# Effect of Soluble P2O5 Form on the Hydration and Hardening of Hemihydrate Phosphogypsum

Authors: Jianwu Zhang; Xiao Wang; Biao Jin; Chunguang Liu; Xiaoting Zhang; Zhixin Li
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

The preparation of building gypsum from phosphogypsum is one of the ways to realize its resource utilization. However, the precondition is first to reveal the influence mechanism of the impurities in phosphogypsum on the properties of plaster. Thus, the influence of the form of soluble P2O5 (H3PO4, H2PO4−, and HPO42−) that is considered the most significant impurity on the hydration and hardening of hemihydrate phosphogypsum was studied in this paper, which included the setting time, mechanical properties, hydration rate, crystal structure, and micromorphology of hydration products. The results showed that both H3PO4 and H2PO4− can accelerate the early hydration of hemihydrate phosphogypsum, shorten the setting time of paste, and improve the mechanical properties of hardened paste. In addition, H3PO4 had a more significant effect than H2PO4−. Different from H3PO4 and H2PO4−, HPO42− significantly delayed the early hydration reaction of hemihydrate phosphogypsum, significantly prolonged the setting time of the paste, and significantly reduced the mechanical properties of the hardened body. The test data of XRD, IR, and SEM showed that two negative effects can be caused when soluble P2O5 existed in the form of HPO42−. On the one hand, HPO42− can lead to the formation of Ca3(PO4)2 insoluble substance in the hydration reaction of hemihydrate phosphogypsum, which significantly hindered the hydration reaction and prolonged the setting time of the paste. On the other hand, HPO42− can change the crystallization habit of the hydration product of CaSO4·2H2O and make it change from needle-columnar to semiflake and irregular shape, resulting in a significant reduction in the mechanical properties of hardened paste.


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## Body

## 1. Introduction

Phosphogypsum is an industrial by-product produced in the process of wet production of phosphoric acid [1, 2]. According to statistics, 4–6 tons of phosphogypsum can be produced for each ton of phosphoric acid [3]. Currently, global emissions of phosphogypsum have exceeded 6 billion tons and are growing at about 150 million tons per year, according to relevant data. The annual output of phosphogypsum in China’s phosphoric acid industry has exceeded 70 million tons, but the utilization ratio is only 40% according to China Phosphate and Compound Fertilizer Industry Association. The stock of phosphogypsum stockpiles has reached 5 hundred million tons in China at present, which not only occupies a lot of land resources but also causes serious environmental pollution to air, groundwater, and soil.Similar to natural gypsum, the main chemical component of phosphogypsum is CaSO4·2H2O, which is usually over 90 wt. %. The production of building gypsum is one of the main ways of resource utilization of phosphogypsum [4–7]. However, the application performance of phosphogypsum is often significantly inferior to that of natural gypsum due to harmful impurities such as P, F, and organic matter [8–13]. Therefore, to explore the influence of harmful impurities in phosphogypsum in order to implement effective impurity removal and purification is always a hot topic in the field of phosphogypsum research but is also one of the key issues in the efficient utilization of phosphogypsum resources.Many studies have showed that phosphorus impurities have the greatest impact on the properties of hemihydrate phosphogypsum, among many types of impurities [14–16]. Phosphorus in phosphogypsum mainly exists in three forms: soluble P2O5, eutectic P2O5, and insoluble P2O5, among which soluble P2O5 has the most significant effect. Soluble P2O5 exists in four forms: H3PO4, HPO42−, HPO42−, and PO43−, which are produced by the ionization of phosphoric acid in an aqueous solution. Due to the high concentration of Ca2+ in gypsum and the low solubility of Ca3 (PO4) 2 as an insoluble salt, the concentration of PO43− in the system is low. Therefore, soluble P2O5 in phosphogypsum mainly exists in three forms: H3PO4, HPO42−, and HPO42−. Many studies have shown that the presence of soluble P2O5 can significantly hinder the hydration, condensation, and hardening process of hemihydrate phosphogypsum and reduce the strength of its hardened body [17–20]. The influence mechanism of soluble P2O5 is that it can combine with calcium ions in a liquid phase to produce insoluble calcium phosphate and cover the surface of gypsum particles, which hinders the hydration reaction of gypsum [21]. However, previous studies on the influence of soluble P2O5 on phosphogypsum performance often do not distinguish the form of soluble P2O5 since a single form of soluble P2O5 is difficult to separate from phosphogypsum, which is obviously not enough to help people fully understand the internal influence mechanism of soluble P2O5. In addition, most studies on the influence of soluble P2O5 on building phosphogypsum mainly focus on macroscopic properties, while there are still few studies on the influence mechanism of soluble P2O5 on the hydration rate of hemihydrate phosphogypsum and the crystal structure of hydration products.Based on the above research deficiencies, this paper intended to study the influence of a soluble P2O5 form on the hydration reaction rate, crystal structure, and micromorphology characteristics of hemihydrate phosphogypsum by introducing different forms of soluble P2O5, establish the relationship between the hydration hardening characteristics of hemihydrate phosphogypsum and its physical and mechanical properties under the influence of soluble P2O5 form, and reveal the internal influence mechanism of soluble P2O5.


