is a viable method for its use [8]. Chen et al. [9] used a series of experiments to verify the performance of cemented paste backfill. The results showed that adding construction demolition waste to phosphogypsum improved the workability of cemented paste backfill, shortened the initial and final setting times and provided long-term support for the underground stope. Phosphogypsum is also accessible for road construction. Amrani et al. [10] observed that use of the stabilized mix such as clayey soil, fly ash, lime, calcareous material and a special hydraulic road binder as the base material in road pavement can improve the mechanical properties of the materials, while ensuring a neutralization of acidity generated by phosphogypsum. Zhao et al. looked at the use of phosphogypsum as a road foundation binder. The results demonstrated that phosphogypsum might deliver enough sulfate ions while also extending the binder’s setting time [11]. In addition, phosphogypsum can be used to make self-leveling mortar [12].

Phosphogypsum is more commonly utilized in construction. According to Liu et al. [13], the use of modified phosphogypsum in super-sulphated cements increased compressive strength and pore structure. Through studies, Huang et al. [14] discovered that employing phosphogypsum to make Belite–calcium sulfoaluminate cement may not only save gypsum but also lower the calcination temperature. In addition to lowering the calcination temperature, phosphogypsum can substitute natural gypsum in the cement retarding effect. Impurities in phosphogypsum can be removed to improve its use in the cement industry. Impurities in phosphogypsum can be eliminated using a Ca(OH)₂-based chemical treatment, according to Neto et al. [15], reducing reaction delay time and increasing initial strength. Phosphogypsum can also be used as a cementitious material in the construction of buildings. To increase its performance, Jin et al. [16] dehydrated phosphogypsum into Beta-hemihydrate phosphogypsum and added super-sulphated cement. According to the findings, adding sulphoaluminate cement to gypsum improved compressive strength, flexural strength, and water coefficient, and also increased the phosphogypsum utilization rate.

The potential of untreated phosphogypsum inclusion in composite cementitious materials is discussed in this study, which is based on the numerous previous references for research discussion. The effect of phosphogypsum on the mechanical properties and hydration hardening characteristics of composite cementitious materials is investigated, with reaction degree, non-evaporable water, porosity, hydration products, mechanical properties, pore size distributions and microstructure all being considered. At the same time, considering the problems of impurities and heavy metal elements in phosphogypsum, this study used ultrasonic circulating water washing to treat phosphogypsum at the early stage of the experiment. The amount of impurities in phosphogypsum decreased after treatment, and the water washing solution obtained can be reused. This innovative improvement can not only effectively improve the performance of phosphogypsum composite materials, but also avoid the secondary pollution and high cost caused by pretreatment. The main goal of this study is to use a large amount of phosphogypsum to make an energy-saving, low-carbon, environmentally friendly and long-lasting composite cementitious material with industrial applications.

2. Materials and Methods

2.1. Experimental Materials

The grade 42.5R ordinary Portland cement, produced by the XiNan cement factory, is used in this study. It has a specific surface area of 385 m²/kg and a specific gravity of 3.15 g/cm³, and compressive strengths of 18.7, 29.8, and 44.5 MPa at 3, 7, and 28 days, respectively, according to Chinese standard GB175-2007 [17].

Standard sand with a high silica concentration is used to make a long-lasting composite cementitious material mortar. The particle sizes are kept below 2.35 mm, and it is made by China’s Xia Men ISO Standard Sand Co., Ltd in Xiamen City, Fujian Province, China.

The ultrasonic circulating water washed phosphogypsum used in this experiment came from China’s Kailin Chemical Fertilizer Co., Ltd in Guiyang City, Guizhou Province, China. It has a grey-white appearance, a density of 2.37 g/cm³, and starting and final setting times of 35.4 and 42.6 min, respectively. It contains a small amount of impurities such as P₂O₅ and F-; no Pb, Cr, Ni and other heavy metals were detected. Its entire performance index met the standards of the Chinese national standard GB/T 23456-2018 [18] for second-class phosphogypsum.

Table 1. Chemical composition of cement and phosphogypsum (wt.%).

Name	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Na2O	K2O	P2O5	F
Cement	26.1	7.67	4.69	54.87	2.91	2.96	0.46	0.34	-	-
Phosphogypsum	8.64	0.52	0.12	38.74	0.32	50.24	0.22	0.02	0.83	0.35

depicts the chemical composition of cement and phosphogypsum.

