The re-fired magnesia produced by Jinan Ludong Refractory Material Co., Ltd. is calcined and ground into magnesia powder at 1700°C for the preparation of magnesia phosphate cement. The powder is brownish yellow. Potassium dihydrogen phosphate (KH₂PO₄) produced by Sinophosphate Chemical Reagent Co., LTD. is a white powder, easily soluble in water at room temperature. Prior to experimentation, the sludge obtained from the desiccation project at Yuecheng Reservoir in Cixian County, Handan City, Hebei Province, requires activation. The activation process involves drying the sludge in a drying box, followed by ball milling to produce a brown and yellow powder. Subsequently, the powder undergoes calcination in a high-temperature furnace before being subjected to ball milling again. The chemical composition of the resulting material is detailed in the table below. The chemical composition of the material is shown in Tables 1, 2 (Lv et al., 2024a; Lv et al., 2024b; Lv et al., 2024c).

P.O.42.5 ordinary Portland cement, produced by the Tangshan Yanshan Cement Factory, was selected for the concrete preparation. Crushed stone and river sand were used as aggregates, with the stone having a uniform particle size range of 5–25 mm and the river sand featuring continuous grading with a maximum particle size of 5 mm. The chemical composition of the cement is presented in Table 3 below.

Due to the small particle size of the sludge, even a small amount can significantly affect the flow behavior of the MPC system. Therefore, in this study, while varying the sludge content in different groups, the flowability of each group was kept consistent. The MG-P molar ratio of the magnesium phosphate cement in the sludge-free group was maintained at 8.5, and the combined mass of the sludge and MgO in the sludge-containing groups was controlled to 2.5, ensuring uniform fluidity across all groups. The specific proportions of MPC with varying sludge contents used in the experiment are presented in Table 4. Additionally, uncoated steel bars (Group C) were prepared as a control group.

The concrete design strength of the drawing specimen is C40 concrete, and the mix ratio per cubic meter of concrete is calculated according to JG55-2011“Ordinary Concrete Mix ratio Design Regulations”, as shown in Table 5 below.

Production of magnesium phosphate cement: Magnesium phosphate cement is prepared according to the modified mix ratio detailed in Table 4. Initially, the required amounts of magnesium oxide, phosphate, and sludge are weighed according to the ratios specified. These materials are then placed into a cement slurry mixer and mixed at a slow speed for 60 s to ensure thorough and uniform blending of the dry components. Next, water, measured according to the mix ratio, is added to the mixer and stirred at a slow speed for another 60 s. The mixing is paused, and any remaining material adhering to the blade and walls of the pot is scraped down into the mixer by using a shovel. The mixture is then stirred for an additional 60 s to achieve uniform consistency. After mixing, the modified magnesium phosphate cement is ready for use.

Coating steel bar production: Given the rapid setting and hardening characteristics of magnesium phosphate cement, the coating process must be executed promptly. First, the prepared magnesium phosphate cement is introduced into a semi-circular PVC pipe. The steel bar is then rotated within the PVC pipe to ensure a complete and even coating of the magnesium phosphate cement on its surface. Finally, the mixing pot and PVC pipe are thoroughly cleaned to remove any residual material.

Coating of steel bars: Due to the rapid setting and hardening of MPC, the coating process should be performed swiftly, with the entire operation completed within 5 min. First, the prepared MPC is placed into a semi-circular PVC pipe, and the steel bar is rotated within the PVC pipe to ensure that the surface is uniformly coated with MPC. The consistent flowability of the slurry ensures a uniform coating thickness of approximately 1 mm. The coated steel bars are then cured under indoor air conditions (25°C ± 2°C, RH 70% ± 5%). Finally, the mixing bowl and PVC pipe are thoroughly cleaned.

