Reuse of Phosphogypsum Pretreated with Water Washing as Aggregate for Cemented Backfill

Reuse of Phosphogypsum Pretreated with Water Washing as Aggregate for Cemented Backfill | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Reuse of Phosphogypsum Pretreated with Water Washing as Aggregate for Cemented Backfill Yanan Zhou, Xibing Li, Ying Shi, Quanqi Zhu, Jing Du This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1172069/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 4 You are reading this latest preprint version Abstract Phosphogypsum (PG) is reused as aggregate in the cemented backfill of the mines, which effectively improves the PG reutilization efficiency. However, the massive impurities contained in aggregate PG would adversely affect the hydration of binder, and therefore deteriorating strength development of backfill. This research starts with the feasibility study on pretreating the aggregate PG with the water washing method. Based on the most economical principle of the water demand, the optimal conditions for washing PG were determined at a stirring time of 5 min and a solid-liquid ratio of 1:0.5. Then, the original and the pretreated PG were made into the backfill. Compared to using the original PG, the backfill slurry using the pretreated PG had better fluidity performance, such as the lower slurry viscosity, the higher bleeding rate and the shorter setting times. Furthermore, with the pretreated aggregate PG, the strength of the backfill was significantly enhanced by more than 8 times, which was due to the removal of impurities on the surface of PG in the pretreatment. Finally, the environmental behavior of the cemented backfill was investigated. Using the pretreated PG as aggregate, concentrations of PO 4 3− -P and F − in the bleeding water and leachates of backfill could meet the Chinese standard for integrated wastewater discharge, which significantly reduced the adverse effect of PG on the environment. The results extend the reuse of PG as aggregate in a more environmental-friendly way, meeting the needs for sustainable mines. Phosphogypsum Water washing Pretreatment Waste reuse Cemented PG backfill Impurity reduction Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1 Introduction Cemented backfill is an effective means to increase ore recovery, improve safety conditions, and reduce surface disposal of solid wastes. As a typical solid waste, phosphogypsum (PG) is the by-product generated during the exploitation of phosphate resources (Moreira et al. 2018, Sahoo and Joseph 2021, Wang et al. 2020). Global production of PG is estimated to be around 100-280 Mt annually, of which China contributes to 25% (Amrani et al. 2020, Ding et al. 2019). Currently, PG is recycled as additives in building materials, soil modifiers and cement productions, but with a limited utilization rate of 15% (Lin et al. 2012, Rashad 2017, Wang 2020). In 2008, Li et al. (2008) innovatively proposed a cemented backfill technique with PG as aggregate, effectively improving PG utilization rate up to 60%. In the cemented PG backfill process, the aggregate PG (over 80% by dry weight) is mixed with binder and water to a heterogeneous backfill slurry, which is then pumped to the underground mined-out areas. The slurry gradually dewaters and consolidates, building up strength to support the rock walls in the underground mines. As the primary backfill material, aggregate PG is mainly composed of CaSO 4 ·2H 2 O, and it also contains large quantities of impurities such as residual acids, phosphates, fluorides and heavy metals. Previous studies have shown that the impurities might seriously deteriorate the hydration process of the backfill and cause serve environmental pollution. Li and Fall (2018) added sulfate in slag-cemented paste backfill and found that high sulfate content negatively impacted the early age strength and self-desiccation of the backfill. Chen et al. (2020) explored the effects of chloride on the mechanical properties of gangue-cemented paste backfill. The results showed that the early strength of backfill decreased obviously when the initial chlorine content was more than 40‰. Zhou et al. (2020) prepared cemented backfill using PG with various phosphate contents and demonstrated that the 120d strength decreased from 2.04 to 0.30 MPa as the dissolved phosphate in PG increased from 29 to 377 mmol/kg. Another concern was the potential environmental hazards due to the high phosphate content in PG. When the phosphate content in PG exceeded 87 mmol/kg, the backfill would cause phosphate pollution to the environment. Furthermore, it is worth noting that PG is a hyperacidic solid waste with a pH value usually within 3, compared with other neutral filling aggregates (Sahoo and Joseph 2021, Wu et al. 2021). However, hydration reactions commonly occur under strongly alkaline conditions (pH>11.5) (Song and Jennings 1999). Therefore, the residual acids in PG would neutralize hydroxyl ions of binder and interfere with the hydration reaction of the backfill, which in turn disturbs the strength development of the backfill. As a result, it is necessary to pretreat solid wastes to mitigate the adverse effects in their secondary utilization. Actually, several studies have found that pretreatment of solid waste can effectively improve the workability of cementation and reduce environmental pollution. Shao et al. (2020) treated corn cob aggregates with cement paste to substitute sand in cement mortars, which improved the ductility of cement mortars. Singh (2002) depicted that the pretreated PG (treating with 3-4% aqueous citric acid) could be used as an additive in place of mineral gypsum for the manufacture of ordinary Portland cement and Portland slag cement. Mao et al. (2020) washed fly ash with water, and it was found that the consolidation rate of heavy metals was above 92% in the treated fly ash. When it was prepared as cementing materials, the consolidation rate was further increased to over 99%, resulting in the leaching concentration of heavy metals was far lower than the national standard limit. Based on these results, the pretreatment of aggregate PG should be considered to reduce the impurities content, therefore ensuring safety for the mining and environment. The PG pretreatment protocols are currently main as follows: chemical, thermal and physical treatments (Ennaciri et al. 2019, Moalla et al. 2017, Smadi et al. 1999). Chemical and thermal treatments of PG can effectively reduce the soluble impurities and organic matters, but the operation process is cumbersome and costly. Generally, the physical treatment of PG, especially water washing, is still preferred in the industry due to its simple operation. Singh et al. (1996) washed PG at a volume proportion of 1:3 for three durations of 30, 50 and 65 min, and found that 63.0% of phosphates, 66.1% of fluorides and 80.7% of organic matters could be removed. Subsequently, Zhao et al. (2017) washed PG with a mass ratio of PG to water of 1:10 for 30 min, and the results showed a reduction in soluble phosphates from 0.79–0.46%, fluorides from 0.87–0.61% and magnesium from 0.09% to 0, respectively. These findings indicate that impurities can be reduced by washing PG with varying solid-liquid (S/L) ratios and stirring times. However, previous studies usually washed PG for only one time to calculate washing efficiency. In fact, multiple washes can achieve a continuous reduction of impurities in solid waste (Mao et al. 2020, Wang et al. 2021). Therefore, the removal efficiency of impurities in PG under different washing conditions should be investigated. The purpose of this study is to further explore the effect of pretreatment aggregate on the cemented backfill process. By considering different stirring durations the number of washing times and the S/L ratios, the optimal condition of water washing pretreatment of PG was determined. Following this, the original PG with different initial pH values was collected as the control group. Subsequently, the original and pretreated PG were made into cemented backfill. The properties of the backfill slurry, the strength and microstructure of hardened backfills, and the resultant surrounding environment impacts were investigated. 2 Materials And Methods 2.1 Raw materials This study evaluated representative samples of PG and composite binder in Guizhou, China. The binder is composed of yellow phosphorous slag: fly ash: cement clinker in 4:1:1, and 16-20% lime of the yellow phosphorous slag mass ratio is added. The main chemical compositions (measured by X-ray fluorescence; Bruker, Switzerland )and physical properties (measured by a particle size analyzer; Malvern Instruments, UK)of PG with different pH values were investigated through the toxicity leaching test, as listed in Table 1 . Table 1 Impurity concentrations and physical properties of PG and binder. Impurity concentration Raw PG Binder PG-1 PG-2 PG-3 PG-4 PG-5 pH 1.75 1.99 2.63 3.52 4.99 13.16 PO 4 3− (mg/L) 4840 4000 1510 252 20 0.04 F − (mg/L) 1641 1103 521 509 182 16 SO 4 2− (mg/L) 10885 10042 4419 2313 1359 487 TDS (ppt) 7.89 3.55 2.79 1.63 0.73 4.91 Physical property D 10 \(\left({\mu }\text{m}\right)\) 13.56 11.94 11.94 10.61 15.61 6.02 D 30 \(\left({\mu }\text{m}\right)\) 42.79 29.18 33.15 36.10 37.76 13.73 D 60 \(\left({\mu }\text{m}\right)\) 92.05 55.24 81.02 67.08 81.31 30.18 C u =D 60 /D 10 6.79 4.63 6.79 4.04 5.21 5.01 C c =D 30 2 /(D 60 *D 10 ) 1.47 1.29 1.14 1.17 1.12 1.04 2.2 Water washing design and cemented backfill procedures In this study, different methods were used to wash the PG. Due to the strong acidity of PG-1 with an initial pH value of 1.75, which was selected to study the effects of stirring time and S/L ratio on the water washing of PG. The PG with initial pH values of 1.75, 1.99 and 2.63 (PG-1, PG-2 and PG-3) were selected to study the effects of water washing on PG with different initial pH values. And PG-4 and PG-5 with initial pH values of 3.52 and 4.99 were selected as the control group. The weight of dry PG, wet PG, water for the first washing and each subsequent washing is shown in Table 2 . Table 2 Mix design of water washing pretreatment of PG samples. PG No. S/L ratio Weight of PG Weight of water Dry (g) Wet (g) First washing (g) Next washing (g) PG-1 1:0.5 100 149.10 0.90 50.00 1:1 100 149.10 50.90 100.00 1:1.5 100 149.10 100.90 150.00 1:2 100 149.10 150.90 200.00 PG-2 1:0.5 100 123.44 26.56 50.00 1:1 100 123.44 76.56 100.00 1:1.5 100 123.44 126.56 150.00 1:2 100 123.44 176.56 200.00 PG-3 1:0.5 100 100.33 49.67 50.00 1:1 100 100.33 99.67 100.00 1:1.5 100 100.33 149.67 150.00 1:2 100 100.33 199.67 200.00 Backfill slurry was prepared by mixing the PG, binder and deionized water with a mass proportion of 5:1:6. In accordance with the experiment scheme, PG and deionized water were first mixed evenly in the stirred vessel to prevent blocking. Then the binder was slowly poured into the PG mixture and stirred homogeneously at 200 rpm/min for 30 min. Next, the backfill slurry was injected into plastic molds with internal dimensions of 40 mm × 40 mm × 40 mm. There was a 0.2 mm small hole at the bottom of the mold to drain the excess water in the slurry. After the slurry was set, the hardened samples were taken out of the molds and cured into a chamber maintained with a constant temperature of 20±2°C and humidity of 90±5%. The flow diagram for this work is shown in Fig. 1 . 2.3 Test methods 2.3.1 Bleeding rate The bleeding rate was measured according to the Chinese standard GB/T 50080-2016. The backfill slurry was injected into a container with a lid and then placed on a vibrator for 20 s to make the slurry denser. The bleeding water was drawn with a syringe at every 30 min until no more water was secreted for three consecutive times. The bleeding rate was calculated by using Eq. ( 1 ): $$B=\frac{Vw}{\left(\frac{W}{G}\right)Gw}\times 100$$ 1 Where B is the bleeding rate (%), Vw is the mass of bleeding water in the container (g), W is the total mass of water in the backfill slurry (g), G is the total mass of backfill slurry (g), Gw is the mass of backfill slurry in the container (g). 2.3.2 Apparent viscosity The apparent viscosity is one of the essential rheological properties of backfill slurry(Atesok et al. 2002), which affects a series of actual conditions such as slurry transportation and pumping. The apparent viscosity of slurry was evaluated according to ASTM D2196-18 by using a DV-1 digital viscometer (Brookfield, USA). Due to the continuous hydration reaction of the slurry, in order to ensure the reliability of the test data, the measurement should be conducted immediately after slurry preparation. 