2.2. Sample Preparation

The hydration hardening process of phosphogypsum in composite cementitious materials was investigated in this study, using the mortar mix provided in

Table 2. Mix of mortar with cement and red mud.

NO.	Cement/g	Phosphogypsum/g	Standard Sand/g	Water/g
PG0	450	0	1350	225
PG1	405	45	1350	225
PG2	315	135	1350	225
PG3	225	225	1350	225
PG4	135	315	1350	225
PG5	45	405	1350	225

. All components (cement, phosphogypsum, standard sand, and water) were calculated by mass, and 54 mortar specimens were made, after being stirred by a mortar mixer, with a specimen size of 160 mm × 40 mm × 40 mm. After curing at room temperature for 1 day, all specimens were demolded and cured for 3 days, 7 days, and 28 days at constant room temperature (20 ± 1 °C) and a relative humidity of 95%. Following that, the mechanical characteristics, pore size distributions, microstructure and porosity of mortar specimens were investigated.

Six different mixtures were created by mixing various concentrations of phosphogypsum: 0% (control), 10%, 30%, 50%, 70%, and 90%. 50 percent of the entire cement mass was supplied. All materials (containing cement, phosphogypsum and water) were calculated by mass, and 18 paste specimens were made after evenly mixing; these were placed in 18 centrifuge tubes of 10 mL and cured over 95 percent relative humidity for 3 days, 7 days, and 28 days at room constant temperature (20 ± 1 °C). The degree of reaction, hydration product, and non-evaporable water of composite cementitious material paste specimens were investigated.

2.3. Test Methods

2.3.1. Reaction Degree and Non-Evaporable Water

The difference in mass between 105 °C and 1000 °C is due to the paste’s non-evaporable water content and the loss of dehydrated phosphogypsum [19]. The formula for calculating the non-evaporable water content is shown in Formula (1). According to the Chinese national standard GB/T 12960-2007 [20], the reaction degree of phosphogypsum in a hardened paste was tested using solution selective dissolution. The formula for calculating the reaction degree is shown in Formula (2).

$$w_{ne}=\frac{\left(m_1-m_2\right)/m_1-\left(1-\beta \right)L_c-\beta L_{rm}}{1-\left(1-\beta \right)L_c-\beta L_{rm}}$$

(1)

$${\alpha }_{rm}=1-\frac{w_B/\left(1-w_{ne}\right)-\left(1-\beta \right)w_{c,e}}{\beta w_{rm,e}}$$

(2)

where: w_ne is the non-evaporable water content of paste, m₁ is the mass of sample heated at 105 °C for 24 h, m₂ is the mass of sample heated at 1000 °C for 2.5 h, L_c and L_rm are the ignition losses of cement and phosphogypsum, and β is the phosphogypsum mass fraction. The mass fractions of composite cementitious material residue, cement residue, and phosphogypsum are denoted by w_B, w_c,e and w_rm,e, respectively.

2.3.2. Strength

Regarding the flexural strength experiments, one group of three specimens (with dimensions of 160 mm × 40 mm × 40 mm) were measured at a loading rate of 0.5 kN/s using the three-point method, and the average of three flexural strength specimens was determined as the final value. Six compressive strength specimens (the compressed area is 40 mm × 40 mm) were then tested at a loading rate of 2.4 kN/s according to Chinese national standard GB/T 17671-2005, and the final value was determined as the average of the six specimens.

2.3.3. Phase Analysis

Using an X-ray diffractometer, the mineral composition of the produced cement phosphogypsum paste may be precisely examined (XRD). The X-ray diffractometer X’Pert PRO MPD with copper palladium was used, operating at a voltage of 40 kV and an emission current of 40 mA, with a scanning rate of 0.02° per step and 10 ° per minute, and an experimental range of 10–90°. The various chemicals were identified by the Made Jade software using the PDF-2 database as references. Using a TM4000 Series scanning electron microscope, the microstructure of a mortar sample can be carefully examined (SEM).

2.3.4. Porosity and Pore Size Distribution Analysis

Mercury intrusion porosimeter (MIP, Model is Autopore II 9220) studies were conducted to determine the porosity and pore size distribution of hardened mortar containing 0~90% phosphogypsum at 3 days and 28 days, with a maximum mercury intrusion pressure of 300 MPa.

