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Article

Mechanical Behavior and Pollutant Stabilization of Modified Basalt Fiber-Reinforced Bio-Cemented Phosphogypsum

by
Gan Nan
1,
Jiaming Zhang
1,* and
Kai Liu
2
1
Faculty of Engineering, China University of Geosciences (Wuhan), 388 Lumo Road, Wuhan 430074, China
2
Central Southern China Electric Power Design Institute Co., Ltd. of China Power, Engineering Consulting Group, 12 Zhongnan 2nd Road, Wuchang District, Wuhan 430074, China
*
Author to whom correspondence should be addressed.
Buildings 2026, 16(2), 455; https://doi.org/10.3390/buildings16020455
Submission received: 26 August 2025 / Revised: 18 September 2025 / Accepted: 22 September 2025 / Published: 22 January 2026
(This article belongs to the Section Building Materials, and Repair & Renovation)
Browse Figures
Versions Notes

Abstract

To facilitate the large-scale recycling of phosphogypsum (PG) as a construction material and mitigate the environmental safety concerns associated with its stockpiling or discharge, this study proposes an innovative approach. The method employs modified (acid-treated) basalt fibers (MBF) synergistically combined with microbially induced carbonate precipitation (MICP) technology for PG solidification. This synergistic MBF–MICP treatment not only enhances the strength and further improves the toughness of the solidified PG but also effectively immobilizes heavy metals within the PG matrix. Bacterial attachment tests conducted on fibers subjected to various pretreatment conditions revealed that the maximum bacterial adhesion occurred on fibers treated with a 1 mol/L acid concentration for 2 h at 40 °C. However, MICP mineralization experiments performed on these pretreated fibers determined the optimal pretreatment conditions for mineralization efficiency to be an acid concentration of 0.93 mol/L, a treatment duration of 0.96 h, and a temperature of 30 °C. Unconfined compressive strength (UCS) tests and calcium carbonate content measurements identified the optimal reinforcement parameters for MBF–MICP-solidified PG as a fiber length of 9 mm and a fiber dosage of 0.4%. Furthermore, comparative analysis demonstrated that the UCS and toughness of MBF–MICP-solidified PG were superior to those of bio-cemented PG specimens treated with unmodified fibers or without any fiber reinforcement. It was found by scanning electron microscopy that there was an obvious phosphogypsum particle-fiber-calcium carbonate precipitation interface in the sample, and the fiber had a bridging effect. Finally, heavy metal leaching tests conducted on the solidified PG confirmed that the leached heavy metal concentrations were below the detection limit, complying with national discharge standards.