## 2. Materials and Test Methods

### 2.1. Materials

Phosphogypsum was purchased from Henan Qiangnai New Material Co., Ltd. Under laboratory conditions, phosphogypsum was washed with deionized water until neutrality to remove soluble impurities. The chemical composition of phosphogypsum before and after water washing is shown in Table1. It can be seen that phosphogypsum contained harmful impurities such as phosphorus and fluorine. After full water washing, the total P2O5 content in phosphogypsum decreased from 0.78% before water washing to 0.64%, indicating that the soluble P2O5 content in phosphogypsum was 0.14%. Phosphoric acid was bought from Yantai Shuangshuang Chemical Co., Ltd, which had a H3PO4 concentration of 85 wt. %. The analytical pure reagents of NaH2PO4 and Na2HPO4 for the experiment were purchased from Tianjin Hengxing Chemical Reagent Co., Ltd. The polycarboxylic acid water reducer (PCE) was purchased from Shandong bock Chemical Co., Ltd. The deionized water was always used in the experiment.Table 1
Chemical composition of phosphogypsum before and after water washing.
ConditionCaOFe2O3Al2O3FSO3SiO2P2O5R2OMgOBefore washing38.280.581.500.3548.258.750.780.950.19After washing39.490.231.360.34549.127.510.640.840.14


### 2.2. Test Methods

#### 2.2.1. Preparation of Hemihydrate Phosphogypsum

First, the washed phosphogypsum was dried at 40°C and then calcined at 150°C for 12 h. Finally, the experimental hemihydrate phosphogypsum can be obtained after 7 d aging treatment of the calcined powder. According to the Chinese Standard GB/T9776-2008, the water consumption for a standard consistency of the hemihydrate phosphogypsum powder and its change with the content of PCE were determined, as shown in Table2. The standard consistency of the experimental phosphogypsum is up to 97% water. The addition of PCE can effectively reduce the standard consistency of hemihydrate phosphogypsum. The water consumption for standard consistency of phosphogypsum is significantly reduced to 71% when the optimum PCE content is 0.8%.Table 2
Test results of water consumption for standard consistency.
PCE dosage (%)00.20.40.60.81.0Water for standard consistency (wt%)978274727171


#### 2.2.2. Setting Time and Mechanical Property Test

First, in order to investigate the influence of the soluble P2O5 form on the setting time and mechanical properties of hemihydrate phosphogypsum, the content of H3PO4, NaH2PO4, and Na2HPO4 was determined to be 0.23%, 0.24%, and 0.28%, respectively, according to the content of soluble P2O5 in phosphogypsum before and after washing. Second, each form of the soluble P2O5 reagent was weighed and pre-dissolved to the mixing water measured by 71 wt. % of the standard water consumption for consistency at the optimal dosage of PCE. Then, the hemihydrate phosphogypsum was added to the mixing water and stirred to make a uniform paste. The initial setting time and final setting time of paste were measured by the vicar apparatus. The prepared paste was molded into blocks of 40 mm × 40 mm × 160 mm. After 1 d age, the wet flexural strength and wet compressive strength of each hardened paste were tested. At the same time, parts of the formed samples were dried in an oven at 40°C until they were completely dried, which was used for the test of dry flexural and dry compressive strength of the hardened paste. The samples with H3PO4, NaH2PO4, and Na2HPO4 were named P-H3PO4, P−H2PO4−, and P−HPO42−, respectively. The sample without the soluble P2O5 reagent was marked as control.


#### 2.2.3. Hydration Degree Test

According to the method described in Section2.2.2, each sample pastes were prepared. After reaching the specified age, the hydration was stopped immediately with anhydrous ethanol for 24 h, and then the samples were dried in an oven at 40°C. Finally, the dried samples were placed in an oven at 250°C for low-temperature heat treatment to constant weight. According to the mass loss of the sample before and after heat treatment, the content of crystal water was calculated to characterize the hydration degree of the sample.


#### 2.2.4. XRD, IR, and SEM Tests

After the dry strength test, the fractured sample was used for XRD, IR, and SEM characterization. The phase composition and structure of hydration product crystals of each sample were tested by a Bruker D8 Advance X-ray diffractometer. The test range was 5–50° at a scanning rate of 0.02°/min. The infrared spectrum data of each hydration sample were collected by the Fourier infrared absorption spectrometer with the model of Nicolet SDX CFTIR. After the fractured samples were coated with gold, the SEM images were captured on a Quanta-450 SEM at 15 kV.