3. Results and Discussion

3.1. Reaction Degree of Phosphogypsum

The reaction degree of phosphogypsum in composite cementitious material paste comprising 10% to 90% phosphogypsum is shown in Figure 1. As can be seen, a low reaction degree of phosphogypsum was demonstrated. Clearly, the reaction degree of phosphogypsum decreases dramatically as phosphogypsum incorporation is increased, which is equivalent to linear reduction. For example, the reaction degree of phosphogypsum at 3 days is 14.36%, 12.67%, 10.69%, 8.36% and 7.08% with 10%, 30%, 50%, 70% and 90% phosphogypsum, respectively. Based on this, it is not difficult to conclude that the reaction degree of phosphogypsum in composite cementitious material is greatly influenced by the content of phosphogypsum when the other parameters in the mixing proportion remained unchanged. The fundamental cause of this occurrence is that the inclusion of phosphogypsum reduces the concentration of cement, which reduces calcium hydroxide excitation, further reducing phosphogypsum activity. As a result, the lower the reaction degree, the higher the phosphogypsum incorporation. The curing time was extended from 3 to 28 days, and the phosphogypsum reaction degree gradually rose from 14.36% to 24.61% Although phosphogypsum incorporation rose from 10% to 90%, the change rule of the phosphogypsum reaction degree with increased curing time remained consistent. As a result, the phosphogypsum reaction degree in all combinations continued to rise with increased curing time. Even though the curing time is extended, the change rule remained: the higher the incorporation, the lower the activity. This phenomenon is developed around composite cementitious material as a result of a significant number of phosphogypsum calcium ions being dissolved, and a large amount of calcium hydroxide being produced, which increased the phosphogypsum reaction degree. In comparison with ground granulated blast-furnace slag, the reaction degree of phosphogypsum in all combinations is low. According to earlier research [3,4] and experimental findings in this study, it was observed that the reaction degree of industrial waste (such as ground granulated blast-furnace slag, fly ash, and lithium slag) were connected to the mineral composition and cement reaction process.

3.2. Non-Evaporable Water Content of Composite Cementitious Material

The non-evaporable water content of composite cementitious material paste comprising 10% to 90% phosphogypsum is shown in Figure 2. It is noticed that the non-evaporable water content of composite cementitious material was increased initially and then was decreased with the addition of phosphogypsum, clearly reaching its highest level with phosphogypsum inclusion of 10% among all mixtures. The non-evaporable water content of the composite cementitious material paste was steadily dropped in comparison with the paste with 0% phosphogypsum, displaying a quasi-linear relationship. The lower the non-evaporable water content of the paste, the more phosphogypsum was added. For example, the non-evaporable water content of the composite cementitious material paste at 3 days was 14.19%, 14.28%, 10.87%, 8.65%, 6.76%, and 5.88%, respectively, with 0%, 10%, 30%, 50%, 70%, and 90% phosphogypsum. The main cause of this phenomenon is the low phosphogypsum reaction degree. The more phosphogypsum is employed, the less cement is utilized. The excitation impact of calcium hydroxide is reduced. As a result, the more phosphogypsum was added, the lower the non-evaporable water content of the paste became, which was consistent with the reaction degree experiment. Furthermore, when the curing time was extended from 3 to 28 days, the non-evaporable water content of the composite cementitious material paste rose in all combinations. Except for the 10% phosphogypsum inclusion, the non-evaporable water content of the paste with 30%–90% was lower than that of pure cement paste for all curing ages and mixes. For example, the non-evaporable water content of the composite cementitious material paste was 20.21%, 21.13%, 18.28%, 17.95%, 15.48%, and 11.19% after 28 days with 0%, 10%, 30%, 50%, 70%, and 90% phosphogypsum, respectively; in addition, it was at its highest with phosphogypsum incorporation of 10%. Because the reaction degree of phosphogypsum was low, despite showing certain pozzolanic activity due to the dissolution of calcium ions and the presence of sulfate radical in phosphogypsum, the non-evaporable water content of composite cementitious material decreased steadily as the incorporation of phosphogypsum was increased from 10% to 90%.