2.3.3 Setting time The initial setting time (IST) and final setting time (FST) of the backfill slurry were determined according to the Chinese standard GB/T 1346-2001 with a Viact apparatus. The prepared backfill slurry was first poured into a Vicat mold, and then the mold was gently shaken several times to scrape off the excess slurry. Finally, measuring and recording IST and FST at regular intervals with a Vicat needle. 2.3.4 Uniaxial compressive strength Uniaxial compressive strength (UCS) is an effective and straightforward method to evaluate the quality of backfill. According to Chinese standard JGJ/T 70-2009, the UCS tests were carried out on the cemented backfill samples cured for 28d with a displacement rate of 0.1 mm/min using a servo-hydraulic machine (Hualong, China). Three samples were used for each UCS test, and the average values were calculated. 2.3.5 Microstructural analysis The scanning electron microscope (SEM) analysis was carried out with HELIOS NamoLab 600i (FEI, USA) to analyze the microstructure and element types of PG and backfill samples. After UCS tests, the broken samples were immediately placed into the anhydrous ethanol solution to prevent hydration reaction. Then the samples were dried at 40°C in a drying oven until a constant weight was obtained. Due to the inferior conductivity of PG and backfill samples, the surface of the samples was coated with gold (Au) for 240s to satisfy the conductivity requirements. 2.3.6 Toxicity leaching test and chemical analysis In order to investigate the concentration of impurities in PG and backfill samples, the toxicity leaching test was conducted according to HJ 557-2010. After curing for 28d, the backfill samples were grounded and sieved through a 3.0 mm screen. The powders were mixed with deionized water in a container at a mass proportion of 1:10 and shaken at 110 rpm/min on a rotary shaker for 8h. Then the mixtures were placed on the table for 16h. Finally, the mixtures were filtered through a 0.45 mm filter, and the leachates were collected for further analysis. The pH value of PG, bleeding water and the leachate of toxicity leaching test was measured by pH meter (Ohaus, US). The concentrations of SO 4 2− and PO 4 3− -P were determined by ammonium molybdate tetrahydrate spectrophotometry (Shimadzu, Japan). The total dissolved solids (TDS) and the concentration of F − were measured by TDS meter (Ohaus, US) and fluorine ion-selective electrode (Leici, China), respectively. 3 Results And Discussion 3.1 Effect of water washing conditions on PG Due to the production processes and stockpile environment, different quantities of impurities are contained in PG. Meanwhile, the types of impurities are also affected by residual acids in PG (Tao et al. 2016, Wen et al. 2019). In this study, the pH value is used as an index to evaluate washing efficiency. The following studies aim to provide an optimal stirring time and S/L ratio for the actual water washing of PG. 3.1.1 Effect of stirring time on PG PG and deionized water with a mass ratio of 1:2 were mixed thoroughly with a stirrer at a speed of 200 rpm/min. The solution was taken out at 1, 2, 5, 10, 30, 60, 120 and 240 min, respectively. After centrifugation, the supernatant was collected to measure the pH value and the concentrations of PO 4 3− -P, F − , and SO 4 2− . As presented in Fig. 2 (a), with the first 1 min of stirring, the pH value of PG reached 1.86 and remained stable afterward. The increase of pH value was mainly due to the residual acids absorbed on the surface of the PG crystals, which were easily detached from the PG surface and escaped into the solution during the stirring process. In addition, it can be seen from Fig. 2 (b) that the concentration of impurities showed evident changes within the 5 min. This undulation of impurities concentrations was due to complicated chemical reactions occurring in the PG solution, such as dissolution and recrystallization of CaSO 4 ·2H 2 O, the ion-exchanges of PO 4 3− -P, F − , and SO 4 2− (Liu et al. 2019). Then, the impurities reached an equilibrium state after 5 min, and the concentrations of PO 4 3− -P, F − and SO 4 2− stabilized at about 3500 mg/L, 1200 mg/L and 14000 mg/L, respectively. The TDS also remained at about 8800 ppm within 5 min (seen in Fig. 2 (a)), which indicated that the dissolved ions absorbed on the PG surface had been well diffused into the solution. In general, it can be inferred that the optimal stirring time for water washing PG is 5 min in this study. 3.1.2 Effect of solid-liquid ratio on PG The water demand affects the labor and material resources that an enterprise needs to invest. In the actual water washing process, the S/L ratio may be directly related to the water demand. In this study, the water demand is defined as the ratio of water consumed for washing PG to the dry weight of PG. Therefore, PG was washed with different S/L ratios of 1:0.5, 1:1, 1:1.5 and 1:2 for 5 min each time until the pH reached a pre-designated value. According to the previous research results and the accumulated experiences, when the pH value of PG is about 5.00, it has little influence on the cemented PG backfill technique (Li et al. 2018, Min et al. 2019). As clearly shown in Fig. 3 (a), the pH value increased along with the water demand. The gradual growth of pH value was owing to the removal of residual acids by water washing. With a pH value of 5.00 as the target, washing PG with the S/L ratio of 1:0.5 required a water demand of 14. While the water demand of washing PG with the S/L ratio of 1:1, 1:1.5 and 1:2 was1.3, 1.5, and 1.6 times than that of 1:0.5, respectively. As regards the changes in impurity concentrations during water washing, Figs. 3 (b-d) present the variation curves of PO 4 3− -P, F − and SO 4 2− concentrations with water demand. It can be seen that the concentrations of all impurities decreased dramatically in the early washing times, and over 80% of impurities were removed at the first 8 washing demands. Then the pace of changes gradually slowed down. It was worth noting that with the S/L ratio increased from 1:0.5 to 1:2, the removal efficiencies of PO 4 3− -P, F − and SO 4 2− changed slightly. According to the above results, it can be inferred that when the PG is washed multiple times with a lower S/L ratio, a more rapid increase in the pH value of PG and a more significant reduction in the impurities concentration can be achieved with the minimum water demand. Therefore, the S/L ratio of 1:0.5 can be considered as the optimal ratio with acceptable efficiency in this study. The morphological structure of the PG with and without pretreatment was observed by SEM analysis. Fig. 4 (a) is the SEM image of the original PG with a pH value of 1.75, and Fig. 4 (b) is the SEM image of pretreated PG with a pH value of 5.15. It is well known that the PG crystals are plate-like structures (Li et al. 2017). Obviously, large quantities of small irregular particles were absorbed on the surface of PG crystals that could be directly identified in Fig. 4 (a). In comparison, as shown in Fig. 4 (b), water washing did not change the structure of PG crystals. However, the amount of irregular particles initially attached on the PG crystals significantly reduced, and the surface became smooth. To further understand the composition of irregular particles, EDS analysis was performed. The results showed that massive Ca, O and S were detected in the irregular particles, and a certain amount of F, P, K, Al, and Si were also measured (seen in Fig. 4 (d) and Fig. 4 (e)). Therefore, it is considered that these small particles attached to the PG surface might be impurity particles. The SEM images also confirm that water washing could effectively remove the impurities. 3.1.3 Effect of initial pH value of PG Three batches of PG (PG-1, PG-2 and PG-3) with an initial pH value of 1.75, 1.99, and 2.63 were selected to study the effect of water washing on the initial pH value of PG. The PG was washed by an S/L ratio of 1:0.5 and a stirring time of 5 min as determined from the above tests until the pH value of PG was 5.00. As presented in Fig. 5 (a), for PG with initial pH of 2.63, the pH was raised to 3.00 only by washing 6 times. And after 20 times of washing, the pH value was higher than 5.00. However, for the original PG with initial pH of 1.75 and 1.99, 28 and 24 times of washing were needed to raise the pH to 5.00. It is evident that PG with a lower pH value contained more H + , and more water was needed to remove the acidity and raise the pH value of PG. Thus, PG with a lower initial pH value needs more washing times to reach the specified pH value. Regarding the impurity concentrations in the original PG, it can be seen that the higher the initial pH of PG, the fewer impurities were observed. This might be attributed to that some impurities were removed under the different stockpiles environments and weathering factors, resulting in fewer impurities contents in the original PG (Tayibi et al. 2009). The effect of washing times on PO 4 3- -P, F - and SO 4 2- concentration are shown in Figs. 5 (b-d), respectively. The impurities concentration dropped rapidly before washing 10 times, meaning that excessive soluble impurities on the PG surface can be easily dissolved in the liquid. However, the descending rate gradually decreased in the following washes, leading to a corresponding decrease in the removal efficiency. Compared to PG-1 and PG-2, PG-3 needed fewer washing times to remove the impurities. The 5% difference in removal efficiency between two adjacent washes is defined as the stabilization of PG in this study. As seen in Fig. 5 (b), the concentration of PO 4 3- -P in PG-3 was stabilized only by 6 washing times. While for PG-1 and PG-2, 15 and 8 washing times were needed, respectively. For F - , more washes were needed to stabilize, and it was 16 washes for PG-3, which had the lowest initial F - content. It indicated that F - would be released continuously in PG in the long run. Eventually, when PG were all washed to the pH value of 5.00, the impurities in the washing solution was about varied from 5~ 8 mg/L of PO 4 3- -P, 6~75 mg/L of F - and 1400~1750 mg/L of SO 4 2- . This result infers that even after multiple washing times, the concentrations of F - and SO 4 2- in PG remain high, posing potential environmental hazards if without further treatment. 3.2 Backfill slurry properties of purified PG In order to investigate the influence of pretreated PG on the properties of backfill slurry, PG-1, PG-2 and PG-3 were washed to pH values of 3.50 and 5.00, respectively. The washing conditions are based on the most economical principle of the water demand determined by the above tests (the optimal washing S/L ratio of 1:0.5 and washing time of 5 min). In addition, PG-4 and PG-5 with an initial pH of 3.52 and 4.99 were selected as control groups. The experimental results of viscosity, bleeding rate and setting times (IST and FST) are presented in Table 3 . Table 3 Characteristics of backfill slurries prepared using PG with different pH values. Batch No. Viscosity (mPa·s) Bleeding Rate (%) IST (h) FST (h) PG-1-O a 769 29.13 - - PG-1-P b -3.50 619 58.00 90 134 PG-1-P-5.00 182 64.21 85 118 PG-2-O 707 33.65 - - PG-2-P-3.50 437 68.97 66 74 PG-2-P-5.00 180 73.78 54 66 PG-3-O 490 44.22 100 140 PG-3-P-3.50 404 56.90 78 116 PG-3-P-5.00 116 88.57 65 76 PG-4-O 397 40.21 64 75 PG-5-O 157 58.48 52 74 a Original; b Pretreatment. 3.2.1 Variation of viscosity in backfill slurry In the backfilling process, the slurry is usually mixed on the ground surface and then pumped into the goaf through the pipeline. Excessive viscosity of the slurry may cause a series of problems in slurry mixing, pumping and transportation (Wu et al. 2014). Fig. 6 shows the variation of slurry viscosity with different pH values of PG. Using original PG as aggregate, the viscosity of backfill slurry decreased from 769 mPa·s to 490 mPa·s, with the increase in the pH value of original PG from 1.75 to 2.63. The decrease indicates that the pH value of aggregate has a significant effect on the backfill slurry. As in this study, the viscosity decreased about 75% when the PG (PG-1, PG-2 and PG-3) was washed to a pH value of about 5.00. These decreases may be explained by the fact that the surface of PG crystals becomes smooth after the residual acids are washed out, reducing the number of direct crystal-crystal contacts and increasing the thickness of the lubricating film around the crystal (Ness et al. 2018). Thereby, the friction force and pressure differential resistance are continuously reduced during the slurry flow process, manifested as a decrease in viscosity. 