## 2.1. Materials

Phosphogypsum was purchased from Henan Qiangnai New Material Co., Ltd. Under laboratory conditions, phosphogypsum was washed with deionized water until neutrality to remove soluble impurities. The chemical composition of phosphogypsum before and after water washing is shown in Table1. It can be seen that phosphogypsum contained harmful impurities such as phosphorus and fluorine. After full water washing, the total P2O5 content in phosphogypsum decreased from 0.78% before water washing to 0.64%, indicating that the soluble P2O5 content in phosphogypsum was 0.14%. Phosphoric acid was bought from Yantai Shuangshuang Chemical Co., Ltd, which had a H3PO4 concentration of 85 wt. %. The analytical pure reagents of NaH2PO4 and Na2HPO4 for the experiment were purchased from Tianjin Hengxing Chemical Reagent Co., Ltd. The polycarboxylic acid water reducer (PCE) was purchased from Shandong bock Chemical Co., Ltd. The deionized water was always used in the experiment.Table 1
Chemical composition of phosphogypsum before and after water washing.
ConditionCaOFe2O3Al2O3FSO3SiO2P2O5R2OMgOBefore washing38.280.581.500.3548.258.750.780.950.19After washing39.490.231.360.34549.127.510.640.840.14


## 2.2. Test Methods

### 2.2.1. Preparation of Hemihydrate Phosphogypsum

First, the washed phosphogypsum was dried at 40°C and then calcined at 150°C for 12 h. Finally, the experimental hemihydrate phosphogypsum can be obtained after 7 d aging treatment of the calcined powder. According to the Chinese Standard GB/T9776-2008, the water consumption for a standard consistency of the hemihydrate phosphogypsum powder and its change with the content of PCE were determined, as shown in Table2. The standard consistency of the experimental phosphogypsum is up to 97% water. The addition of PCE can effectively reduce the standard consistency of hemihydrate phosphogypsum. The water consumption for standard consistency of phosphogypsum is significantly reduced to 71% when the optimum PCE content is 0.8%.Table 2
Test results of water consumption for standard consistency.
PCE dosage (%)00.20.40.60.81.0Water for standard consistency (wt%)978274727171


### 2.2.2. Setting Time and Mechanical Property Test

First, in order to investigate the influence of the soluble P2O5 form on the setting time and mechanical properties of hemihydrate phosphogypsum, the content of H3PO4, NaH2PO4, and Na2HPO4 was determined to be 0.23%, 0.24%, and 0.28%, respectively, according to the content of soluble P2O5 in phosphogypsum before and after washing. Second, each form of the soluble P2O5 reagent was weighed and pre-dissolved to the mixing water measured by 71 wt. % of the standard water consumption for consistency at the optimal dosage of PCE. Then, the hemihydrate phosphogypsum was added to the mixing water and stirred to make a uniform paste. The initial setting time and final setting time of paste were measured by the vicar apparatus. The prepared paste was molded into blocks of 40 mm × 40 mm × 160 mm. After 1 d age, the wet flexural strength and wet compressive strength of each hardened paste were tested. At the same time, parts of the formed samples were dried in an oven at 40°C until they were completely dried, which was used for the test of dry flexural and dry compressive strength of the hardened paste. The samples with H3PO4, NaH2PO4, and Na2HPO4 were named P-H3PO4, P−H2PO4−, and P−HPO42−, respectively. The sample without the soluble P2O5 reagent was marked as control.


### 2.2.3. Hydration Degree Test

According to the method described in Section2.2.2, each sample pastes were prepared. After reaching the specified age, the hydration was stopped immediately with anhydrous ethanol for 24 h, and then the samples were dried in an oven at 40°C. Finally, the dried samples were placed in an oven at 250°C for low-temperature heat treatment to constant weight. According to the mass loss of the sample before and after heat treatment, the content of crystal water was calculated to characterize the hydration degree of the sample.


### 2.2.4. XRD, IR, and SEM Tests

After the dry strength test, the fractured sample was used for XRD, IR, and SEM characterization. The phase composition and structure of hydration product crystals of each sample were tested by a Bruker D8 Advance X-ray diffractometer. The test range was 5–50° at a scanning rate of 0.02°/min. The infrared spectrum data of each hydration sample were collected by the Fourier infrared absorption spectrometer with the model of Nicolet SDX CFTIR. After the fractured samples were coated with gold, the SEM images were captured on a Quanta-450 SEM at 15 kV.