Figure 3 depicts the relationship between non-evaporable water content and the composite cementitious material reaction degree, which exhibits a good linear relationship. The linear equation of y = 0.96827 + 1.05412x, R² = 0.97509 can be obtained through 15 points in the scatter plot. The non-evaporable water content may be predicted simply by the reaction degree of composite cementitious material containing 0~90%, based on these easy-to-find experimental results.

3.3. Porosity of Composite Cementitious Material

At 3 and 28 days, the porosity of the hardened composite cementitious material mortar containing 0% to 90% phosphogypsum is shown in Figure 4. It is clear that the porosity was dropping initially and then increased with phosphogypsum content from 0% to 90%; however, this rise and decrease ratio was relatively minor. It could be argued that the porosity is essentially controlled at 18–22%, with the greatest difference being less than 4%. In addition to the mechanical qualities, non-evaporable water content and reaction degree, the porosity of mortar containing 10% phosphogypsum was the lowest of all the mixers.

The porosity of mortar containing 0–90% phosphogypsum significantly decreased after the curing time was increased from 3 to 28 days, and the porosity of mortar containing 10% phosphogypsum was lower than that of mortar containing 0% phosphogypsum (which was also the minimum porosity for all mixes). For three days, the porosity was about 61.10%, 62.11%, 67.71%, 72.95%, 80.39%, and 79.28%. As a result, it is clear that the higher the phosphogypsum incorporation, the higher the porosity should be.

3.4. Pore Size Distributions of Composite Cementitious Material

The pore size distributions of hardened composite cementitious material mortar with 0% to 90% phosphogypsum at 3 and 28 days are shown in Figure 5. The volume of the capillary pore of >200 nm is roughly 25% of the overall gel pore volume for hardened cement mortar with 0% phosphogypsum; comparable changes are observed in the rest of the mix proportions, including 10% to 90% phosphogypsum. The capillary pore volume of >200 nm and the overall gel pore volume of mortar containing 10% phosphogypsum were both lower than that of cement mortar, whereas the volume of the capillary pore of 20–100 nm was larger. The volume of the capillary pore of >200 nm, 100–200 nm, 20–100 nm, and 20 nm constantly rose and even exceeded that of cement mortar when phosphogypsum incorporation was over 10%. As a result, the more the phosphogypsum was incorporated, the larger the volume of the capillary pore of >200 nm, 100–200 nm, 20–100 nm, and 20 nm should be.

The change rule of four types of gel pores in hardened composite cementitious material mortar is largely consistent with the curing age of three days after 28 days of curing. For pure cement mortar, the volume of the capillary pore >200 nm was around 25% of the overall gel pore volume, while the total gel pore volume of mortar containing 10% phosphogypsum was the lowest of the mixers. As a result, the pore size distributions of hardened mortar could be refined as curing time passed, demonstrating the continuing hydration process in a phosphogypsum mortar.

3.5. Hydration Products of Composite Cementitious Material

Figure 6 shows the hydration products of a composite cementitious material paste containing 10% to 90% phosphogypsum after 3 days. The hydration products of paste containing 0% phosphogypsum are mostly calcium silicate hydrate (amorphous, cannot be confirmed by XRD), portlandite, quartz, calcite, and hatrurite, as illustrated in Figure 6a, with no clear distinctive peak of ettringite. When phosphogypsum incorporation increased from 0% to 90%, the characteristic peak of portlandite clearly decreased and even vanished, while the characteristic peaks of gypsum, hatrurite, and calcite clearly increased and even reached their strongest. When phosphogypsum incorporation is 90%, the characteristic peak of gypsum reached the strongest among all mixers. Similarly, the distinctive peak of calcium oxide fluoride grew steadily as phosphogypsum incorporation climbed from 0 to 90%; however, the characteristic peak was not visible. These observations showed that phosphogypsum was involved in the secondary hydration reaction, but its activity was minimal. When phosphogypsum incorporation was quite high, the reaction degree of phosphogypsum was limited, and most of it was still filled, resulting in a prominent gypsum characteristic peak.