3.2.2 Variation of bleeding rate in backfill slurry The bleeding rate affects the durability and strength of the hardened backfill, which is one of the main physical properties (Yim et al. 2013). As shown in Fig. 7 and Table 3 , the pretreatment of PG significantly affected the bleeding rate of the slurry. With the increase in the pH during the washing process, the bleeding rate of three groups PG-1, PG-2 and PG-3 increased significantly by 120%, 119% and 100%, respectively. This increase was caused by the reduction of residual acids and impurities in PG, which reduced the viscosity of the backfill slurry and weakened the free-water absorption capacity of the slurry. Therefore, the macroscopic performance is the gradual increase in the bleeding rate. For the original PG, as the initial pH value increase from 1.75 to 2.63, the bleeding rate increased by 52%, which is also attributed to this reason. The variation of viscosity and bleeding rate indicates that the water washing pretreatment can effectively improve the fluidity and transportation of the backfill slurry. 3.2.3 Variation of setting time in backfill slurry The setting time affects the cementation and early strength of the backfill in the backfilling process (Chen et al. 2012). The initial setting time (IST) and final setting time (FST) of the backfill slurry are presented in Fig. 8 and Table 3 . The slurry prepared from the original PG-1 and PG-2 was not completely set within 7d, so the IST and FST were not measured. A possible explanation for this finding could be that the initial pH value of PG-1 and PG-2 are relatively low. With binder addition into PG, the binder first reacted with PG in a neutralization reaction, consequently slowing down the hydration reaction and prolonging the setting time (Li et al. 2017). For the backfill slurry prepared from pretreated PG, the binder could more rapidly and more extensively participate in the hydration reaction, thus shortening the setting times. It can be evidently seen from the figure that when the pH value of PG-1 and PG-2 was washed to 3.50, the setting times were greatly shortened. As the pH value increased to 5.00, the IST of PG-1 and PG-2 continued to reduce for 5h and 12h, and the FST was reduced by 16h and 8h, respectively. As the pH value of PG-3 was washed to 5.00, the IST and FST was reduced by 35% and 46%. Overall, the results of setting times can be concluded that the increase in pH value of pretreated PG facilitates the solidification of the backfill slurry into the hardened backfill. 3.3 Strength of pretreated PG-based cemented backfill The backfill slurry is pumped into the goaf and then cemented into a hardened backfill with a certain strength, and the strength directly affects the stability of the stope (Li et al. 2019). Herein, the 28d strength of cemented PG backfill with and without pretreatment was measured, as shown in Fig. 9 . Figure 9 (a) shows that the pretreatment of aggregate PG could well enhance the strength of the backfill. The backfill strength was enhanced significantly by 8.1 times, 6.2 times and 2.7 times by pretreating the three batches of aggregates. This increase in the backfill strength can be explained by the following three reasons. For one thing, it is known that the backfill strength derives from the overlap and tight bonding of aggregate PG and hydration products of the binder (Fall and Pokharel 2010). The pretreatment smoothed the PG surface and facilitated the overlap of hydration products and aggregate. For another, PG contains residual acid, which could consume the alkalinity of the binder and reduce the hydration products. As mentioned in 3.4.1, the pH value of the backfill slurry prepared from PG-1-O and PG-2-O are both around 8, causing the 28d strength of hardened backfill to be less than 0.15 MPa. Water washing could remove the majority of residual acids in PG. As the pH value of the pretreated PG reached 3.50 and 5.00, the pH value of the slurry reached around 12.8, which could ensure the proceed of the hydration reaction. The third reason for pretreatment enhancing the backfill strength was the reduction of soluble impurities. The excessive anions in PG would react with Ca 2+ of the binder, forming insoluble precipitations attached to the hydration products, therefore lowering the quality of hydration products. As shown in Fig. 3 and Fig. 5 , the content of impurities in PG decreased significantly after pretreatment, so the quality of hydration products improved and backfill strength increased accordingly. In addition, when PG with relatively low initial pH value (1.76, 1.99 and 2.63) was washed to pH 3.50, an evident increase in strength was observed, indicating proper water washing could well enhance the strength development of backfill. However, when the pH value increased from 3.5 to 5.0, a very slight increase was observed for 28d backfill strength. This result indicates that excessive pretreatment of aggregate was unhelpful for backfill strength. Figure 9 (b) shows the backfill strength prepared from PG washed to the same pH value. Two PG with an initial pH of 3.52 and 4.99 were selected as the control group. When PG with different initial pH was washed to the same pH value, the backfill obtained a similar 28d strength. For example, using PG with a pH value of 3.50 as aggregate (four batches, pretreated or original), the 28 d backfill strength was similar at about 0.9 MPa. This result indicates that pH value could be used as an index to evaluate the quality of PG. To save costs, the degree of water washing should be controlled within a reasonable range based on the actual mining method of the mines. The SEM images shown in Fig. 10 describe an overview of the microscopic observations of the backfill prepared by PG-2 with and without pretreatment. A large number of exposed plate-like PG crystals can be seen in Fig. 10 (a), interspersed with a small amount of C-S-H gel and ettringite (AFt). When PG was pretreated, the content of hydration products noticeably increased, leading to an increase in strength by 4.7 and 5.2 times. Therefore, the water washing of aggregate can effectively improve the strength of cemented PG backfill. 3.4 Environmental behavior of purified PG in the backfill process The majority of impurities could be either removed by water washing pretreatment or solidified/stabilized (S/S) by hydration reactions of the binder. However, it remains to be explored whether the bleeding water of backfill slurry and the leaching water of cemented backfill carries unconsolidated impurities and escapes into the groundwater (Li et al. 2019). Therefore, it is necessary to comprehensively understand the environmental behavior of impurities in the backfill process. 3.4.1 Impurities in bleeding water Figure 11 depicts the concentration of PO 4 3- -P, F - and SO 4 2- in the bleeding water prepared from PG with and without pretreatment. By comparing the PO 4 3- -P concentration in the PG and the bleeding water, it was found that the hydration reaction was able to consolidate 99% of PO 4 3- -P, which has also been demonstrated by the previous study (Li et al. 2017). However, relatively high PO 4 3- -P concentrations (35.84 mg/L and 30.15 mg/L) were observed in the bleeding water with PG-1-O and PG-2-O. When the aggregate PG was water washed, the concentration of PO 4 3- -P in all bleeding water was reduced to less than 0.5 mg/L, as shown in Figure 11 (a). As for F - , the concentration of F - decreased to 4~6 mg/L (seen in Figure 11 (b)) after pretreatment. Further, as can be seen in Figure 11 (c), with the increase of pH value of pretreated PG, the concentration of SO 4 2- in the bleeding water of PG-1 and PG-2 groups gradually decreased from 4000~5000 mg/L to about 1300 mg/L. Overall, the water washing of PG could well remove the impurities, leading to lower concentrations of impurities in bleeding water. In addition, it can also be observed that when the pH of PG was washed to about 3.50, the impurities in bleeding water could be maintained at relatively stable levels. Among them, the concentration of PO 4 3- -P and F - have met the Chinese standard GB8978-1996 for integrated wastewater discharge (F - concentration < 10mg/L and PO 4 3- -P < 0.5mg/L). 3.4.2 Impurities in backfill leachate The toxicity leaching test was conducted on the backfills cured for 28d, and the degree of S/S of impurities in cemented PG backfill was investigated, as shown in Table 4 . The PO 4 3- -P concentrations in all leachate were less than 0.5 mg/L, and the F - concentrations were less than 10 mg/L (except for PG-1-O), which met the Chinese standard for integrated wastewater discharge. The SO 4 2- concentrations were controlled at around 30 mg/L, except for PG-1-O with SO 4 2- concentration up to 104 mg/L. Almost 100% of PO 4 3- -P, more than 99.3% of F - and SO 4 2- in pretreated PG were consolidated. This also proved that the backfill prepared with pretreated PG could significantly alleviate the environmental pollution of PG. Table 4 Experimental data of the backfill leachate. Batch No. PO 4 3- (mg/L) F - (mg/L) SO 4 2- (mg/L) PG-1-O 0.12 12 104 PG-1-P-3.50 0.05 4 28 PG-1-P-5.00 0.08 4 30 PG-2-O 0.02 8 29 PG-2-P-3.50 0.03 1 32 PG-2-P-5.00 0.09 1 29 PG-3-O 0.03 2 27 PG-3-P-3.50 0.02 2 29 PG-3-P-5.00 0.02 2 26 4 Conclusion The purpose of this study was to investigate the effects of stirring times, S/L ratio and initial pH value of PG on the mechanical properties and environmental behavior of the backfill using the water-washed PG as aggregate. The experimental results show that the pretreatment of aggregate could effectively improve the performance of backfill. The following conclusions can be drawn: (1) The PG pretreatment process was optimized for backfill, including 5 min stirring time, the S/L ratio of 1:0.5. (2) Using pretreated PG as aggregate effectively improved the workability of the backfill slurry and enhanced the strength development of the hardened backfill. (3) Water washing pretreatment significantly reduced the impurities content in the bleeding water and the leachates of backfill. Eventually, almost 100% of PO 4 3- -P, more than 99.3% of F - and SO 4 2- in PG had been fixed into the backfill. (4) The wastewater generated after washing PG could be treated first. For example, by adding the common CaO directly to the wastewater, which is relatively easy to operate. The treated water could still be used to wash PG, realizing the circulation of water resources. (5) In practice, it is recommended to use the pH value of PG as a parameter for selecting the pretreatment method to meet the mechanical and environmental requirements of the backfill in mines. Declarations Funding This work was supported by the State Key Research Development Program of China (Grant No. 2018YFC1800400), and the National Natural Science Foundation of China (Grant No. 72088101). Ethical approval, consent to participate and consent to publish Not applicable. Competing interest The authors declare that they have no conflict of interest. Author contribution Yanan Zhou: Conceptualization; Methodology; Data curation; Writing - original draft. Xibing Li: Supervision; Writing - review & editing. Ying Shi: Methodology; Writing - review & editing; Funding acquisition. Quanqi zhu: Writing - review & editing; Validation. Jing du: Formal analysis. Availability of data and materials The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request. References Amrani M, Taha Y, Kchikach A, Benzaazoua M, Hakkou R (2020) Phosphogypsum recycling: New horizons for a more sustainable road material application. Journal of Building Engineering 30 Atesok G, Boylu F, Sirkeci AA, Dincer H (2002) The effect of coal properties on the viscosity of coal–water slurries. Fuel 81, 1855-1858 Chen C, Gu XY, Chen YX, Luo T (2012) Effect of Different Grinding Time on the Performance of Gypsum Mould. Applied Mechanics & Materials 155-156, 955-959 Chen SJ, Du ZW, Zhang Z, Zhang HW, Xia ZG, Feng F (2020) Effects of chloride on the early mechanical properties and microstructure of gangue-cemented paste backfill. Construction and Building Materials 235 Ding WJ, Chen QJ, Sun HJ, Peng TJ (2019) Modified mineral carbonation of phosphogypsum for CO 2 sequestration. Journal of CO 2 Utilization 34, 507-515 Ennaciri Y, Zdah I, Alaoui-Belghiti H, El , Bettach M (2019) Characterization and purification of waste phosphogypsum to make it suitable for use in the plaster and the cement industry. Chemical Engineering Communications 207, 382-392 Fall M, Pokharel M (2010) Coupled effects of sulphate and temperature on the strength development of cemented tailings backfills. Cement & Concrete Composites 32, 819-828 Li W, Fall M (2018) Strength and self-desiccation of slag-cemented paste backfill at early ages: Link to initial sulphate concentration. Cement and Concrete Composites 89, 160-168 Li XB, Zhou ZL, Zhao GY, Liu ZX (2008) Utilization of phosphogypsum for backfilling, way to relieve its environmental impact. Gospodarka Surowcami Mineralnymi 24, 223-232 Li XB, Du J, Gao L, He SY, Gan L, Sun C, Shi Y (2017) Immobilization of phosphogypsum for cemented paste backfill and