1. Introduction

Phosphogypsum (PG) is a bulk industrial solid waste generated during the wet-process phosphoric acid production [1,2]. Approximately 5 tons of PG are produced per ton of phosphoric acid manufactured [3,4,5]. The underlying chemical reaction mechanism is shown in Equation (1). The primary chemical composition of PG is calcium sulfate dihydrate (CaSO4·2H2O), accompanied by impurities such as phosphorus, fluorine, and heavy metals. The global utilization rate of PG is approximately 15% [6,7]. Its random stacking or discharge not only occupies land resources but also causes soil pollution and eutrophication of water bodies, which seriously threatens human life, activities, health, and safety.
C a 5 F ( P O 4 ) 3 + 5 H 2 S O 4 + 10 H 2 O → 3 H 3 P O 4 + 5 C a S O 4 · 2 H 2 O + H F
Given the environmental safety concerns arising from the stockpiling or discharge of PG, the treatment and resource recovery of PG are imperative. Currently, the primary approaches for PG disposal and resource utilization encompass four key areas: building materials [8,9]; production of agricultural amendments [10,11]; chemical feedstocks [12,13]; and rare earth element (REE) extraction [14,15]. Within the building materials sector, PG serves as a substitute for natural gypsum [16,17], a cement set retarder [18,19], mine backfill material [20,21], and subgrade filler [22,23]. In agriculture, PG is primarily utilized as a plant fertilizer [24,25] and soil remediation material [26,27]. Within the chemical industry, PG serves as a potential feedstock for the production of chemical compounds such as potassium sulfate, sulfuric acid, ammonium sulfate, and calcium carbonate [28,29]. Regarding rare earth element extraction, PG retains approximately 85% of the REEs originally present in the phosphate rock feedstock, and these elements can be recovered from PG via an acid leaching processes [30,31]. While the aforementioned approaches are significant for PG treatment and resource recovery, they suffer from drawbacks including high costs and low utilization rates [6,7], with the potential for secondary pollution. Consequently, there is an urgent need to develop low-cost, high-efficiency, and environmentally benign disposal technologies to address the challenges associated with PG stockpiling.
It is well established that gypsum is in high demand as a building material for engineering construction. Consequently, recycling PG as a building material represents an effective approach to addressing PG stockpiling issues. However, untreated PG exhibits low strength, high acidity, and contains impurities such as phosphorus, fluorine, and heavy metals [32,33], rendering it unsuitable for direct use in construction applications. As an environmentally benign technology, microbially induced carbonate precipitation (MICP) has been extensively applied in diverse areas including ground improvement [34,35], liquefaction mitigation [36,37], soil permeability reduction [38,39], rock fracture remediation [40,41], and heavy metal-contaminated soil remediation [42,43]. The technology produces urease through the biological activities of bacteria. The carbonate produced by urea in the environment under the catalysis of urease is combined with calcium ions in the environment to produce calcium carbonate precipitation with cementing properties, thus cementing the loose soil particles into a whole and improving the overall engineering performance of the soil. Studies have demonstrated that microbially induced carbonate precipitation technology can enhance the strength and impermeability of PG [43,44]. However, MICP-solidified PG specimens exhibit brittle fracture behavior [45,46], leading to rapid loss of overall structural integrity in construction products. This compromises their impermeability and erosion resistance, which in turn may trigger accelerated release of impurities such as phosphorus, fluorine, and heavy metals from PG into the natural environment. In recent years, to address the brittle fracture issue in MICP-solidified specimens, numerous researchers have employed fiber reinforcement combined with MICP technology for soil solidification [47,48,49]. This approach not only enhances the toughness of MICP-treated soils but also improves their strength and impermeability. Li et al. [50] used basalt fiber as a reinforcing material and found that adding basalt fiber can improve the stability and toughness of microbial hardened sand. Wang et al. [51] studied the mechanical properties of calcareous sand solidified by basalt fiber combined with MICP. The results show that the combination of basalt fiber and MICP can improve its tensile strength. Basalt fiber (BF), recognized for its advantages including high strength, corrosion resistance, low cost, and environmental friendliness, has been widely employed in microbially induced carbonate precipitation (MICP) for soil reinforcement or cementitious material enhancement [52,53]. However, the smooth surface structure and low surface free energy of basalt fibers result in poor interfacial bonding with the matrix [54,55,56]. This consequently undermines the positive contribution of BF to the engineering performance of MICP-solidified materials to some extent. Therefore, to enhance the toughening effect of basalt fiber-reinforced MICP-solidified PG, surface modification of the basalt fibers is imperative.
To date, the majority of research has focused on the effects of fiber length, fiber dosage, and fiber type on microbially induced carbonate precipitation (MICP)-solidified soils. However, studies investigating the influence of modified basalt fibers (MBF) on MICP, and specifically on the synergistic solidification of PG using MBF combined with MICP technology, have received limited attention. To facilitate the large-scale recycling of PG as a construction material and mitigate the environmental safety concerns associated with its stockpiling or discharge, this study employed acid treatment for basalt fiber modification. The modified basalt fibers (MBF) were then synergistically combined with MICP (MBF–MICP) to solidify PG, with the aim of identifying the optimal modification conditions for the basalt fibers. The influence of modified fiber length, modified fiber dosage, and the effect of fiber modification on the mechanical properties of MBF–MICP-solidified PG were analyzed to determine the optimal fiber reinforcement conditions. Toxicity leaching tests were conducted to comparatively analyze the heavy metal leaching concentrations from PG before and after solidification. Scanning electron microscopy (SEM) was utilized to investigate the mechanism underlying the synergistic solidification of PG by modified fibers and MICP. This study not only provides an effective method for modifying basalt fibers but also offers a green, cost-effective technology for the large-scale recycling and utilization of PG solid waste in the construction sector.

2. Testing Material

2.1. Raw Material

The PG utilized in this study was sourced from the PG stockpile of a fertilizer plant in Yicheng City, Hubei Province, China. The chemical composition of the PG is presented in Table 1, while its physical properties are detailed in Table 2. The basalt fiber employed was procured from Haining Anjie Composite Materials Co., Ltd. Anjie Composite Materials Co., Ltd., Haining, Zhejiang, China, with its physical characteristics outlined in Table 3. The PG and basalt fiber schematic diagram is shown in Figure 1.

2.2. Preparation of Bacterial Solution and Cementing Fluid

The present study employed Bacillus pasteurii. Initially, the culture medium was placed in an autoclave for sterilization under conditions of 103.4 kPa and 121.3 °C for 15–30 min. Subsequently, the activated bacterial culture was inoculated into the sterilized medium at a 1% ratio and cultivated in a shaker incubator at 30 °C with agitation of 160 rpm for 24–36 h. The binding solution was prepared by mixing 1 M urea and 1 M calcium chloride in a 1:1 volumetric ratio. The bacterial culture process is shown in Figure 2.

3. Test Method

3.1. Fiber Pretreatment

The present study investigated the effects of modified basalt fiber on bacterial adhesion capacity and microbial-induced calcite precipitation (MICP). To this end, basalt fibers were pretreated with diluted sulfuric acid, with three critical parameters evaluated: acid concentration, treatment duration, and temperature. The acid concentrations tested were 0, 0.5, 1, 1.5, and 2 mol/L; treatment durations were 0, 0.5, 1, 2, and 4 h; and temperatures were 20, 30, 40, 50, and 60 °C.