## 2.2.1. Preparation of Hemihydrate Phosphogypsum

First, the washed phosphogypsum was dried at 40°C and then calcined at 150°C for 12 h. Finally, the experimental hemihydrate phosphogypsum can be obtained after 7 d aging treatment of the calcined powder. According to the Chinese Standard GB/T9776-2008, the water consumption for a standard consistency of the hemihydrate phosphogypsum powder and its change with the content of PCE were determined, as shown in Table2. The standard consistency of the experimental phosphogypsum is up to 97% water. The addition of PCE can effectively reduce the standard consistency of hemihydrate phosphogypsum. The water consumption for standard consistency of phosphogypsum is significantly reduced to 71% when the optimum PCE content is 0.8%.Table 2
Test results of water consumption for standard consistency.
PCE dosage (%)00.20.40.60.81.0Water for standard consistency (wt%)978274727171


## 2.2.2. Setting Time and Mechanical Property Test

First, in order to investigate the influence of the soluble P2O5 form on the setting time and mechanical properties of hemihydrate phosphogypsum, the content of H3PO4, NaH2PO4, and Na2HPO4 was determined to be 0.23%, 0.24%, and 0.28%, respectively, according to the content of soluble P2O5 in phosphogypsum before and after washing. Second, each form of the soluble P2O5 reagent was weighed and pre-dissolved to the mixing water measured by 71 wt. % of the standard water consumption for consistency at the optimal dosage of PCE. Then, the hemihydrate phosphogypsum was added to the mixing water and stirred to make a uniform paste. The initial setting time and final setting time of paste were measured by the vicar apparatus. The prepared paste was molded into blocks of 40 mm × 40 mm × 160 mm. After 1 d age, the wet flexural strength and wet compressive strength of each hardened paste were tested. At the same time, parts of the formed samples were dried in an oven at 40°C until they were completely dried, which was used for the test of dry flexural and dry compressive strength of the hardened paste. The samples with H3PO4, NaH2PO4, and Na2HPO4 were named P-H3PO4, P−H2PO4−, and P−HPO42−, respectively. The sample without the soluble P2O5 reagent was marked as control.


## 2.2.3. Hydration Degree Test

According to the method described in Section2.2.2, each sample pastes were prepared. After reaching the specified age, the hydration was stopped immediately with anhydrous ethanol for 24 h, and then the samples were dried in an oven at 40°C. Finally, the dried samples were placed in an oven at 250°C for low-temperature heat treatment to constant weight. According to the mass loss of the sample before and after heat treatment, the content of crystal water was calculated to characterize the hydration degree of the sample.


## 2.2.4. XRD, IR, and SEM Tests

After the dry strength test, the fractured sample was used for XRD, IR, and SEM characterization. The phase composition and structure of hydration product crystals of each sample were tested by a Bruker D8 Advance X-ray diffractometer. The test range was 5–50° at a scanning rate of 0.02°/min. The infrared spectrum data of each hydration sample were collected by the Fourier infrared absorption spectrometer with the model of Nicolet SDX CFTIR. After the fractured samples were coated with gold, the SEM images were captured on a Quanta-450 SEM at 15 kV.


## 3. Results and Discussion

### 3.1. Setting Time

Figure1 shows the effect of the soluble P2O5 form on the setting time of building phosphogypsum. For the control sample, the initial setting time and final setting time were 11 min and 17 min, respectively. The addition of H3PO4 can significantly accelerate the setting and hardening rate of the paste, and the initial setting time and final setting time were shortened to 7 min and 12 min, respectively. Similar to H3PO4, the addition of H2PO4− can also promote the setting and hardening of the paste, but its effect is less than that of H3PO4. Different from the effect of soluble P2O5 in the form of H3PO4 and H2PO4−, the addition of HPO42− significantly delayed the setting and hardening rate of paste. The test data showed that the initial setting time and final setting time of P−HPO42− sample were significantly prolonged to 21 min and 27 min, respectively, compared with the control sample.Figure 1
Influence of soluble P2O5 form on setting time of hemihydrate phosphogypsum.


### 3.2. Mechanical Properties

Figure2 shows the effect of different forms of soluble P2O5 on the compressive strength of hemihydrate phosphogypsum. For the control sample, the wet compressive strength and dry compressive strength of hardened pastes were 3.8 MPa and 6.8 MPa, respectively. After the addition of H3PO4, the wet compressive strength and dry compressive strength of hardened paste increased to 5.3 MPa and 9.5 MPa, respectively, which increased by 39.5% and 39.7% compared with the control sample, respectively. The addition of H2PO4- also can improve the compressive strength of hemihydrate phosphogypsum, but the effect was not as significant as H3PO4. The wet compressive strength and dry compressive strength of the P−H2PO4− sample increased by 28.9% and 25%, respectively, compared with the control sample. Different from H3PO4 and H2PO4−, the addition of HPO42− significantly reduced the strength of hemihydrate phosphogypsum. The data showed that the wet compressive strength and dry compressive strength of the P−HPO42− sample were only 3.6 MPa and 3.1 MPa, respectively, which were reduced by 5.3% and 20.5%, respectively, compared with the control sample. Figure 3 shows the flexural strength test results. It can be seen that the variation law of flexural strength and compressive strength was similar. Similarly, the addition of H3PO4 and H2PO4− can improve the wet and dry flexural strength of hemihydrate phosphogypsum, and the effect of H3PO4 is more significant than H2PO4−. The addition of HPO42− can significantly reduce the flexural strength of the hardened paste.Figure 2
Influence of soluble P2O5 form on the compressive strength of hemihydrate phosphogypsum.Figure 3
Influence of soluble P2O5 form on the flexural strength of hemihydrate phosphogypsum.