its environmental effect. Journal of Cleaner Production 156, 137-146 Li XB, Liu B, Yao JR, Shi Y, Li DY, Du SL, He ZG, Gao L, Wang XM, Zhao GY, Liu ZX, Li QY (2018) Theory and practice of green mine backfill with whole phosphate waste. The Chinese Journal of Nonferrous Metals 9, 1845-1865 Li XB, Zhou YN, Zhu QQ, Zhou ST, Min CD, Shi Y (2019) Slurry preparation effects on the cemented phosphogypsum backfill through an orthogonal experiment. Minerals 9 Lin ZS, Shi HD, Beguedou E (2012) Effect of Polycarboxylate Superplasticizer on Properties of Phosphogypsum-Based Cement. Key Engineering Materials 509, 13-19 Liu YK, Zhang QL, Chen QS, Qi CC, Su Z, Huang ZD (2019) Utilisation of Water-Washing Pre-Treated Phosphogypsum for Cemented Paste Backfill. Minerals 9 Mao YP, Wu H, Wang WL, Jia MH, Che XC (2020) Pretreatment of municipal solid waste incineration fly ash and preparation of solid waste source sulphoaluminate cementitious material. Journal of hazardous materials 385, 121580 Min CD, Li XB, He SY, Zhou ST, Zhou YN, Yang S, Shi Y (2019) Effect of mixing time on the properties of phosphogypsum-based cemented backfill. Construction and Building Materials 210, 564-573 Moalla R, Gargouri M, Khmiri F, Kamoun L, Zairi M (2017) Phosphogypsum purification for plaster production: A process optimization using full factorial design. Environmental Engineering Research 23, 36-45 Moreira RH, Queiroga FS, Paiva HA, Medina NH, Fontana G, Guazzelli MA (2018) Extraction of natural radionuclides in TENORM waste phosphogypsum. Journal of Environmental Chemical Engineering 6, 6664-6668 Ness C, Mari R, Cates ME (2018) Shaken and stirred: Random organization reduces viscosity and dissipation in granular suspensions. Science Advances 4, 3296 Rashad AM (2017) Phosphogypsum as a construction material. Journal of Cleaner Production 166, 732-743 Sahoo P, Joseph J (2021) Radioactive Hazards in Utilization of Industrial By-Products: Comprehensive Review. Journal of Hazardous, Toxic, and Radioactive Waste 25 Shao K, Du Y X, Zhou F (2020) Feasibility of using treated corn cob aggregates in cement mortars. Construction and Building Materials 271, 121575 Singh M, Garg M, Verma CL, Handa SK, Kumar R (1996) An improved process for the purification of phosphogypsum. Construction and Building Materials 10, 597-600 Singh M (2002) Treating waste phosphogypsum for cement and plaster manufacture. Cement & Concrete Research 32, 1033-1038 Smadi MM, Haddad RH, Akour AM (1999) Potential use of phosphogypsum in concrete. Cement and Concrete Research 29, 1419-1425 Song S, Jennings HM (1999) Pore solution chemistry of alkali-activated ground granulated blast-furnace slag. Cement and Concrete Research 29, 159-170 Tao WD, Fattah KP, Huchzermeier MP (2016) Struvite recovery from anaerobically digested dairy manure: A review of application potential and hindrances. Journal of environmental management 169, 46-57 Tayibi H, Choura M, Lo´pez FlA, Alguacil FJ, Lopez-Delgado A (2009) Environmental impact and management of phosphogypsum. Journal of environmental management 90, 2377-2386 Wang CQ, Mei XD, Zhang C, Liu DS, Xu FL (2020) Mechanism study on co-processing of water-based drilling cuttings and phosphogypsum in non-autoclaved aerated concrete. Environmental Science and Pollution Research 27, 23364-23368 Wang JM (2020) Utilization effects and environmental risks of phosphogypsum in agriculture: A review. Journal of Cleaner Production 276 Wang Q, Li J, Zhang JJ, Wu P, Lyu X, Hu SG, Qiu J, Liu XD, Yu HG (2021) Reuse of the soda sludge dealt with water washing as a supplementary material for the synthesis of clinker binders. Journal of Cleaner Production 295 Wen YZ, Mao YF, Kang ZF, Luo QH (2019) Application of an ammonium ion-selective electrode for the real-time measurement of ammonia nitrogen based on pH and temperature compensation. Measurement 137, 98-101 Wu D, Cai SJ, Huang G (2014) Coupled effect of cement hydration and temperature on rheological properties of fresh cemented tailings backfill slurry. Transactions of Nonferrous Metals Society of China 24, 2954-2963 Wu FH, Zhao CY, Qu GF, Liu S, Ren YC, Chen BJ, Li JY, Liu LL (2021) A Critical Review of the Typical By-product Clean Ecology Links in the Chinese Phosphorus Chemical Industry in China: Production Technologies, Fates and Future Directions. Journal of Environmental Chemical Engineering Yim HJ, Kim JH, Kwak HG, Kim JK (2013) Evaluation of internal bleeding in concrete using a self-weight bleeding test. Cement & Concrete Research 53, 18-24 Zhao HT, Bao WJ, Sun ZH, Li SG, Li HQ, Lin WG (2017) Deep removal of impurities from phosphogypsum. Chemical Industry and Engineeing Process 36, 1240-1246 (in Chinese) Zhou ST, Li XB, Zhou YN, Min CD, Shi Y (2020) Effect of phosphorus on the properties of phosphogypsum-based cemented backfill. Journal of hazardous materials 399, 122993 Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 17 Jan, 2022 Reviewers invited by journal 17 Jan, 2022 Editor assigned by journal 21 Dec, 2021 First submitted to journal 14 Dec, 2021 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-1172069","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":77188550,"identity":"3b0b6eb5-bc6a-4b92-acfb-5ed1ed105ca9","order_by":0,"name":"Yanan 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Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Jing","middleName":"","lastName":"Du","suffix":""}],"badges":[],"createdAt":"2021-12-15 02:37:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1172069/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1172069/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":17534583,"identity":"cf75905b-ce5f-4757-9926-54aa30b72127","added_by":"auto","created_at":"2022-01-21 15:31:56","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":74515,"visible":true,"origin":"","legend":"\u003cp\u003eFlow diagram of the experiment.\u003c/p\u003e","description":"","filename":"fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/067b2567590abf6efbc5e3e9.png"},{"id":17534586,"identity":"f6f79d11-ab46-4338-88cb-bd956f26d051","added_by":"auto","created_at":"2022-01-21 15:31:57","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":42157,"visible":true,"origin":"","legend":"\u003cp\u003eVariations of PG with different stirring times: (a) pH value and TDS, (b) concentrations of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, F\u003csup\u003e-\u003c/sup\u003e and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/38ea2d6f46369866b8590465.png"},{"id":17534584,"identity":"3bf71779-32bb-420e-82c5-41756c284ced","added_by":"auto","created_at":"2022-01-21 15:31:56","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":102830,"visible":true,"origin":"","legend":"\u003cp\u003eVariations of PG with different S/L ratio: (a) pH, (b) PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, (c) F\u003csup\u003e-\u003c/sup\u003e, (d) SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/457dd8a0770e96f16bea4307.png"},{"id":17534585,"identity":"7d8ce18a-d769-4673-b734-f089e657f55a","added_by":"auto","created_at":"2022-01-21 15:31:56","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":337796,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of PG: (a) original PG-1 with a pH value of 1.75, (b) pretreated PG-1 with a pH value of 5.15, (c) EDS of PG, (d) EDS of impurity-1, (e) EDS of impurity-2.\u003c/p\u003e","description":"","filename":"fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/807ed91d6c540cc83a6538a6.png"},{"id":17534581,"identity":"d0bcdb36-fca1-4a6e-80a9-b4e5b54bb98b","added_by":"auto","created_at":"2022-01-21 15:31:55","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":83125,"visible":true,"origin":"","legend":"\u003cp\u003eVariations of pretreated PG with different initial pH value: (a) pH, (b) PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, (c) F\u003csup\u003e-\u003c/sup\u003e, (d) SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/ac979a7e25d1b560fe8dea3a.png"},{"id":17534580,"identity":"b0bdc805-d1d3-416b-994b-367ce52471e6","added_by":"auto","created_at":"2022-01-21 15:31:55","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":88559,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of viscosity in backfill slurry.\u003c/p\u003e","description":"","filename":"fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/6316ddb8c1c35a60b21ff9ee.png"},{"id":17534590,"identity":"33085fcb-e0b5-4e5e-8c25-f44710f16dc4","added_by":"auto","created_at":"2022-01-21 15:31:57","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":124348,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of bleeding rate in backfill slurry.\u003c/p\u003e","description":"","filename":"fig7.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/3f231a5368edfc35a786f9bb.png"},{"id":17534591,"identity":"7c4af808-eb72-4b65-9b46-9642b82b03bb","added_by":"auto","created_at":"2022-01-21 15:31:58","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":250493,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of setting time in backfill slurry: (a) IST, (b) FST.\u003c/p\u003e","description":"","filename":"fig8.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/38d44a25e86533e2a6d588e3.png"},{"id":17534582,"identity":"357374fc-f6a7-41b3-81ba-1221d0a732fe","added_by":"auto","created_at":"2022-01-21 15:31:55","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":209917,"visible":true,"origin":"","legend":"\u003cp\u003eUnconfined compressive strength of 28d backfill prepared by PG with and without pretreatment.\u003c/p\u003e","description":"","filename":"fig9.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/cb88916139cb3dc1283a4353.png"},{"id":17534587,"identity":"1a94866e-7209-4c03-b5ac-6950ec0a60ff","added_by":"auto","created_at":"2022-01-21 15:31:57","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":380805,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of cemented PG backfill samples: (a) PG-2-O, (b) PG-2-P-3.50, (c) PG-2-P-5.00.\u003c/p\u003e","description":"","filename":"fig10.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/9bdd4e5e370c48916bb525d7.png"},{"id":17534589,"identity":"bf62d111-65e7-4c0a-bb9f-fad37f1681ec","added_by":"auto","created_at":"2022-01-21 15:31:57","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":150233,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of impurities in bleeding water: (a) PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, (b) F\u003csup\u003e-\u003c/sup\u003e, (c) SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"fig11.png","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/f6dbd523588916e90d6c658f.png"},{"id":17534595,"identity":"25ed0487-608e-4d15-a905-512ef67d8d01","added_by":"auto","created_at":"2022-01-21 15:32:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":978040,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1172069/v1/8637244e-5e56-40bb-9233-32a7f0a0b4dd.pdf"}],"financialInterests":"","formattedTitle":"\u003cp\u003eReuse of Phosphogypsum Pretreated with Water Washing as Aggregate for Cemented Backfill\u003c/p\u003e","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eCemented backfill is an effective means to increase ore recovery, improve safety conditions, and reduce surface disposal of solid wastes. As a typical solid waste, phosphogypsum (PG) is the by-product generated during the exploitation of phosphate resources (Moreira et al. 2018, Sahoo and Joseph 2021, Wang et al. 2020). Global production of PG is estimated to be around 100-280 Mt annually, of which China contributes to 25% (Amrani et al. 2020, Ding et al. 2019). Currently, PG is recycled as additives in building materials, soil modifiers and cement productions, but with a limited utilization rate of 15% (Lin et al. 2012, Rashad 2017, Wang 2020). In 2008, Li et al. (2008) innovatively proposed a cemented backfill technique with PG as aggregate, effectively improving PG utilization rate up to 60%. In the cemented PG backfill process, the aggregate PG (over 80% by dry weight) is mixed with binder and water to a heterogeneous backfill slurry, which is then pumped to the underground mined-out areas. The slurry gradually dewaters and consolidates, building up strength to support the rock walls in the underground mines.