3.2. Adhesion Ability of Fiber to Bacteria

To investigate the adhesion capacity of modified basalt fibers toward bacterial bacteria, 0.5 g of basalt fibers pretreated under different conditions were weighed and placed into a conical flask containing 50 mL of Bacillus pasteurii culture. The flask was then incubated in a shaker at 30 °C and 160 r/min for oscillation periods of 1, 12, 24, 36, and 48 h. After oscillation, the fibers were retrieved and transferred into centrifuge tubes with 20 mL of sterile physiological saline. The tubes were subsequently placed in a shaker at 30 °C and 160 r/min for 20 min of agitation. The eluate was then collected for OD600 measurement.

3.3. Effect of Fiber on MICP

(1)
Single-factor test of the effect of fiber on MICP
Firstly, 0.2 g of basalt fibers subjected to different pretreatment conditions were weighed and placed into centrifuge tubes. Subsequently, 20 mL of bacterial suspension and 20 mL of cementation solution were sequentially added to each tube to initiate the microbially induced carbonate precipitation (MICP) reaction. Following the MICP reaction, the tubes were centrifuged at 4000 rpm for 10 min. This centrifugation step was repeated twice. The supernatant was then filtered using filter paper. Both the filtered paper and the centrifuge tubes containing the precipitate were dried in an oven at 60 °C for 48 h. Their masses were measured separately. The mass of calcium carbonate precipitate M enerated was then calculated as follows:
M = M 2 + M 4 − M 1 + M 3
where M1 is the initial mass of the centrifuge tube, M2 is the mass of the centrifuge tube after drying, M3 is the mass of the filter paper before filtration, and M4 is the mass of the filter paper after drying.
(2)
Response surface test of the effect of fiber on MICP
Based on the results of single-factor experiments, using calcium carbonate precipitation yield as the evaluation criterion, the response surface methodology was employed to determine the optimal modification conditions for basalt fibers.

3.4. Mechanical Test of Modified Fiber Combined with MICP-Solidified PG

(1)
Indoor mechanical test
Based on the results of the basalt fiber modification experiments, according to the standard for geotechnical testing methods [57], unconfined compressive strength (UCS) tests were conducted to investigate the influencing factors of modified basalt fiber content, fiber length, and confining pressure. The experimental design primarily focused on the variables of modified basalt fiber content and length, as outlined in the specific test protocol presented in Table 4. The apparatus used in the unconfined compressive strength test is shown in Figure 3.
(2)
Sample preparation steps
① An appropriate amount of PG was dried for standby, and the basalt fiber under the optimal modification conditions was used for standby; ② the PG and the modified basalt fiber were divided into three equal parts, and each equal part of the PG and the modified basalt fiber were uniformly mixed; ③ the mixed sample was divided into three layers and loaded into the mold. In the process of loading, the layer was pulled between the layers, and finally the sample was pressed. The sample size was 39.1 mm and the height was 80 mm. A filter paper and a permeable stone were placed at each end of the sample, and then a rubber plug with a hole was plugged at both ends; ④ the bacterial solution with 1 times pore volume was injected into the sample at a rate of 5 mL/min by a peristaltic pump, and then stood for 4 h, and then the cementing solution with 1 times pore volume was injected into the sample at a rate of 5 mL/min. Grouting was performed 10 times at intervals of 24 h. After the grouting was completed, deionized water was injected into the sample at the same grouting rate to terminate the MICP mineralization reaction. The grouting device is shown in Figure 4.
(3)
Determination of calcium carbonate content in specimens: specimens were dried to constant weight after solidification. The mass of calcite precipitated during the MICP reaction was determined by comparing the weight of the specimen before and after solidification. The calcium carbonate content was calculated by dividing this precipitated mass by the initial mass of the specimen (prior to solidification), expressed as a percentage (%).

4. Results and Discussion

4.1. Analysis of Adhesion Characteristics of Modified Basalt Fiber to Bacteria

4.1.1. Analysis of Bacteria Attachment Time-History Curve

The bacterial adhesion behavior on acid-treated basalt fibers is demonstrated in Figure 5. As shown in Figure 5a–c, under different acid treatment conditions, the bacterial concentration adhering to the fibers generally exhibits an initial increase followed by a subsequent decrease over time, with the bacterial OD600 (bacterial cell density) reaching its peak at 38 h. The possible reason for this phenomenon may be that bacteria need nutrients in the process of growth and reproduction, but as the bacteria reproduce, the nutrients in the medium are gradually consumed, resulting in cell death and decreased bacterial concentration. At the same time, with the passage of bacterial breeding time, bacterial metabolic waste will gradually accumulate, which may lead to acidification or enhanced toxicity of culture, thereby reducing bacterial concentration. From Figure 5a, it can be observed that fibers treated with 0 mol/L (distilled water) demonstrated bacterial adhesion capacity, while fibers treated with all other acid concentrations exhibited higher adhesion capacities compared to the distilled water-treated fibers. This is attributed to the etching effect of acid on the fiber surface, which creates irregular grooves and provides attachment sites for bacteria. Additionally, Figure 5a indicates that the maximum bacterial OD600 value on fibers occurs at an acid treatment concentration of 1 mol/L. As shown in Figure 5b, the highest bacterial OD600 value is achieved at an acid treatment duration of 2 h, while Figure 5c reveals that the optimal acid treatment temperature for maximizing bacterial OD600 values is 40 °C.