### 3.3. Hydration Degree

The effect of different forms of P2O5 on the crystalline water content of hemihydrate phosphogypsum is shown in Figure 4. It can be seen that, in the initial hydration stage, the form soluble P2O5 of H3PO4 and H2PO4− can effectively accelerate the hydration reaction of hemihydrate phosphogypsum, and the role of H3PO4 in promoting hydration is significantly higher than that of H2PO4−. Different from H3PO4 and H2PO4−, soluble P2O5 in the form of HPO42− can significantly delay the early hydration reaction of the hemihydrate phosphogypsum. For example, after hydration for 30 min, the content of crystal water in P-H3PO4 and P−H2PO4− samples was 18.1% and 16.4%, respectively, which was significantly higher than of the control sample. However, the crystalline water content of P−HPO42− sample was only 12.3% at this time, which was significantly lower than that of the control sample. After 24 h hydration, the crystal water content of each sample was similar, which was close to the theoretical crystal water content of dihydrate gypsum crystals, indicating that the hemihydrate gypsum in each sample has been completely hydrated. By comparing the data in Figures 2 and 5, it can be seen that the effect of soluble P2O5 form on the hydration rate of hemihydrate phosphogypsum was consistent with its effect on the setting time.Figure 4
Influence of soluble P2O5 form on the content of water of crystallization of hemihydrate phosphogypsum at different hydration times.Figure 5
XRD spectra of each sample.


### 3.4. XRD Analysis

Figure5 shows the XRD spectrum of each hydration sample. It can be seen that the hydration products of each 9 are calcium sulfate dihydrate crystals. Moreover, the peak shape of the diffraction peak in each sample was very sharp, which indicated that the crystal crystallization degree of the hydration products of dihydrate gypsum in each sample was high. The difference was that the peak intensity of the three strong peaks of dihydrate gypsum crystal in the P-H3PO4 sample was significantly lower than that of the control, P−H2PO4−, and P−HPO42− samples. Generally speaking, the intensity of the crystal diffraction peak was directly proportional to the number of crystals and crystallinity. According to the test results of the hydration rate, the amount of the dihydrate gypsum crystal in each sample was the same after 1 d hydration. Therefore, it can be inferred that the existence of soluble P2O5 may affect the hydration crystallization process of hemihydrate phosphogypsum and then significantly change the crystallization degree of the crystal surface corresponding to the three strong peaks in dihydrate gypsum crystals.In order to further explain the influence of soluble P2O5, the (020) crystal plane diffraction peak was taken as the research object for in-depth analysis and description, as shown in Figure 6. It can be seen that the diffraction angles of the crystal plane diffraction peaks of each sample (020) were different. Through the peak seeking function of jade analysis software, the diffraction angles of (020) crystal plane of the control sample, P-H3PO4, P−H2PO4−, and P−HPO42− can be determined to be 11.63°, 11.585°, 11.643°, and 11.679°, respectively. This phenomenon may be caused by the formation of CaHPO4·2H2O solid solution in a dihydrate gypsum crystal due to the existence of soluble P2O5. Many studies showed that CaHPO4·2H2O and CaSO4·2H2O belong to a monoclinic system and have relatively similar lattice constants. Therefore, CaHPO4·2H2O can enter the CaSO4·2H2O lattice to form a solid solution under certain conditions. In order to facilitate the analysis, the diffraction angles of the diffraction peaks of the (020) crystal plane of pure CaSO4·2H2O and pure CaHPO4·2H2O are calibrated in Figure 6, which were 11.588° and 11.68°, respectively. For the control sample, the diffraction angle of (020) crystal plane was between CaSO4·2H2O and CaHPO4·2H2O crystals. It can be seen that there was a solid solution of CaHPO4·2H2O in the hydration process of the control sample. The source of CaHPO4·2H2O in the control sample was mainly related to its eutectic phosphorus. Relevant research studies showed that the eutectic P2O5 still existed when phosphogypsum was calcined at 150°C. Therefore, when hemihydrate phosphogypsum reacted with water, part of HPO42− dissolved from its lattice may reparticipate in the crystallization process of dihydrate gypsum. For the P-H3PO4 sample, the diffraction angle of the (020) crystal plane was very close to that of pure CaSO4·2H2O. This may be due to the fact that H3PO4 was difficult to deeply ionize and produce more HPO42- ions in the reaction liquid phase as a weak electrolyte, which resulted in the crystal surface diffraction peak site of P-H3PO4 being closer to pure CaSO4·2H2O. For P-H2PO4-, the shift of diffraction peak position to the right may also be related to the significant ionization of H2PO4- ions. For P−HPO42−, the directly doped HPO42− ions can significantly participate in the formation of calcium sulfate dihydrate, resulting in the crystal diffraction peak position more inclined to the direction of CaHPO4·2H2O.Figure 6
X-ray diffraction peak of the (020) crystal plane of CaSO4·2H2O in each sample.Accordingly, the eutectic phenomenon in the control sample, P-H2PO4-, and P−HPO42− significantly inhibited the growth of gypsum crystals along the c-axis and relatively increased the growth rate and crystallinity of (020) crystal planes, which resulted in the diffraction peak intensity of (020) crystal planes in these three groups being significantly higher than that of P-H3PO4.