\u003c/p\u003e \u003cp\u003eAs the primary backfill material, aggregate PG is mainly composed of CaSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO, and it also contains large quantities of impurities such as residual acids, phosphates, fluorides and heavy metals. Previous studies have shown that the impurities might seriously deteriorate the hydration process of the backfill and cause serve environmental pollution. Li and Fall (2018) added sulfate in slag-cemented paste backfill and found that high sulfate content negatively impacted the early age strength and self-desiccation of the backfill. Chen et al. (2020) explored the effects of chloride on the mechanical properties of gangue-cemented paste backfill. The results showed that the early strength of backfill decreased obviously when the initial chlorine content was more than 40\u0026permil;. Zhou et al. (2020) prepared cemented backfill using PG with various phosphate contents and demonstrated that the 120d strength decreased from 2.04 to 0.30 MPa as the dissolved phosphate in PG increased from 29 to 377 mmol/kg. Another concern was the potential environmental hazards due to the high phosphate content in PG. When the phosphate content in PG exceeded 87 mmol/kg, the backfill would cause phosphate pollution to the environment. Furthermore, it is worth noting that PG is a hyperacidic solid waste with a pH value usually within 3, compared with other neutral filling aggregates (Sahoo and Joseph 2021, Wu et al. 2021). However, hydration reactions commonly occur under strongly alkaline conditions (pH\u0026gt;11.5) (Song and Jennings 1999). Therefore, the residual acids in PG would neutralize hydroxyl ions of binder and interfere with the hydration reaction of the backfill, which in turn disturbs the strength development of the backfill. As a result, it is necessary to pretreat solid wastes to mitigate the adverse effects in their secondary utilization.\u003c/p\u003e \u003cp\u003eActually, several studies have found that pretreatment of solid waste can effectively improve the workability of cementation and reduce environmental pollution. Shao et al. (2020) treated corn cob aggregates with cement paste to substitute sand in cement mortars, which improved the ductility of cement mortars. Singh (2002) depicted that the pretreated PG (treating with 3-4% aqueous citric acid) could be used as an additive in place of mineral gypsum for the manufacture of ordinary Portland cement and Portland slag cement. Mao et al. (2020) washed fly ash with water, and it was found that the consolidation rate of heavy metals was above 92% in the treated fly ash. When it was prepared as cementing materials, the consolidation rate was further increased to over 99%, resulting in the leaching concentration of heavy metals was far lower than the national standard limit. Based on these results, the pretreatment of aggregate PG should be considered to reduce the impurities content, therefore ensuring safety for the mining and environment.\u003c/p\u003e \u003cp\u003eThe PG pretreatment protocols are currently main as follows: chemical, thermal and physical treatments (Ennaciri et al. 2019, Moalla et al. 2017, Smadi et al. 1999). Chemical and thermal treatments of PG can effectively reduce the soluble impurities and organic matters, but the operation process is cumbersome and costly. Generally, the physical treatment of PG, especially water washing, is still preferred in the industry due to its simple operation. Singh et al. (1996) washed PG at a volume proportion of 1:3 for three durations of 30, 50 and 65 min, and found that 63.0% of phosphates, 66.1% of fluorides and 80.7% of organic matters could be removed. Subsequently, Zhao et al. (2017) washed PG with a mass ratio of PG to water of 1:10 for 30 min, and the results showed a reduction in soluble phosphates from 0.79\u0026ndash;0.46%, fluorides from 0.87\u0026ndash;0.61% and magnesium from 0.09% to 0, respectively. These findings indicate that impurities can be reduced by washing PG with varying solid-liquid (S/L) ratios and stirring times. However, previous studies usually washed PG for only one time to calculate washing efficiency. In fact, multiple washes can achieve a continuous reduction of impurities in solid waste (Mao et al. 2020, Wang et al. 2021). Therefore, the removal efficiency of impurities in PG under different washing conditions should be investigated.\u003c/p\u003e \u003cp\u003eThe purpose of this study is to further explore the effect of pretreatment aggregate on the cemented backfill process. By considering different stirring durations the number of washing times and the S/L ratios, the optimal condition of water washing pretreatment of PG was determined. Following this, the original PG with different initial pH values was collected as the control group. Subsequently, the original and pretreated PG were made into cemented backfill. The properties of the backfill slurry, the strength and microstructure of hardened backfills, and the resultant surrounding environment impacts were investigated.\u003c/p\u003e"},{"header":"2 Materials And Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Raw materials\u003c/h2\u003e \u003cp\u003eThis study evaluated representative samples of PG and composite binder in Guizhou, China. The binder is composed of yellow phosphorous slag: fly ash: cement clinker in 4:1:1, and 16-20% lime of the yellow phosphorous slag mass ratio is added. The main chemical compositions (measured by X-ray fluorescence; Bruker, Switzerland )and physical properties (measured by a particle size analyzer; Malvern Instruments, UK)of PG with different pH values were investigated through the toxicity leaching test, as listed in Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eImpurity concentrations and physical properties of PG and binder.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eImpurity concentration\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"5\" nameend=\"c6\" namest=\"c2\"\u003e \u003cp\u003eRaw PG\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eBinder\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePG-1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePG-2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePG-3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePG-4\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003ePG-5\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003epH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e13.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4840\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1510\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e252\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eF\u003csup\u003e\u0026minus;\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1641\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1103\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e521\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e509\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e182\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10885\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10042\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4419\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2313\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1359\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e487\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTDS (ppt)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4.91\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhysical property\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003csub\u003e10\u003c/sub\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\left({\\mu }\\text{m}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e13.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e11.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e11.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e15.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e6.02\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003csub\u003e30\u003c/sub\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\left({\\mu }\\text{m}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e42.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e29.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e33.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e36.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e37.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e13.73\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003csub\u003e60\u003c/sub\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\left({\\mu }\\text{m}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e92.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e55.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e81.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e67.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e81.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e30.18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003csub\u003eu\u003c/sub\u003e=D\u003csub\u003e60\u003c/sub\u003e/D\u003csub\u003e10\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e4.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e5.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e5.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003csub\u003ec\u003c/sub\u003e=D\u003csub\u003e30\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e/(D\u003csub\u003e60\u003c/sub\u003e*D\u003csub\u003e10\u003c/sub\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Water washing design and cemented backfill procedures\u003c/h2\u003e \u003cp\u003eIn this study, different methods were used to wash the PG. Due to the strong acidity of PG-1 with an initial pH value of 1.75, which was selected to study the effects of stirring time and S/L ratio on the water washing of PG. The PG with initial pH values of 1.75, 1.99 and 2.63 (PG-1, PG-2 and PG-3) were selected to study the effects of water washing on PG with different initial pH values. And PG-4 and PG-5 with initial pH values of 3.52 and 4.99 were selected as the control group. The weight of dry PG, wet PG, water for the first washing and each subsequent washing is shown in Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMix design of water washing pretreatment of PG samples.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003ePG No.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eS/L ratio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003eWeight of PG\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e \u003cp\u003eWeight of water\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDry (g)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eWet (g)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFirst washing (g)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNext washing (g)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003ePG-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e149.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e50.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e149.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e50.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e100.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e149.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e100.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e150.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e149.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e150.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e200.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003ePG-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e123.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e26.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e50.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e123.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e76.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e100.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e123.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e126.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e150.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e123.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e176.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e200.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003ePG-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e100.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e49.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e50.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e100.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e99.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e100.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e100.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e149.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e150.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e100.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e199.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e200.