Drawing test piece production: For the preparation of drawing test pieces for reinforced concrete, use steel bars with a diameter of 10 mm and a length of 500 mm, and pour concrete into molds to create cubes measuring 150 mm × 150 mm × 150 mm. Insert one end of the steel bar into the mold to a depth of 100 mm, with the other end extending 250 mm, and set the protective layer thickness to 75 mm. To minimize experimental errors, place a PVC pipe of 50 mm length at each end of the steel bar. The effective bonding length between the steel bar and concrete is set to 5 d (where d is the diameter of the steel bar; thus, 5 d equals 50 mm). The specific mold is depicted in Figure 1. First, apply a release agent to the inside of the plastic mold. Next, accurately weigh the required amounts of cement, aggregate, and sand according to the mix ratio, and combine them in the concrete mixing pot. Perform a dry mix for 90 s to ensure uniform distribution of the dry materials. Add the measured water, and mix until the concrete achieves the desired flow and consistency. Pour the mixed concrete into the pre-oiled plastic mold, position the mold in the center of a shaking table, and use a scraper to remove excess concrete while vibrating the mold to ensure proper compaction.

The central drawing test device used a WAW-1000A microcomputer controlled electro-hydraulic servo universal testing machine to load the specimen and customized the drawing fixture with matching size according to the specifications of the loading device. Before loading, the specimen was passed through the drawing fixture, the upper end of the loading test machine tightened the fixture, and the lower end of the clamping steel bar loading device is shown in Figure 2 below. The loading mode was set as displacement loading, and a dial gage was fixed at the free end of the reinforced concrete specimen to test the relative slip between the reinforcement and concrete. During the experiment, when the free slip end exceeds the specified value, the concrete cracks or the steel bar is pulled apart, the loading is stopped and the load and the dial indicator are recorded at this time, and then the loading of the next specimen is continued.

Scanning electron microscopy (SEM) was used to analyze the magnesium phosphate cement modified with different amounts of sludge and to explain the influence on the adhesive properties at the microscopic level. The EDS point spraying method was used to analyze the chemical element composition of the modified magnesium phosphate cement and to explain the effect of the modified magnesium phosphate cement with different amounts of sludge.

During the experimental loading, all 24 pull-out specimens of circular steel bars exhibited central pull-out failure, as illustrated in Figure 3. This indicates that there was a slip between the steel bars and the concrete, and the PVC pipes at both ends of the steel bars also experienced some degree of slip. This failure mode suggests that the friction force between the steel bars and the concrete was insufficient to overcome the applied tensile stress, leading to the direct pull-out of the steel bars due to low bond strength. For the 24 ribbed steel bar pull-out specimens, those with a coating on the surface also displayed central pull-out failure. However, the specimens with uncoated surfaces exhibited splitting failure, primarily due to the high mechanical bite force between the ribs of the ribbed steel bars and the concrete. This high bond strength between the steel bars and concrete is maintained until the tensile stress of the concrete exceeds its tensile and cracking strength, resulting in splitting failure. When the surface of the ribbed steel bars is coated, the mechanical bite force between the ribs and the steel bars is disrupted. In all groups, there was no evidence of peeling or detachment between the magnesium phosphate cement coating and the steel bars, indicating that the bond strength between the magnesium phosphate cement and the steel bars is robust. This suggests that the coating adheres well to the steel bar surface, providing effective protection (Yang et al., 2002; Li et al., 2021).

Since the central drawing test was used for test loading in all experiments and the length of the bonding section between reinforcement and concrete was small, the bonding stress between the reinforcement and concrete was approximately regarded as uniformly distributed, and the following Formula 1–1 was used to calculate the bonding stress. The labels MS-0, MS-5, MS-10, MS-15, MS-20, MS-25, and MS-30 represent the sludge contents of 0, 5%, 10%, 15%, 20%, 25%, and 30%, respectively. τ = P π dl . (1) In the above formula, τ represents the calculated bond stress, and the unit is MPa.

P represents the measured drawing force, and the unit is N.

d represents the diameter of the coated steel bar, expressed in mm.

l is the length of the effective bonding section, expressed in mm.