### 3.5. IR Analysis

In order to further analyze the influence mechanism of the form of soluble P2O5, the infrared spectrum data of each hydration sample were tested, as shown in Figure 7. The stretching vibration peaks of crystalline water hydroxyl in calcium sulfate dihydrate were detected near 3547 cm−1 and 3045 cm−1. The peak near 1685 cm−1 and 1621 cm−1 was the bending vibration peak of S-O in CaSO4. For the sample of control, P−H2PO4−, and P−HPO42−, the characteristic absorption peak of eutectic phosphorus was also detected near 845 cm−1, which was consistent with the analysis results of XRD. In addition, the characteristic absorption peak of PO43− was also detected near 1036 cm−1 in control and P−HPO42− sample, indicating that insoluble Ca3(PO4)2 was generated in the hydration process of these two groups of samples. The reason why the control sample and P−HPO42− sample had a longer setting time than other group may be related to this phenomenon. Many studies have proved that the formation of Ca3(PO4)2 can effectively cover the surface of hemihydrate gypsum crystal and delay its hydration process [21, 22].Figure 7
Infrared spectra of each sample.


### 3.6. Scanning Electron Microscopy

Figure8 shows the microtopography photograph of each sample. For the control sample, the hydrated dihydrate gypsum crystal mainly showed a coarse columnar structure, and a part of the form of the product was irregular plate. In addition, there was an obvious parallel superposition between the columnar crystal products. Obviously different from the control sample, the dihydrate gypsum crystal formed in the P-H3PO4 sample was significantly needle columnar, and each crystal showed a high degree of overlapping and interleaving, which reflected relatively higher strength of the hardened body. For the P−H2PO4− sample, the morphology of hydration products was mainly a needle column and an irregular shape. The crystal morphology of hydration products in the P−HPO42− sample was mostly plate-like or irregular, and only a few crystals were columnar. These morphological characteristics significantly affect the crystal skeleton effect of dihydrate gypsum in the P-HPO42- sample, resulting in significantly poor mechanical properties of its hardened body.Figure 8
Microscopic morphologies of each sample: (a) control; (b) P-H3PO4; (c) P−H2PO4−; (d) P−HPO42−.
(a)(b)(c)(d)


## 3.1. Setting Time

Figure1 shows the effect of the soluble P2O5 form on the setting time of building phosphogypsum. For the control sample, the initial setting time and final setting time were 11 min and 17 min, respectively. The addition of H3PO4 can significantly accelerate the setting and hardening rate of the paste, and the initial setting time and final setting time were shortened to 7 min and 12 min, respectively. Similar to H3PO4, the addition of H2PO4− can also promote the setting and hardening of the paste, but its effect is less than that of H3PO4. Different from the effect of soluble P2O5 in the form of H3PO4 and H2PO4−, the addition of HPO42− significantly delayed the setting and hardening rate of paste. The test data showed that the initial setting time and final setting time of P−HPO42− sample were significantly prolonged to 21 min and 27 min, respectively, compared with the control sample.Figure 1
Influence of soluble P2O5 form on setting time of hemihydrate phosphogypsum.


## 3.2. Mechanical Properties

Figure2 shows the effect of different forms of soluble P2O5 on the compressive strength of hemihydrate phosphogypsum. For the control sample, the wet compressive strength and dry compressive strength of hardened pastes were 3.8 MPa and 6.8 MPa, respectively. After the addition of H3PO4, the wet compressive strength and dry compressive strength of hardened paste increased to 5.3 MPa and 9.5 MPa, respectively, which increased by 39.5% and 39.7% compared with the control sample, respectively. The addition of H2PO4- also can improve the compressive strength of hemihydrate phosphogypsum, but the effect was not as significant as H3PO4. The wet compressive strength and dry compressive strength of the P−H2PO4− sample increased by 28.9% and 25%, respectively, compared with the control sample. Different from H3PO4 and H2PO4−, the addition of HPO42− significantly reduced the strength of hemihydrate phosphogypsum. The data showed that the wet compressive strength and dry compressive strength of the P−HPO42− sample were only 3.6 MPa and 3.1 MPa, respectively, which were reduced by 5.3% and 20.5%, respectively, compared with the control sample. Figure 3 shows the flexural strength test results. It can be seen that the variation law of flexural strength and compressive strength was similar. Similarly, the addition of H3PO4 and H2PO4− can improve the wet and dry flexural strength of hemihydrate phosphogypsum, and the effect of H3PO4 is more significant than H2PO4−. The addition of HPO42− can significantly reduce the flexural strength of the hardened paste.Figure 2
Influence of soluble P2O5 form on the compressive strength of hemihydrate phosphogypsum.Figure 3
Influence of soluble P2O5 form on the flexural strength of hemihydrate phosphogypsum.