4.1.2. Analysis of the Maximum Attachment of Fiber to Bacteria

Figure 6 illustrates the relationship between pretreatment parameters and the maximum bacterial adhesion capacity of modified basalt fibers. The bacterial adhesion concentration under three pretreatment conditions (acid concentration, treatment duration, and temperature) exhibits a unimodal trend, initially increasing and then decreasing as the pretreatment conditions are altered. As in Figure 6a–c, From Figure 6a–c, it can be seen that under the influence of different acid treatment concentrations, the maximum attachment concentration OD600 of modified basalt fiber is 0.512, and the corresponding acid treatment concentration is 1 mol/L; under the influence of different acid treatment time, the maximum attachment concentration OD600 of modified basalt fiber is 0.502, and the corresponding acid treatment time is 2 h. Under the influence of different acid treatment temperatures, the maximum adhesion concentration (OD600) of modified basalt fiber is 0.502, corresponding to an acid treatment temperature of 40 °C.

4.2. Effect of Basalt Fiber with Different Pretreatment Conditions on MICP Single Factor Test

The influence of basalt fiber on MICP under varying acid pretreatment conditions is illustrated in Figure 7. As demonstrated in Figure 7a–c, the maximum calcium carbonate precipitation was achieved at an acid concentration of 1 mol/L, a treatment duration of 1 h, and a temperature near room temperature.

4.3. Response Surface Test of the Effect of Basalt Fiber on MICP Under Different Acid Treatment Conditions

4.3.1. Experimental Design and Results of Response Surface Method

Based on the results of single-factor analysis, the response surface methodology was employed to design the experimental framework. Following the Box–Behnken design principle, a three-factor, three-level experimental analysis was conducted, with calcium carbonate precipitation yield serving as the response variable. The specific experimental parameters are detailed in Table 5. The experimental results corresponding to the parameter settings in Table 5 are summarized in Table 6.

4.3.2. Response Surface Regression Model Establishment and Variance Analysis

The experimental data in Table 6 were statistically analyzed using Design-Expert software 11.0, which yielded a quadratic polynomial regression model correlating the independent variables (acid treatment concentration, time, and temperature) with the dependent variable (calcium carbonate production):
Y = 0.069 + 0.1465A + 0.3615B + 0.1097C − 4.8 × 10−16AB − 0.001AC − 0.003BC − 0.072A2 − 0.132B2 − 0.00182C2
where A is the concentration of acid treatment (mol/L), B is the time of acid treatment (h), C is the temperature of acid treatment (°C), and Y is the amount of calcium carbonate (g).
The response surface regression model analysis of variance results is presented in Table 7. As shown in Table 7, the determination coefficient (R2) of the regression model is 0.9659, indicating that the model exhibits good regression properties and can accurately predict the amount of calcium carbonate production. The p-value of the mismatch term in the regression model is greater than 0.05, indicating that the error between the predicted value of the model and the measured value is not significant. This suggests that the regression model is reliable in predicting the amount of calcium carbonate production. The signal-to-noise ratio of the model is 12.904 (greater than 4), indicating that the model has high prediction accuracy and good reliability. Therefore, the model is suitable for predicting and analyzing calcium carbonate production in MICP by fibers under different acid treatment conditions.
It can be seen from Table 7 that the p values for acid treatment concentration, acid treatment time, and acid treatment temperature were 0.004, 0.002, and 0.001, respectively. The p values of the three experimental factors were all less than 0.05, indicating that acid treatment concentration, acid treatment time, and acid treatment temperature had a significant effect on the amount of calcium carbonate produced.