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eBackfill slurry was prepared by mixing the PG, binder and deionized water with a mass proportion of 5:1:6. In accordance with the experiment scheme, PG and deionized water were first mixed evenly in the stirred vessel to prevent blocking. Then the binder was slowly poured into the PG mixture and stirred homogeneously at 200 rpm/min for 30 min. Next, the backfill slurry was injected into plastic molds with internal dimensions of 40 mm \u0026times; 40 mm \u0026times; 40 mm. There was a 0.2 mm small hole at the bottom of the mold to drain the excess water in the slurry. After the slurry was set, the hardened samples were taken out of the molds and cured into a chamber maintained with a constant temperature of 20\u0026plusmn;2\u0026deg;C and humidity of 90\u0026plusmn;5%. The flow diagram for this work is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Test methods\u003c/h2\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.3.1 Bleeding rate\u003c/h2\u003e \u003cp\u003eThe bleeding rate was measured according to the Chinese standard GB/T 50080-2016. The backfill slurry was injected into a container with a lid and then placed on a vibrator for 20 s to make the slurry denser. The bleeding water was drawn with a syringe at every 30 min until no more water was secreted for three consecutive times. The bleeding rate was calculated by using Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e):\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$B=\\frac{Vw}{\\left(\\frac{W}{G}\\right)Gw}\\times 100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003eB\u003c/em\u003e is the bleeding rate (%), \u003cem\u003eVw\u003c/em\u003e is the mass of bleeding water in the container (g), \u003cem\u003eW\u003c/em\u003e is the total mass of water in the backfill slurry (g), \u003cem\u003eG\u003c/em\u003e is the total mass of backfill slurry (g), \u003cem\u003eGw\u003c/em\u003e is the mass of backfill slurry in the container (g).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.3.2 Apparent viscosity\u003c/h2\u003e \u003cp\u003eThe apparent viscosity is one of the essential rheological properties of backfill slurry(Atesok et al. 2002), which affects a series of actual conditions such as slurry transportation and pumping. The apparent viscosity of slurry was evaluated according to ASTM D2196-18 by using a DV-1 digital viscometer (Brookfield, USA). Due to the continuous hydration reaction of the slurry, in order to ensure the reliability of the test data, the measurement should be conducted immediately after slurry preparation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.3.3 Setting time\u003c/h2\u003e \u003cp\u003eThe initial setting time (IST) and final setting time (FST) of the backfill slurry were determined according to the Chinese standard GB/T 1346-2001 with a Viact apparatus. The prepared backfill slurry was first poured into a Vicat mold, and then the mold was gently shaken several times to scrape off the excess slurry. Finally, measuring and recording IST and FST at regular intervals with a Vicat needle.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e2.3.4 Uniaxial compressive strength\u003c/h2\u003e \u003cp\u003eUniaxial compressive strength (UCS) is an effective and straightforward method to evaluate the quality of backfill. According to Chinese standard JGJ/T 70-2009, the UCS tests were carried out on the cemented backfill samples cured for 28d with a displacement rate of 0.1 mm/min using a servo-hydraulic machine (Hualong, China). Three samples were used for each UCS test, and the average values were calculated.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e2.3.5 Microstructural analysis\u003c/h2\u003e \u003cp\u003eThe scanning electron microscope (SEM) analysis was carried out with HELIOS NamoLab 600i (FEI, USA) to analyze the microstructure and element types of PG and backfill samples. After UCS tests, the broken samples were immediately placed into the anhydrous ethanol solution to prevent hydration reaction. Then the samples were dried at 40\u0026deg;C in a drying oven until a constant weight was obtained. Due to the inferior conductivity of PG and backfill samples, the surface of the samples was coated with gold (Au) for 240s to satisfy the conductivity requirements.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e2.3.6 Toxicity leaching test and chemical analysis\u003c/h2\u003e \u003cp\u003eIn order to investigate the concentration of impurities in PG and backfill samples, the toxicity leaching test was conducted according to HJ 557-2010. After curing for 28d, the backfill samples were grounded and sieved through a 3.0 mm screen. The powders were mixed with deionized water in a container at a mass proportion of 1:10 and shaken at 110 rpm/min on a rotary shaker for 8h. Then the mixtures were placed on the table for 16h. Finally, the mixtures were filtered through a 0.45 mm filter, and the leachates were collected for further analysis.\u003c/p\u003e \u003cp\u003eThe pH value of PG, bleeding water and the leachate of toxicity leaching test was measured by pH meter (Ohaus, US). The concentrations of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e-P were determined by ammonium molybdate tetrahydrate spectrophotometry (Shimadzu, Japan). The total dissolved solids (TDS) and the concentration of F\u003csup\u003e\u0026minus;\u003c/sup\u003e were measured by TDS meter (Ohaus, US) and fluorine ion-selective electrode (Leici, China), respectively.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3 Results And Discussion","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Effect of water washing conditions on PG\u003c/h2\u003e \u003cp\u003eDue to the production processes and stockpile environment, different quantities of impurities are contained in PG. Meanwhile, the types of impurities are also affected by residual acids in PG (Tao et al. 2016, Wen et al. 2019). In this study, the pH value is used as an index to evaluate washing efficiency. The following studies aim to provide an optimal stirring time and S/L ratio for the actual water washing of PG.\u003c/p\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1 Effect of stirring time on PG\u003c/h2\u003e \u003cp\u003ePG and deionized water with a mass ratio of 1:2 were mixed thoroughly with a stirrer at a speed of 200 rpm/min. The solution was taken out at 1, 2, 5, 10, 30, 60, 120 and 240 min, respectively. After centrifugation, the supernatant was collected to measure the pH value and the concentrations of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e-P, F\u003csup\u003e\u0026minus;\u003c/sup\u003e, and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAs presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a), with the first 1 min of stirring, the pH value of PG reached 1.86 and remained stable afterward. The increase of pH value was mainly due to the residual acids absorbed on the surface of the PG crystals, which were easily detached from the PG surface and escaped into the solution during the stirring process. In addition, it can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) that the concentration of impurities showed evident changes within the 5 min. This undulation of impurities concentrations was due to complicated chemical reactions occurring in the PG solution, such as dissolution and recrystallization of CaSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO, the ion-exchanges of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e-P, F\u003csup\u003e\u0026minus;\u003c/sup\u003e, and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e (Liu et al. 2019). Then, the impurities reached an equilibrium state after 5 min, and the concentrations of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e-P, F\u003csup\u003e\u0026minus;\u003c/sup\u003e and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e stabilized at about 3500 mg/L, 1200 mg/L and 14000 mg/L, respectively. The TDS also remained at about 8800 ppm within 5 min (seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a)), which indicated that the dissolved ions absorbed on the PG surface had been well diffused into the solution. In general, it can be inferred that the optimal stirring time for water washing PG is 5 min in this study.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2 Effect of solid-liquid ratio on PG\u003c/h2\u003e \u003cp\u003eThe water demand affects the labor and material resources that an enterprise needs to invest. In the actual water washing process, the S/L ratio may be directly related to the water demand. In this study, the water demand is defined as the ratio of water consumed for washing PG to the dry weight of PG. Therefore, PG was washed with different S/L ratios of 1:0.5, 1:1, 1:1.5 and 1:2 for 5 min each time until the pH reached a pre-designated value. According to the previous research results and the accumulated experiences, when the pH value of PG is about 5.00, it has little influence on the cemented PG backfill technique (Li et al. 2018, Min et al. 2019).\u003c/p\u003e \u003cp\u003eAs clearly shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), the pH value increased along with the water demand. The gradual growth of pH value was owing to the removal of residual acids by water washing. With a pH value of 5.00 as the target, washing PG with the S/L ratio of 1:0.5 required a water demand of 14. While the water demand of washing PG with the S/L ratio of 1:1, 1:1.5 and 1:2 was1.3, 1.5, and 1.6 times than that of 1:0.5, respectively. As regards the changes in impurity concentrations during water washing, Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b-d) present the variation curves of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e-P, F\u003csup\u003e\u0026minus;\u003c/sup\u003e and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e concentrations with water demand. It can be seen that the concentrations of all impurities decreased dramatically in the early washing times, and over 80% of impurities were removed at the first 8 washing demands. Then the pace of changes gradually slowed down. It was worth noting that with the S/L ratio increased from 1:0.5 to 1:2, the removal efficiencies of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e-P, F\u003csup\u003e\u0026minus;\u003c/sup\u003e and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e changed slightly. According to the above results, it can be inferred that when the PG is washed multiple times with a lower S/L ratio, a more rapid increase in the pH value of PG and a more significant reduction in the impurities concentration can be achieved with the minimum water demand. Therefore, the S/L ratio of 1:0.5 can be considered as the optimal ratio with acceptable efficiency in this study.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe morphological structure of the PG with and without pretreatment was observed by SEM analysis. Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (a) is the SEM image of the original PG with a pH value of 1.75, and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (b) is the SEM image of pretreated PG with a pH value of 5.15. It is well known that the PG crystals are plate-like structures (Li et al. 2017). Obviously, large quantities of small irregular particles were absorbed on the surface of PG crystals that could be directly identified in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a). In comparison, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b), water washing did not change the structure of PG crystals. However, the amount of irregular particles initially attached on the PG crystals significantly reduced, and the surface became smooth. To further understand the composition of irregular particles, EDS analysis was performed. The results showed that massive Ca, O and S were detected in the irregular particles, and a certain amount of F, P, K, Al, and Si were also measured (seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d) and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(e)). Therefore, it is considered that these small particles attached to the PG surface might be impurity particles. The SEM images also confirm that water washing could effectively remove the impurities.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e \u003ch2\u003e3.1.3 Effect of initial pH value of PG\u003c/h2\u003e \u003cp\u003eThree batches of PG (PG-1, PG-2 and PG-3) with an initial pH value of 1.75, 1.99, and 2.63 were selected to study the effect of water washing on the initial pH value of PG. The PG was washed by an S/L ratio of 1:0.5 and a stirring time of 5 min as determined from the above tests until the pH value of PG was 5.00. As presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a), for PG with initial pH of 2.63, the pH was raised to 3.00 only by washing 6 times. And after 20 times of washing, the pH value was higher than 5.00. However, for the original PG with initial pH of 1.75 and 1.99, 28 and 24 times of washing were needed to raise the pH to 5.00. It is evident that PG with a lower pH value contained more H\u003csup\u003e+\u003c/sup\u003e, and more water was needed to remove the acidity and raise the pH value of PG. Thus, PG with a lower initial pH value needs more washing times to reach the specified pH value.