## 3.3. Hydration Degree

The effect of different forms of P2O5 on the crystalline water content of hemihydrate phosphogypsum is shown in Figure 4. It can be seen that, in the initial hydration stage, the form soluble P2O5 of H3PO4 and H2PO4− can effectively accelerate the hydration reaction of hemihydrate phosphogypsum, and the role of H3PO4 in promoting hydration is significantly higher than that of H2PO4−. Different from H3PO4 and H2PO4−, soluble P2O5 in the form of HPO42− can significantly delay the early hydration reaction of the hemihydrate phosphogypsum. For example, after hydration for 30 min, the content of crystal water in P-H3PO4 and P−H2PO4− samples was 18.1% and 16.4%, respectively, which was significantly higher than of the control sample. However, the crystalline water content of P−HPO42− sample was only 12.3% at this time, which was significantly lower than that of the control sample. After 24 h hydration, the crystal water content of each sample was similar, which was close to the theoretical crystal water content of dihydrate gypsum crystals, indicating that the hemihydrate gypsum in each sample has been completely hydrated. By comparing the data in Figures 2 and 5, it can be seen that the effect of soluble P2O5 form on the hydration rate of hemihydrate phosphogypsum was consistent with its effect on the setting time.Figure 4
Influence of soluble P2O5 form on the content of water of crystallization of hemihydrate phosphogypsum at different hydration times.Figure 5
XRD spectra of each sample.


## 3.4. XRD Analysis

Figure5 shows the XRD spectrum of each hydration sample. It can be seen that the hydration products of each 9 are calcium sulfate dihydrate crystals. Moreover, the peak shape of the diffraction peak in each sample was very sharp, which indicated that the crystal crystallization degree of the hydration products of dihydrate gypsum in each sample was high. The difference was that the peak intensity of the three strong peaks of dihydrate gypsum crystal in the P-H3PO4 sample was significantly lower than that of the control, P−H2PO4−, and P−HPO42− samples. Generally speaking, the intensity of the crystal diffraction peak was directly proportional to the number of crystals and crystallinity. According to the test results of the hydration rate, the amount of the dihydrate gypsum crystal in each sample was the same after 1 d hydration. Therefore, it can be inferred that the existence of soluble P2O5 may affect the hydration crystallization process of hemihydrate phosphogypsum and then significantly change the crystallization degree of the crystal surface corresponding to the three strong peaks in dihydrate gypsum crystals.In order to further explain the influence of soluble P2O5, the (020) crystal plane diffraction peak was taken as the research object for in-depth analysis and description, as shown in Figure 6. It can be seen that the diffraction angles of the crystal plane diffraction peaks of each sample (020) were different. Through the peak seeking function of jade analysis software, the diffraction angles of (020) crystal plane of the control sample, P-H3PO4, P−H2PO4−, and P−HPO42− can be determined to be 11.63°, 11.585°, 11.643°, and 11.679°, respectively. This phenomenon may be caused by the formation of CaHPO4·2H2O solid solution in a dihydrate gypsum crystal due to the existence of soluble P2O5. Many studies showed that CaHPO4·2H2O and CaSO4·2H2O belong to a monoclinic system and have relatively similar lattice constants. Therefore, CaHPO4·2H2O can enter the CaSO4·2H2O lattice to form a solid solution under certain conditions. In order to facilitate the analysis, the diffraction angles of the diffraction peaks of the (020) crystal plane of pure CaSO4·2H2O and pure CaHPO4·2H2O are calibrated in Figure 6, which were 11.588° and 11.68°, respectively. For the control sample, the diffraction angle of (020) crystal plane was between CaSO4·2H2O and CaHPO4·2H2O crystals. It can be seen that there was a solid solution of CaHPO4·2H2O in the hydration process of the control sample. The source of CaHPO4·2H2O in the control sample was mainly related to its eutectic phosphorus. Relevant research studies showed that the eutectic P2O5 still existed when phosphogypsum was calcined at 150°C. Therefore, when hemihydrate phosphogypsum reacted with water, part of HPO42− dissolved from its lattice may reparticipate in the crystallization process of dihydrate gypsum. For the P-H3PO4 sample, the diffraction angle of the (020) crystal plane was very close to that of pure CaSO4·2H2O. This may be due to the fact that H3PO4 was difficult to deeply ionize and produce more HPO42- ions in the reaction liquid phase as a weak electrolyte, which resulted in the crystal surface diffraction peak site of P-H3PO4 being closer to pure CaSO4·2H2O. For P-H2PO4-, the shift of diffraction peak position to the right may also be related to the significant ionization of H2PO4- ions. For P−HPO42−, the directly doped HPO42− ions can significantly participate in the formation of calcium sulfate dihydrate, resulting in the crystal diffraction peak position more inclined to the direction of CaHPO4·2H2O.Figure 6
X-ray diffraction peak of the (020) crystal plane of CaSO4·2H2O in each sample.Accordingly, the eutectic phenomenon in the control sample, P-H2PO4-, and P−HPO42− significantly inhibited the growth of gypsum crystals along the c-axis and relatively increased the growth rate and crystallinity of (020) crystal planes, which resulted in the diffraction peak intensity of (020) crystal planes in these three groups being significantly higher than that of P-H3PO4.