4.3.3. Analysis of Response Surface Method Test Results

Figure 8 shows the interaction of acid treatment concentration, acid treatment time, and acid treatment temperature on the production of calcium carbonate in MICP. Figure 8a,b show the relationship between acid treatment concentration and acid treatment time and calcium carbonate production. It can be seen from Figure 8a that the contour map is oval, indicating an interaction between acid treatment time and acid treatment concentration. It can be seen from Figure 8b that the amount of calcium carbonate production increases initially and then decreases with increasing acid treatment concentration. The appropriate acid treatment concentration can increase calcium carbonate production to a greater extent. This may be because the concentration is too low. The modification of the fiber surface is not apparent, and the concentration is too high to disrupt the fiber surface, thereby weakening the promoting effect on the formation of calcium carbonate. At the same time, it can also be seen from the diagram that the production of calcium carbonate initially increases and then decreases over time. The appropriate acid treatment time will be more conducive to increasing calcium carbonate production.
Figure 8c,d show the relationship between acid treatment concentration, acid treatment temperature, and calcium carbonate production. It can be seen from Figure 8c that the contour lines in the figure are oval, indicating that the interaction between acid treatment concentration and acid treatment temperature is significant, which can jointly affect the amount of calcium carbonate produced. Under the condition of a particular acid treatment concentration, the longer the acid treatment time, the faster the rate of calcium carbonate production reduction. From Figure 8d, it can be seen that the production of calcium carbonate initially increases and then decreases with increasing temperature, reaching a maximum value at the optimal temperature.
Figure 8e,f show the relationship between acid treatment time, acid treatment temperature, and calcium carbonate production. It can be seen from Figure 8e that the contour map is approximately circular, indicating that the interaction between acid treatment time and acid treatment temperature is not significant. From Figure 8f, the production of calcium carbonate increases first and then decreases with the increase in acid treatment time and acid treatment temperature.

4.3.4. The Optimum Treatment Conditions of Basalt Fiber and the Verification of Response Surface Regression Model

Through the analysis and optimization of the test results using Design-Expert software 11.0, the optimum pretreatment conditions for basalt fiber were obtained as follows: the acid treatment concentration was 0.93 mol/L, the acid treatment time was 0.96 h, and the acid treatment temperature was 30 °C. Under these conditions, the yield of calcium carbonate was 1.92 g. According to the theoretical conditions, three parallel experiments were carried out to verify the theoretical conditions, and the average amount of calcium carbonate produced was 1.95 g. It can be seen that the error between the experimental value and theoretical value is relatively small, indicating that the model can accurately predict the amount of calcium carbonate production.

4.4. Effect of Modified Fiber on Mechanical Properties of MBF-MICP Solidified PG

4.4.1. Effect of Modified Basalt Fiber on Unconfined Compressive Strength of Solidified PG

The relationship between fiber content n and the unconfined compressive strength of solidified PG is shown in Figure 9a. From Figure 9a, it can be seen that, under a certain fiber length L, the unconfined compressive strength of solidified PG increases first and then decreases with the increase in fiber content. When the fiber content is 0.4%, the unconfined compressive strength of the solidified PG is the largest. The minimum unconfined compressive strength of fiber combined with MICP-solidified PG is 480.87 MPa, while the unconfined compressive strength of fiber-free solidified PG is 451.49 MPa, which is 6.51% higher than that of fiber-free solidified PG, indicating that the addition of fiber can promote the strength of MICP-solidified PG. It can also be seen from Figure 9 that the effect of excessively high or low fiber content on the strength of solidified PG is reduced. The primary reason is that when the fiber content is lower than the optimal content of 0.4%, the increase in fiber content is conducive to the formation of a more effective reinforced network structure and the improvement of the strength of the sample. When the fiber content exceeds the optimal fiber content, due to the mutual adsorption between the fibers, it is easy to form aggregates, which enhances the inhomogeneity of the sample and affects the reinforcement effect of the fiber on MICP-solidified PG [53,58].
The relationship between fiber length and the unconfined compressive strength of solidified PG is shown in Figure 9b. It can be seen from Figure 9b that when the fiber content is constant, the unconfined compressive strength of solidified PG increases initially and then decreases with the increase in fiber length. When the fiber length is 9 mm, the unconfined compressive strength of the solidified PG is the largest. It can also be seen from Figure 9b that the effect of too long or too short fiber on the strength of solidified PG is reduced. When the fiber length is long, the fiber can form an effective ‘bridging’ structure between the PG particles, enhance the bonding force between the calcium carbonate precipitation product and the matrix particles, and improve the stress transfer efficiency, thereby improving the overall strength. When the fiber length is too long, the fiber is prone to entanglement, folding, or uneven orientation, resulting in uneven distribution within the matrix and weakening its interfacial anchoring ability [59].

4.4.2. Effect of Modified Fiber on Calcium Carbonate Content of Solidified PG

The relationship between fiber content and calcium carbonate content is illustrated in Figure 10. As shown in Figure 10, when the fiber length is constant, the calcium carbonate content is basically positively correlated with the fiber content, indicating that the incorporation of fibers is conducive to the formation of calcium carbonate. The primary reason is that the presence of fibers enhances the conditions for microbial attachment and heterogeneous nucleation of crystals. With the increase in fiber content, the surface area available for bacterial attachment in the solidification system increases significantly, which is conducive to the enrichment and continuous metabolism of microorganisms on the fiber surface, promotes the hydrolysis reaction of urea, and thus enhances the formation rate of calcium carbonate. At the same time, as a heterogeneous nucleation substrate, the fiber can induce calcium carbonate to grow preferentially on its surface, form more precipitation nuclei, and increase the amount of calcium carbonate produced.
The relationship between fiber length and calcium carbonate production is shown in Figure 11. As shown in Figure 11 when the fiber length is 9 mm, the amount of calcium carbonate is the largest. At the same time, it can be seen from Figure 11 that the amount of calcium carbonate production increases initially with the increase in fiber length but then decreases. When the fiber length is long, its surface area is sufficient to provide more microbial attachment sites, allowing for a uniform distribution in the system. The three-dimensional network structure promotes bacterial enrichment and heterogeneous nucleation of calcium carbonate, thereby increasing the amount of calcium carbonate produced. However, when the fiber length is too long, the fiber is prone to winding, bending, and spatial accumulation in the system, resulting in uneven distribution, reducing the effective specific surface area of the sample, and limiting the adhesion and diffusion of microorganisms, and thus inhibiting the deposition reaction of calcium carbonate [56].