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRegarding the impurity concentrations in the original PG, it can be seen that the higher the initial pH of PG, the fewer impurities were observed. This might be attributed to that some impurities were removed under the different stockpiles environments and weathering factors, resulting in fewer impurities contents in the original PG (Tayibi et al. 2009). The effect of washing times on PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, F\u003csup\u003e-\u003c/sup\u003e and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e concentration are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b-d), respectively. The impurities concentration dropped rapidly before washing 10 times, meaning that excessive soluble impurities on the PG surface can be easily dissolved in the liquid. However, the descending rate gradually decreased in the following washes, leading to a corresponding decrease in the removal efficiency. Compared to PG-1 and PG-2, PG-3 needed fewer washing times to remove the impurities. The 5% difference in removal efficiency between two adjacent washes is defined as the stabilization of PG in this study. As seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b), the concentration of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P in PG-3 was stabilized only by 6 washing times. While for PG-1 and PG-2, 15 and 8 washing times were needed, respectively. For F\u003csup\u003e-\u003c/sup\u003e, more washes were needed to stabilize, and it was 16 washes for PG-3, which had the lowest initial F\u003csup\u003e-\u003c/sup\u003e content. It indicated that F\u003csup\u003e-\u003c/sup\u003e would be released continuously in PG in the long run. Eventually, when PG were all washed to the pH value of 5.00, the impurities in the washing solution was about varied from 5~ 8 mg/L of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, 6~75 mg/L of F\u003csup\u003e-\u003c/sup\u003e and 1400~1750 mg/L of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e. This result infers that even after multiple washing times, the concentrations of F\u003csup\u003e-\u003c/sup\u003e and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003ein PG remain high, posing potential environmental hazards if without further treatment.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Backfill slurry properties of purified PG\u003c/h2\u003e \u003cp\u003eIn order to investigate the influence of pretreated PG on the properties of backfill slurry, PG-1, PG-2 and PG-3 were washed to pH values of 3.50 and 5.00, respectively. The washing conditions are based on the most economical principle of the water demand determined by the above tests (the optimal washing S/L ratio of 1:0.5 and washing time of 5 min). In addition, PG-4 and PG-5 with an initial pH of 3.52 and 4.99 were selected as control groups. The experimental results of viscosity, bleeding rate and setting times (IST and FST) are presented in Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCharacteristics of backfill slurries prepared using PG with different pH values.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBatch No.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eViscosity (mPa\u0026middot;s)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBleeding Rate (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIST (h)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFST (h)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-1-O \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e769\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e29.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-1-P \u003csup\u003eb\u003c/sup\u003e-3.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e619\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e58.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e134\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-1-P-5.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e182\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e64.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e118\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-2-O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e707\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e33.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-2-P-3.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e437\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e68.97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e74\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-2-P-5.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e180\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e73.78\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-3-O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e490\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e44.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e140\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-3-P-3.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e404\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e56.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e78\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e116\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-3-P-5.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e88.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e76\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-4-O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e397\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e40.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e75\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-5-O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e157\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e58.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e74\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e\u003csup\u003ea\u003c/sup\u003e Original; \u003csup\u003eb\u003c/sup\u003e Pretreatment.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Variation of viscosity in backfill slurry\u003c/h2\u003e \u003cp\u003eIn the backfilling process, the slurry is usually mixed on the ground surface and then pumped into the goaf through the pipeline. Excessive viscosity of the slurry may cause a series of problems in slurry mixing, pumping and transportation (Wu et al. 2014). Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the variation of slurry viscosity with different pH values of PG. Using original PG as aggregate, the viscosity of backfill slurry decreased from 769 mPa\u0026middot;s to 490 mPa\u0026middot;s, with the increase in the pH value of original PG from 1.75 to 2.63. The decrease indicates that the pH value of aggregate has a significant effect on the backfill slurry. As in this study, the viscosity decreased about 75% when the PG (PG-1, PG-2 and PG-3) was washed to a pH value of about 5.00. These decreases may be explained by the fact that the surface of PG crystals becomes smooth after the residual acids are washed out, reducing the number of direct crystal-crystal contacts and increasing the thickness of the lubricating film around the crystal (Ness et al. 2018). Thereby, the friction force and pressure differential resistance are continuously reduced during the slurry flow process, manifested as a decrease in viscosity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003e3.2.2 Variation of bleeding rate in backfill slurry\u003c/h2\u003e \u003cp\u003eThe bleeding rate affects the durability and strength of the hardened backfill, which is one of the main physical properties (Yim et al. 2013). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e and Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the pretreatment of PG significantly affected the bleeding rate of the slurry. With the increase in the pH during the washing process, the bleeding rate of three groups PG-1, PG-2 and PG-3 increased significantly by 120%, 119% and 100%, respectively. This increase was caused by the reduction of residual acids and impurities in PG, which reduced the viscosity of the backfill slurry and weakened the free-water absorption capacity of the slurry. Therefore, the macroscopic performance is the gradual increase in the bleeding rate. For the original PG, as the initial pH value increase from 1.75 to 2.63, the bleeding rate increased by 52%, which is also attributed to this reason. The variation of viscosity and bleeding rate indicates that the water washing pretreatment can effectively improve the fluidity and transportation of the backfill slurry.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section3\"\u003e \u003ch2\u003e3.2.3 Variation of setting time in backfill slurry\u003c/h2\u003e \u003cp\u003eThe setting time affects the cementation and early strength of the backfill in the backfilling process (Chen et al. 2012). The initial setting time (IST) and final setting time (FST) of the backfill slurry are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The slurry prepared from the original PG-1 and PG-2 was not completely set within 7d, so the IST and FST were not measured. A possible explanation for this finding could be that the initial pH value of PG-1 and PG-2 are relatively low. With binder addition into PG, the binder first reacted with PG in a neutralization reaction, consequently slowing down the hydration reaction and prolonging the setting time (Li et al. 2017). For the backfill slurry prepared from pretreated PG, the binder could more rapidly and more extensively participate in the hydration reaction, thus shortening the setting times. It can be evidently seen from the figure that when the pH value of PG-1 and PG-2 was washed to 3.50, the setting times were greatly shortened. As the pH value increased to 5.00, the IST of PG-1 and PG-2 continued to reduce for 5h and 12h, and the FST was reduced by 16h and 8h, respectively. As the pH value of PG-3 was washed to 5.00, the IST and FST was reduced by 35% and 46%. Overall, the results of setting times can be concluded that the increase in pH value of pretreated PG facilitates the solidification of the backfill slurry into the hardened backfill.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Strength of pretreated PG-based cemented backfill\u003c/h2\u003e \u003cp\u003eThe backfill slurry is pumped into the goaf and then cemented into a hardened backfill with a certain strength, and the strength directly affects the stability of the stope (Li et al. 2019). Herein, the 28d strength of cemented PG backfill with and without pretreatment was measured, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(a) shows that the pretreatment of aggregate PG could well enhance the strength of the backfill. The backfill strength was enhanced significantly by 8.1 times, 6.2 times and 2.7 times by pretreating the three batches of aggregates. This increase in the backfill strength can be explained by the following three reasons. For one thing, it is known that the backfill strength derives from the overlap and tight bonding of aggregate PG and hydration products of the binder (Fall and Pokharel 2010). The pretreatment smoothed the PG surface and facilitated the overlap of hydration products and aggregate. For another, PG contains residual acid, which could consume the alkalinity of the binder and reduce the hydration products. As mentioned in 3.4.1, the pH value of the backfill slurry prepared from PG-1-O and PG-2-O are both around 8, causing the 28d strength of hardened backfill to be less than 0.15 MPa. Water washing could remove the majority of residual acids in PG. As the pH value of the pretreated PG reached 3.50 and 5.00, the pH value of the slurry reached around 12.8, which could ensure the proceed of the hydration reaction. The third reason for pretreatment enhancing the backfill strength was the reduction of soluble impurities. The excessive anions in PG would react with Ca\u003csup\u003e2+\u003c/sup\u003e of the binder, forming insoluble precipitations attached to the hydration products, therefore lowering the quality of hydration products. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the content of impurities in PG decreased significantly after pretreatment, so the quality of hydration products improved and backfill strength increased accordingly. In addition, when PG with relatively low initial pH value (1.76, 1.99 and 2.63) was washed to pH 3.50, an evident increase in strength was observed, indicating proper water washing could well enhance the strength development of backfill. However, when the pH value increased from 3.5 to 5.0, a very slight increase was observed for 28d backfill strength. This result indicates that excessive pretreatment of aggregate was unhelpful for backfill strength.