The bond strength of modified magnesium phosphate cement with varying sludge contents, applied to smooth round steel bars, is illustrated in Figure 4. Without the magnesium phosphate cement coating, the bond strength of the smooth round steel bar is the lowest, at 2.53 MPa. With the addition of sludge at 0%, 5%, 10%, 15%, 20%, 25%, and 30%, the bond strengths increased to 3.13 MPa, 3.56 MPa, 3.71 MPa, 3.81 MPa, 3.73 MPa, 3.76 MPa, and 3.52 MPa, respectively. The corresponding bond strength increases were 23.7%, 40.7%, 46.6%, 50.6%, 47.4%, 48.6%, and 39.1%, respectively. These data indicate that the inclusion of sludge significantly enhances the bond strength between the modified magnesium phosphate cement-coated steel bars and concrete, with the optimal bond performance achieved at a sludge content of 15%. However, further increases in sludge content have a diminishing effect on bond performance. The observed improvement in bond strength with the magnesium phosphate cement coating is primarily due to the increased friction force between the smooth steel bars and the concrete, which is related to the roughness of the contact surface. The coating introduces surface irregularities and pores, which enhance bond strength. Additionally, research by Tang Hao supports these findings, highlighting that magnesium phosphate cement exhibits strong adhesive properties with steel bars and forms a dense bonding layer on the steel surface, further contributing to the increased bond strength observed with the coating (Tang et al., 2020).

The bond strength of modified magnesium phosphate cement applied to ribbed steel bars with varying sludge contents is depicted in Figure 5. The data clearly show that the bond strength behavior of ribbed steel bars is opposite to that of smooth round steel bars. For ribbed steel bars without coating, the bond strength is the highest at 21.83 MPa. As sludge is added, the bond strength of the ribbed steel bars gradually decreases. Specifically, with sludge contents of 0%, 5%, 10%, 15%, 20%, 25%, and 30%, the bond strengths are 17.35 MPa, 16.17 MPa, 16.09 MPa, 14.88 MPa, 13.37 MPa, 12.96 MPa, and 11.78 MPa, respectively. The reductions in bond strength are 20.5%, 25.9%, 26.3%, 31.8%, 38.8%, 40.6%, and 46.0%, respectively. The significant reduction in the bond strength of coated ribbed steel bars is attributed to the disruption of the mechanical biting force between the ribs and the concrete. In uncoated ribbed steel bars, the bond strength is primarily supported by this mechanical biting force. However, when the modified magnesium phosphate cement coating is applied, it interferes with the biting force, leading to a reliance on friction force and the diminished mechanical bite force, ultimately reducing the overall bond strength (Mathern and Yang, 2021; Chen et al., 2023).

Comparing the two types of steel bars reveals that the adhesive properties of smooth round steel bars improve with the application of modified magnesium phosphate cement coating. Conversely, the adhesive properties of ribbed steel bars decrease with the same coating. This difference is primarily related to the nature of the bond between each type of steel bar and the concrete. While coating ribbed steel bars in engineering applications can enhance their corrosion protection, it may also lead to a reduction in bond strength. Therefore, a careful evaluation and analysis are necessary to balance the benefits of improved corrosion resistance with the potential decrease in bond strength during application.

Figure 6 illustrates the slippage of smooth round steel bars coated with modified magnesium phosphate cement containing varying sludge contents. For smooth round steel bars, the bond stress between the steel bar and the concrete is primarily provided by friction force. The data show that the slip amount for uncoated steel bars is the largest, reaching 0.54 mm. When modified magnesium phosphate cement is applied, the slip amount decreases significantly. Specifically, for sludge contents of 0%, 5%, 10%, 15%, 20%, 25%, and 30%, the slip amounts are 0.27 mm, 0.28 mm, 0.31 mm, 0.26 mm, 0.29 mm, 0.33 mm, and 0.32 mm, respectively. Analysis of the different sludge contents indicates that there is minimal variation in the slip amount across the different groups, suggesting that the sludge content has little impact on the slip of the modified magnesium phosphate cement-coated round steel bars. During the test loading process, there is no significant slip observed in the initial stages; however, as loading progresses, the uncoated group experiences substantial slip due to inadequate bonding force between the steel bar and the concrete. Coating the steel bar surface improves the roughness of the contact surface, thereby reducing slip when the maximum load is reached. Overall, applying a modified magnesium phosphate cement coating to the surface of smooth round steel bars effectively enhances the bond strength between the steel bar and concrete, reducing relative slip. This treatment not only improves the corrosion resistance of the steel bars but also increases the bond strength with the concrete (Xiang and Yang, 2023; He and Lu, 2024a).