4.4.3. Analysis of the Effect of Basalt Fiber Modification on the Formation of Calcium Carbonate in Solidified PG

The relationship between fiber modification and calcium carbonate production is illustrated in Figure 12. As shown in Figure 12, the calcium carbonate production of solidified PG with acid-treated fiber is 17.8%, while the calcium carbonate production of solidified PG with untreated fiber is 15.79%. The calcium carbonate production of solidified PG with acid-treated fiber is 2.01% higher than that with untreated fiber. The calcium carbonate production of solidified PG without fiber is 13.28%, and the calcium carbonate production of solidified PG with acid-treated fiber is 39.83% higher than that without fiber, indicating that the modification of fiber can increase the calcium carbonate production of solidified PG. The primary reason is that the introduction of fibers provides a large number of microbial attachment sites in the matrix, which enhances the colonization ability of bacteria. Additionally, their surfaces act as a heterogeneous nucleation site, promoting the precipitation of calcium carbonate. The surface roughness of the fiber treated with dilute sulfuric acid increased, the hydrophobic impurities were removed, and the nucleation density and crystallization efficiency were significantly improved, resulting in more calcium carbonate. In contrast, the untreated fiber has a specific promotion effect, but its surface is relatively smooth, and the induction effect is limited. However, the MICP-solidified body without fibers lacks an effective heterogeneous nucleation substrate, resulting in a large bacteria dispersion and the lowest mineralization reaction efficiency, which in turn leads to the smallest amount of calcium carbonate.

4.4.4. Analysis of Macroscopic Mechanical Properties of MICP-Solidified PG Modified by Basalt Fiber

The relationship between fiber modification and unconfined compressive strength of MICP-solidified PG is shown in Figure 13. From Figure 13, it can be seen that the unconfined compressive strength of modified fiber bio-cemented PG is the largest, and the unconfined compressive strength of non-doped fiber bio-cemented PG is the smallest. The unconfined compressive strength of modified fiber bio-cemented PG is 71.3% higher than that of non-doped fiber bio-cemented PG and 27.5% higher than that of fiber solidified PG. It demonstrates that modifying basalt fiber is beneficial for enhancing the strength of MICP-solidified PG. The primary reason is that the fiber forms spatial support and ties the skeleton structure within the solidified body, which can effectively inhibit crack propagation and stress concentration, thereby improving the integrity and bearing capacity of the overall structure. Secondly, the surface roughness of the fiber treated with dilute sulfuric acid is significantly improved, which enhances the mechanical interlocking between the fiber and PG particles and calcium carbonate sediments, thereby improving the interfacial bonding strength and forming a more stable composite structure. Moreover, in contrast, the untreated fiber has a relatively smooth surface and a weak binding force with the matrix, and the enhancement effect is limited; however, the MICP-solidified body without fiber lacks an internal support structure, which is prone to shear failure under stress, resulting in the lowest compressive strength.
From Figure 14, it can be seen that when the stress reaches its peak value, the stress of undoped fiber bio-cemented PG decreases rapidly compared with that of fiber bio-cemented PG, exhibiting typical brittle failure characteristics. At the same time, the stress of modified fiber solidified PG decreases slowly compared with that of unmodified PG, indicating that the toughness of modified fiber-solidified PG is better than that of unmodified fiber. The main reason for this phenomenon is that the wettability between the fiber and the matrix after acid treatment is stronger than that of the untreated fiber, so that the bite force between the fiber and the matrix is higher than that of the untreated fiber. Therefore, the residual strength and toughness of the fiber after acid treatment are better than those of the untreated fiber.

4.5. Solidified PG Microstructure Analysis

Figure 15 shows the SEM images of solidified PG under various solidification conditions. From Figure 15a,b, it can be found that the calcium carbonate produced by the simple MCIP-solidified PG is less, and there are many large pores between the PG particles. The degree of compaction is low, which, in turn, reduces the overall strength of the solidified PG. Figure 15c,d show the microstructure of fiber combined with MICP-solidified PG. It is observed that calcium carbonate is formed on the surface of the fiber. There is an obvious PG particle-fiber-calcium carbonate precipitation interface in the microstructure of the whole sample, in which the fiber plays a bridging role. However, the content of calcium carbonate generated on the surface of the fiber is less, which affects the ability of the fiber to cooperate with the PG particles under the external load to a certain extent, and then weakens the toughening effect of the fiber on the solidified PG. Figure 15e,f show the microstructure of the acid-treated fiber combined with MICP-solidified PG. From the diagram, it can be observed that a large number of calcium carbonate precipitates are present on the surface of the fiber, and calcium carbonate adheres and expands outward along the fiber. This may be due to the acid-treated fiber producing a large number of bacterial adsorption sites, which provide favorable conditions for the large generation of calcium carbonate, and then make the synergistic effect of PG particles and the fiber as a whole better. Compared to the untreated fiber, the sample exhibits higher macroscopic strength and lower brittleness.