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(b) shows the backfill strength prepared from PG washed to the same pH value. Two PG with an initial pH of 3.52 and 4.99 were selected as the control group. When PG with different initial pH was washed to the same pH value, the backfill obtained a similar 28d strength. For example, using PG with a pH value of 3.50 as aggregate (four batches, pretreated or original), the 28 d backfill strength was similar at about 0.9 MPa. This result indicates that pH value could be used as an index to evaluate the quality of PG. To save costs, the degree of water washing should be controlled within a reasonable range based on the actual mining method of the mines.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe SEM images shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e describe an overview of the microscopic observations of the backfill prepared by PG-2 with and without pretreatment. A large number of exposed plate-like PG crystals can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e (a), interspersed with a small amount of C-S-H gel and ettringite (AFt). When PG was pretreated, the content of hydration products noticeably increased, leading to an increase in strength by 4.7 and 5.2 times. Therefore, the water washing of aggregate can effectively improve the strength of cemented PG backfill.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Environmental behavior of purified PG in the backfill process\u003c/h2\u003e \u003cp\u003eThe majority of impurities could be either removed by water washing pretreatment or solidified/stabilized (S/S) by hydration reactions of the binder. However, it remains to be explored whether the bleeding water of backfill slurry and the leaching water of cemented backfill carries unconsolidated impurities and escapes into the groundwater (Li et al. 2019). Therefore, it is necessary to comprehensively understand the environmental behavior of impurities in the backfill process.\u003c/p\u003e \u003cdiv id=\"Sec23\" class=\"Section3\"\u003e \u003ch2\u003e3.4.1 Impurities in bleeding water\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e depicts the concentration of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, F\u003csup\u003e-\u003c/sup\u003e and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e in the bleeding water prepared from PG with and without pretreatment. By comparing the PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P concentration in the PG and the bleeding water, it was found that the hydration reaction was able to consolidate 99% of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, which has also been demonstrated by the previous study (Li et al. 2017). However, relatively high PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P concentrations (35.84 mg/L and 30.15 mg/L) were observed in the bleeding water with PG-1-O and PG-2-O. When the aggregate PG was water washed, the concentration of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P in all bleeding water was reduced to less than 0.5 mg/L, as shown in Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(a). As for F\u003csup\u003e-\u003c/sup\u003e, the concentration of F\u003csup\u003e-\u003c/sup\u003e decreased to 4~6 mg/L (seen in Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(b)) after pretreatment. Further, as can be seen in Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(c), with the increase of pH value of pretreated PG, the concentration of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e in the bleeding water of PG-1 and PG-2 groups gradually decreased from 4000~5000 mg/L to about 1300 mg/L. Overall, the water washing of PG could well remove the impurities, leading to lower concentrations of impurities in bleeding water. In addition, it can also be observed that when the pH of PG was washed to about 3.50, the impurities in bleeding water could be maintained at relatively stable levels. Among them, the concentration of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P and F\u003csup\u003e-\u003c/sup\u003e have met the Chinese standard GB8978-1996 for integrated wastewater discharge (F\u003csup\u003e-\u003c/sup\u003e concentration \u0026lt; 10mg/L and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P \u0026lt; 0.5mg/L).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec24\" class=\"Section3\"\u003e \u003ch2\u003e3.4.2 Impurities in backfill leachate\u003c/h2\u003e \u003cp\u003eThe toxicity leaching test was conducted on the backfills cured for 28d, and the degree of S/S of impurities in cemented PG backfill was investigated, as shown in Table \u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P concentrations in all leachate were less than 0.5 mg/L, and the F\u003csup\u003e-\u003c/sup\u003e concentrations were less than 10 mg/L (except for PG-1-O), which met the Chinese standard for integrated wastewater discharge. The SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e concentrations were controlled at around 30 mg/L, except for PG-1-O with SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e concentration up to 104 mg/L. Almost 100% of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, more than 99.3% of F\u003csup\u003e-\u003c/sup\u003e and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e in pretreated PG were consolidated. This also proved that the backfill prepared with pretreated PG could significantly alleviate the environmental pollution of PG.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eExperimental data of the backfill leachate.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBatch No.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csup\u003e-\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-1-O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e104\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-1-P-3.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e28\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-1-P-5.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-2-O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e29\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-2-P-3.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-2-P-5.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e29\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-3-O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e27\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-3-P-3.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e29\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePG-3-P-5.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e26\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eThe purpose of this study was to investigate the effects of stirring times, S/L ratio and initial pH value of PG on the mechanical properties and environmental behavior of the backfill using the water-washed PG as aggregate. The experimental results show that the pretreatment of aggregate could effectively improve the performance of backfill. The following conclusions can be drawn:\u003c/p\u003e \u003cp\u003e(1) The PG pretreatment process was optimized for backfill, including 5 min stirring time, the S/L ratio of 1:0.5.\u003c/p\u003e \u003cp\u003e(2) Using pretreated PG as aggregate effectively improved the workability of the backfill slurry and enhanced the strength development of the hardened backfill.\u003c/p\u003e \u003cp\u003e(3) Water washing pretreatment significantly reduced the impurities content in the bleeding water and the leachates of backfill. Eventually, almost 100% of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e-P, more than 99.3% of F\u003csup\u003e-\u003c/sup\u003e and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e in PG had been fixed into the backfill.\u003c/p\u003e \u003cp\u003e(4) The wastewater generated after washing PG could be treated first. For example, by adding the common CaO directly to the wastewater, which is relatively easy to operate. The treated water could still be used to wash PG, realizing the circulation of water resources.\u003c/p\u003e \u003cp\u003e(5) In practice, it is recommended to use the pH value of PG as a parameter for selecting the pretreatment method to meet the mechanical and environmental requirements of the backfill in mines.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch3\u003eFunding\u003c/h3\u003e\n\u003cp\u003eThis work was supported by the State Key Research Development Program of China (Grant No. 2018YFC1800400), and the National Natural Science Foundation of China (Grant No. 72088101).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval,\u003c/strong\u003e\u003cstrong\u003econsent to participate and consent to publish\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflict of interest.\u003c/p\u003e\n\u003ch3\u003eAuthor contribution\u003c/h3\u003e\n\u003cp\u003e\u003cstrong\u003eYanan Zhou:\u003c/strong\u003e Conceptualization; Methodology; Data curation; Writing - original draft. \u003cstrong\u003eXibing Li:\u003c/strong\u003e Supervision; Writing - review \u0026amp; editing. \u003cstrong\u003eYing Shi:\u003c/strong\u003e Methodology; Writing - review \u0026amp; editing; Funding acquisition. \u003cstrong\u003eQuanqi zhu: \u003c/strong\u003eWriting - review \u0026amp; editing; Validation.\u003cstrong\u003e Jing du: \u003c/strong\u003eFormal analysis.\u003c/p\u003e\n\u003ch3\u003eAvailability of data and materials\u003c/h3\u003e\n\u003cp\u003eThe datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAmrani M, Taha Y, Kchikach A, Benzaazoua M, Hakkou R (2020) Phosphogypsum recycling: New horizons for a more sustainable road material application. 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Journal of hazardous materials 399, 122993\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Phosphogypsum, Water washing, Pretreatment, Waste reuse, Cemented PG backfill, Impurity reduction","lastPublishedDoi":"10.21203/rs.3.rs-1172069/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1172069/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePhosphogypsum (PG) is reused as aggregate in the cemented backfill of the mines, which effectively improves the PG reutilization efficiency. However, the massive impurities contained in aggregate PG would adversely affect the hydration of binder, and therefore deteriorating strength development of backfill. This research starts with the feasibility study on pretreating the aggregate PG with the water washing method. Based on the most economical principle of the water demand, the optimal conditions for washing PG were determined at a stirring time of 5 min and a solid-liquid ratio of 1:0.5. Then, the original and the pretreated PG were made into the backfill. Compared to using the original PG, the backfill slurry using the pretreated PG had better fluidity performance, such as the lower slurry viscosity, the higher bleeding rate and the shorter setting times. Furthermore, with the pretreated aggregate PG, the strength of the backfill was significantly enhanced by more than 8 times, which was due to the removal of impurities on the surface of PG in the pretreatment. Finally, the environmental behavior of the cemented backfill was investigated. Using the pretreated PG as aggregate, concentrations of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e-P and F\u003csup\u003e\u0026minus;\u003c/sup\u003e in the bleeding water and leachates of backfill could meet the Chinese standard for integrated wastewater discharge, which significantly reduced the adverse effect of PG on the environment. The results extend the reuse of PG as aggregate in a more environmental-friendly way, meeting the needs for sustainable mines.\u003c/p\u003e","manuscriptTitle":"Reuse of Phosphogypsum Pretreated with Water Washing as Aggregate for Cemented Backfill","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-01-21 15:31:50","doi":"10.21203/rs.3.rs-1172069/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2022-01-17T21:49:17+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-01-17T16:35:16+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2021-12-21T05:43:18+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2021-12-14T21:35:53+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"6c0a1983-e6f2-4095-9512-0af986d47b20","owner":[],"postedDate":"January 21st, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2022-01-21T15:31:50+00:00","versionOfRecord":[],"versionCreatedAt":"2022-01-21 15:31:50","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-1172069","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1172069","identity":"rs-1172069","version":["v1"]},"buildId":"7rjqhiLT3MXkJMwkYKINL","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}