## 3.5. IR Analysis

In order to further analyze the influence mechanism of the form of soluble P2O5, the infrared spectrum data of each hydration sample were tested, as shown in Figure 7. The stretching vibration peaks of crystalline water hydroxyl in calcium sulfate dihydrate were detected near 3547 cm−1 and 3045 cm−1. The peak near 1685 cm−1 and 1621 cm−1 was the bending vibration peak of S-O in CaSO4. For the sample of control, P−H2PO4−, and P−HPO42−, the characteristic absorption peak of eutectic phosphorus was also detected near 845 cm−1, which was consistent with the analysis results of XRD. In addition, the characteristic absorption peak of PO43− was also detected near 1036 cm−1 in control and P−HPO42− sample, indicating that insoluble Ca3(PO4)2 was generated in the hydration process of these two groups of samples. The reason why the control sample and P−HPO42− sample had a longer setting time than other group may be related to this phenomenon. Many studies have proved that the formation of Ca3(PO4)2 can effectively cover the surface of hemihydrate gypsum crystal and delay its hydration process [21, 22].Figure 7
Infrared spectra of each sample.


## 3.6. Scanning Electron Microscopy

Figure8 shows the microtopography photograph of each sample. For the control sample, the hydrated dihydrate gypsum crystal mainly showed a coarse columnar structure, and a part of the form of the product was irregular plate. In addition, there was an obvious parallel superposition between the columnar crystal products. Obviously different from the control sample, the dihydrate gypsum crystal formed in the P-H3PO4 sample was significantly needle columnar, and each crystal showed a high degree of overlapping and interleaving, which reflected relatively higher strength of the hardened body. For the P−H2PO4− sample, the morphology of hydration products was mainly a needle column and an irregular shape. The crystal morphology of hydration products in the P−HPO42− sample was mostly plate-like or irregular, and only a few crystals were columnar. These morphological characteristics significantly affect the crystal skeleton effect of dihydrate gypsum in the P-HPO42- sample, resulting in significantly poor mechanical properties of its hardened body.Figure 8
Microscopic morphologies of each sample: (a) control; (b) P-H3PO4; (c) P−H2PO4−; (d) P−HPO42−.
(a)(b)(c)(d)


## 4. Conclusions

This paper mainly studies the influence of soluble P in three forms of H3PO4, H2PO4−, and HPO42− on the hydration and hardening of hemihydrate phosphogypsum. The main conclusions were as follows:(1)
Soluble P2O5 in the form of H3PO4 and H2PO4− can effectively accelerate the early hydration reaction of hemihydrate phosphogypsum, and the influence of H3PO4 was greater than that of H2PO4−. Accordingly, the setting time of paste with soluble P2O5 in the form of H3PO4 and H2PO4− was significantly shortened, and similarly, the effect of H3PO4 was more significant than that of H2PO4−. Different from H3PO4 and H2PO4−, HPO42− significantly hindered the early hydration reaction and prolonged the setting time of hemihydrate phosphogypsum.(2)
The test results of mechanical properties showed that when the soluble P2O5 in phosphogypsum mainly existed in the form of H3PO4 and H2PO4−, the mechanical properties of building phosphogypsum can be improved to a certain extent, and the enhancement effect of H3PO4 on the mechanical properties of hardened paste was higher than that of H2PO4−. On the contrary, soluble P2O5 with HPO42− as the main form had a significant negative effect on the mechanical properties of hemihydrate phosphogypsum.(3)
The test results of XRD, IR, and SEM comprehensively reflected that two negative effects can be caused when the soluble P2O5 in phosphogypsum exists in the form of HPO42−. On the one hand, HPO42− can lead to the formation of Ca3(PO4)2 insoluble substance in the hydration reaction of hemihydrate phosphogypsum, which significantly hindered the hydration reaction and prolonged the setting time of the paste. On the other hand, HPO42− can change the crystallization habit of the hydration product of CaSO4·2H2O and make it change from needle-columnar to semiflake and irregular shape, resulting in a significant reduction in the mechanical properties of the hardened paste.



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*Source: 1212649-2022-03-01.xml*