4.6. Leaching Analysis of Heavy Metals in Solidified PG

According to the relevant test specifications [60], the experimental study on the leaching amount of heavy metals in PG before and after solidification was carried out. The leaching concentrations of heavy metals before and after solidification of PG are shown in Table 8. Zn, Pb, and Ba in untreated PG were 0.188 mg/L, 0.075 mg/L, and 1.583 mg/L, respectively, which exceeded the detection limit. However, after MBF–MICP solidification, the leaching concentration of heavy metals in PG is lower than the detection limit, which meets the national emission standard. In the system of basalt fiber combined with MICP-solidified PG after dilute sulfuric acid treatment, the calcium carbonate precipitation induced by microorganisms can not only cement the PG particles but also seal some heavy metal ions. At the same time, the carbonate ions produced in the MICP process are combined with heavy metal ions to form insoluble heavy metal carbonate precipitates. In addition, the surface of basalt fiber after acid treatment has a large number of polar groups [61] and active sites, which can produce a significant adsorption effect on heavy metal ions, and the presence of fiber can also promote the formation and nucleation of calcium carbonate, further enhancing the encapsulation and sealing effect of heavy metals, so as to achieve efficient solidification and stabilization of heavy metals through multiple synergistic effects.

5. Conclusions

In this paper, the effects of basalt fiber modification on bacterial adhesion ability and micro-induced calcium carbonate precipitation were studied. The effects of modified fiber length and modified fiber content on the mechanical properties of MBF–MICP-solidified PG were analyzed. The leaching changes in heavy metals before and after PG solidification were identified, and the micro-solidification mechanism of modified basalt fiber combined with MICP technology was revealed, which provided a new solution for the large-scale utilization of PG. The main conclusions are as follows:
(1)
The adhesion ability of the acid-treated fiber to the bacteria was stronger than that of the untreated fiber. The results of the microbial-induced calcium carbonate precipitation test by fiber modification showed that the optimal conditions for fiber modification were as follows: The acid treatment concentration was 0.93 mol/L, the acid treatment time was 0.96 h, and the acid treatment temperature was 30 °C.
(2)
The calcium carbonate content of MBF–MICP-solidified PG increases with the increase in modified fiber content. When the length of modified fiber is in the range of 6–9 mm, the calcium carbonate content of MBF–MICP-solidified PG increases with the increase in the length of the modified fiber. When the length of modified fiber exceeds 9 mm, the calcium carbonate content of MBF–MICP-solidified PG decreases with the increase in the length of modified fiber. The unconfined compressive strength of MBF–MICP-solidified PG is the optimal reinforcement condition for MBF–MICP-solidified PG when the length of the modified fiber is 9 mm and the content of the modified fiber is 0.4%.
(3)
The calcium carbonate production and unconfined compressive strength of PG solidified by modified fiber combined with MICP technology are higher than those of BF–MICP-solidified PG and MICP-solidified PG. In terms of sample brittleness, the brittleness of PG solidified by modified fiber combined with MICP is lower than that of unmodified fiber, and the brittleness of MICP-solidified PG is the highest.
(4)
The microstructure study found that there was an obvious PG particle-fiber-calcium carbonate precipitation interface in the sample, and the fiber had a bridging effect. There was a large amount of calcium carbonate on the surface of the fiber after acid treatment, and the calcium carbonate adhered and expanded outward along the fiber. Under the action of carbonate precipitation, the encapsulation of microbial-induced calcium carbonate precipitation and the adsorption of polar groups, the leaching amount of heavy metals in PG solidified by MBF–MICP is lower than the detection limit, which can meet the national emission standards.
This study provides an innovative method for PG waste. This method combines modified basalt fiber and MICP technology to solidify/stabilize PG. In the above studies, it is shown that the modification of basalt fiber can improve the strength and toughness of MICP-solidified PG, and the modification of basalt fiber can increase the yield of calcium carbonate, which has a positive effect on stabilizing heavy metal ions in PG. The above research results also show that the leaching amount of heavy metals solidified by modified basalt fiber and MICP technology meets the national emission standards. The proposal of this technology can provide theoretical guidance for large-scale recycling of PG in the construction field.