In comparison with the curing age of 3 days, the hydration products of cement paste are primarily calcium silicate hydrate, portlandite, quartz, calcite, and hatrurite after 28 days of curing. With 10% to 90% phosphogypsum mixed into cement paste, the relative intensity of typical hydration product peaks should be modified. The relative intensity of the distinctive peak of portlandite, for example, was somewhat increased, while the relative intensity of the characteristic peak of quartz and hatrurite was clearly increased, and the relative intensity of the characteristic peak of gypsum was greatly lowered. The fundamental cause of this anomaly is that when the curing time is extended, the cement reaction accelerated and the hydration products (such as portlandite and calcium silicate hydrate) increased, resulting in a small rise in the distinctive peak of portlandite. Furthermore, phosphogypsum, despite its modest activity, can participate in the secondary hydration reaction under the excitation of portlandite. As a result, the gypsum’s unique peak is noticeably weakened.

3.6. Microstructure of Composite Cementitious Material

Figure 7 shows the microstructure of a composite cementitious material mortar containing 10% to 90% phosphogypsum after 3 days. In Figure 7a, hexagonal layered calcium hydroxide, a lot of fibrous calcium silicate hydrate and micropore, a few needle-like ettringite, square particles, and some unhydrated particles are seen wrapped around each other, and the microstructure is relatively dense. The hydration products of mortar with 10% phosphogypsum mainly contained outstanding clustered and fibrous calcium silicate hydrate, a very small amount of needle-like ettringite, short column-like gypsum, and a certain number of pores, which are wrapped around each other, and the microstructure became denser than that of mortar with 0% phosphogypsum. As the amount of phosphogypsum that was mixed in rose from 10% to 90%, clustered calcium silicate hydrate decreased or went away, the length of column-like gypsum, interfacial porosity, and unhydrated particles all increased, and the hydration products piled on top of each other, making the bonding force weaker.

Figure 8 shows scanning electron microscopy pictures of mortar after 28 days of curing. In the microstructure of mortar containing 0% phosphogypsum, clintheriform and layered calcium hydroxide, fibrous calcium silicate hydrate, needle-like ettringite, and a certain number of pores and unhydrated particles wrapped around each other were observed in Figure 8a, and the hydration products were richer in morphology than that of mortar after 3 days. The hydration products of mortar with 10% phosphogypsum primarily contained fibrous calcium silicate hydrate, column-like gypsum, clintheriform and layered calcium hydroxide, and a certain number of pores that are wrapped around one another, resulting in a microstructure that is denser than that of mortar containing 0% phosphogypsum. When phosphogypsum incorporation was increased from 10% to 90%, square hydration products, long and thin gypsum and unhydrated particles grew, clintheriform and layered calcium hydroxide decreased, and the microstructure was generally loose.

3.7. 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.

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 H₃PO₄, Ca(H₂PO₄)₂·H₂O, Ca₃(PO₄)₂, and CaHPO₄·2H₂O [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 Costa [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 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. 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 CaSO₄·2H₂O.

During the hydration process, numerous hydration products, such as clintheriform 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 Figure 9 and Figure 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 Gu [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. Pavlovic 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. Figure 4 and Figure 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.

4. Conclusions

In this study, the effect of mechanical characteristics and hydration mechanisms on composite binder mortar are investigated. The primary experimental findings are as follows:

(1)

As the amount of phosphogypsum is increased, the reaction degree and non-evaporable water are decreased. There is a linear relationship between the two parameters, and their maximum values at 28 days with 10% phosphogypsum are 22.34% and 21.13%, respectively.

(2)

The compressive and flexural strengths of mortar containing 10% phosphogypsum can be up to 49.8 MPa and 9.02 MPa greater, respectively, than those of pure cement mortar.

(3)

In terms of mechanical properties, mortar with 30% phosphogypsum was essentially comparable to cement mortar, which suggested that the dosage of phosphogypsum can be increased to 30%.

(4)

Phosphogypsum can participate in the secondary hydration reaction to create ettringite and consume portlandite, as revealed by XRD studies. In addition, it can contribute to the microaggregate effect, making the microstructure denser.

(5)

According to the MIP and SEM investigations, the maximum amount of phosphogypsum that can lower the overall porosity and refine pore size distributions is 30%. Over 30% phosphogypsum should result in harmful effects.

(6)

After 30% phosphogypsum replaces cement, the cost of mortar is reduced, and the amount of cement in mortar is reduced, which is beneficial to the realization of peak carbon dioxide emissions and carbon neutrality.