Figure 7 illustrates the slippage of ribbed steel bars coated with modified magnesium phosphate cement containing various sludge contents. For ribbed steel bars, the bond stress is primarily provided by the mechanical biting force between the ribs and the concrete. The slip amount for the uncoated group is 1.74 mm. With the application of modified magnesium phosphate cement coating, the slip amount decreases to some extent. Specifically, with sludge contents of 0%, 5%, 10%, 15%, 20%, 25%, and 30%, the slip amounts are 0.91 mm, 1.06 mm, 0.89 mm, 0.88 mm, 0.96 mm, 1.01 mm, and 0.97 mm, respectively. The sludge content has a minimal effect on the slippage of ribbed steel bars, similar to the trend observed with smooth round steel bars. Ribbed steel bars, due to their threaded surface, generate significant mechanical biting force with the concrete. However, when the surface of the steel bars is coated, the uniform coating reduces the mechanical biting force between the coated steel bars and the concrete, which diminishes the bond performance and decreases slip amount. Consequently, the slip amount is reduced when the maximum load is reached (He et al., 2024; He and Lu, 2024b).

When smooth round and ribbed steel bars are uncoated, the slip amount of the smooth round steel bar is relatively small. This is due to the lower bond strength between the smooth round steel bar and the concrete, resulting in minimal slip when the loading process reaches its maximum load. After coating, the slippage of both types of steel bars decreases, indicating that the coating effectively reduces the relative slippage between the steel bars and the concrete. This reduction in slip contributes to improved structural stability.

Figure 8 displays SEM images of modified magnesium phosphate cement with varying sludge contents. The images reveal that struvite, the primary hydration product of magnesium phosphate cement, predominantly exhibits a sheet-like structure. The overall structure consists of a spatial arrangement formed by magnesium oxide and struvite, which has not undergone further hydration. Comparison of SEM images across different sludge contents shows that pure magnesium phosphate cement has notable voids and large pores. At a sludge content of 15%, the overall compaction of the structure significantly improves. However, as the sludge content increases to 30%, the amount of struvite decreases, and some voids and pores become evident. This phenomenon is primarily due to the fact that the main component of sludge is inactive quartz (SiO₂), and the sludge particles are small. On one hand, the inactive sludge can adhere fully to the surface of magnesium oxide, extending the time the water remains in contact with the material, which enhances the hydration reaction. This leads to the formation of more guanolite, increasing the density and reducing porosity. On the other hand, the small particle size of the sludge allows it to function as an inactive filler material, filling gaps between struvite and magnesium oxide. This increases the overall density of the structure and further reduces porosity. However, when the sludge content is too much, it will replace part of the amount of magnesium oxide, resulting in insufficient reaction material and a reduction in the amount of struvite as a hydration product. Through SEM analysis, it is explained that the bond strength of the smooth round steel bar reaches the maximum when the sludge content is 15%, but decreases when the sludge content is 30%.

Figure 9 presents the correlation analysis of modified magnesium phosphate cement with 30% sludge content, conducted via EDS point scanning. The analysis reveals that as the sludge content increases, the proportion of silicon (Si) in the cement matrix increases. This is attributed to the high SiO₂ content in the sludge, which causes a decrease in the magnesium-to-phosphorus (Mg/P) ratio. After 7 days of curing, the Si content in region 1 of the sample with 30% sludge is 18.46%. This indicates that excessive sludge can displace magnesium oxide, leading to an insufficient amount of magnesium oxide and resulting in inadequate hydration reactions. Consequently, there is a reduced formation of struvite, leading to a less complete structure with numerous pores and cracks. Additionally, the excess sludge negatively impacts structural densification and reduces the bond strength of the smooth round steel bars.