Author Contributions

Methodology, G.N., J.Z. and K.L.; Software, G.N. and K.L.; Investigation, G.N. and J.Z.; Data curation, G.N.; Writing—original draft, G.N.; Writing—review & editing, G.N., J.Z. and K.L.; Supervision, J.Z. and K.L.; Funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (No. 42177166).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Raw materials used in the test.
Figure 1. Raw materials used in the test.
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Figure 2. Bacterial culture process.
Figure 2. Bacterial culture process.
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Figure 3. Triaxial compression test instrument.
Figure 3. Triaxial compression test instrument.
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Figure 4. Grouting device diagram.
Figure 4. Grouting device diagram.
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Figure 5. The time travel curve of the attachment of basalt fiber to bacteria after different acid treatments. (a) the maximum bacterial OD600 value on fibers occurs at an acid treatment concentration of 1 mol/L; (b) the highest bacterial OD600 value is achieved at an acid treatment duration of 2 h; (c) the optimal acid treatment temperature for maximizing bacterial OD600 values is 40 °C.
Figure 5. The time travel curve of the attachment of basalt fiber to bacteria after different acid treatments. (a) the maximum bacterial OD600 value on fibers occurs at an acid treatment concentration of 1 mol/L; (b) the highest bacterial OD600 value is achieved at an acid treatment duration of 2 h; (c) the optimal acid treatment temperature for maximizing bacterial OD600 values is 40 °C.
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Figure 6. The relationship between different acid treatment conditions of basalt fiber and the maximum adhesion of bacteria. (a) acid concentration; (b) treatment duration; (c) temperature.
Figure 6. The relationship between different acid treatment conditions of basalt fiber and the maximum adhesion of bacteria. (a) acid concentration; (b) treatment duration; (c) temperature.
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Figure 7. Effect of basalt fiber and MICP under different acid treatment conditions. (a) acid concentration; (b) treatment duration; (c) temperature.
Figure 7. Effect of basalt fiber and MICP under different acid treatment conditions. (a) acid concentration; (b) treatment duration; (c) temperature.
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Figure 8. The relationship between calcium carbonate production and pretreatment conditions. (a,b), the relationship between acid treatment concentration and acid treatment time and calcium carbonate production; (c,d), the relationship between acid treatment concentration, acid treat-ment temperature, and calcium carbonate production. (e,f) the relationship between acid treatment time, acid treatment temper-ature, and calcium carbonate production.
Figure 8. The relationship between calcium carbonate production and pretreatment conditions. (a,b), the relationship between acid treatment concentration and acid treatment time and calcium carbonate production; (c,d), the relationship between acid treatment concentration, acid treat-ment temperature, and calcium carbonate production. (e,f) the relationship between acid treatment time, acid treatment temper-ature, and calcium carbonate production.
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Figure 9. Relationship between content and length of fiber and unconfined compressive strength. (a) The relationship between fiber content n and the unconfined compressive strength of solidified PG (b) The relationship between fiber length and the unconfined compressive strength of solidified PG.
Figure 9. Relationship between content and length of fiber and unconfined compressive strength. (a) The relationship between fiber content n and the unconfined compressive strength of solidified PG (b) The relationship between fiber length and the unconfined compressive strength of solidified PG.
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Figure 10. Relationship between fiber content and calcium carbonate production.
Figure 10. Relationship between fiber content and calcium carbonate production.
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Figure 11. Relationship between fiber length and calcium carbonate content.
Figure 11. Relationship between fiber length and calcium carbonate content.
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Figure 12. Relationship between fiber modification and calcium carbonate production.
Figure 12. Relationship between fiber modification and calcium carbonate production.
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Figure 13. Relationship between fiber modification and unconfined compressive strength of MICP-solidified PG.
Figure 13. Relationship between fiber modification and unconfined compressive strength of MICP-solidified PG.
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Figure 14. Stress–strain curve of solidified PG before and after fiber modification.
Figure 14. Stress–strain curve of solidified PG before and after fiber modification.
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Figure 15. SEM diagram of solidified PG. (a,b) are MICP -olidified PG; (c,d) are untreated fiber combined with MICP to solidify PG; (e,f) are acid-treated fibers combined with MICP to solidify PG.
Figure 15. SEM diagram of solidified PG. (a,b) are MICP -olidified PG; (c,d) are untreated fiber combined with MICP to solidify PG; (e,f) are acid-treated fibers combined with MICP to solidify PG.
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Table 1. Chemical composition of PG.
Table 1. Chemical composition of PG.
ComponentCaOSO3SiO2AI2O3P2O5Fe2O3Na2OK2OMgOOther
33.2346.8210.070.62.350.33--0.016.59
Table 2. Physical properties of PG.
Table 2. Physical properties of PG.
Liquid Limit (%)Plastic Limit (%)Plasticity Index (%)Maximum Dry Density (g/cm3)Optimum Moisture Content (%)Particle Size Distribution
27.8218.729.11.41522.1d10d30d60
8.1817.9348.64
Table 3. Physical properties of basalt fiber.
Table 3. Physical properties of basalt fiber.
Diameter (μm)Tensile Strength (MPa)Elastic Modulus (GPa)Density (g/cm3)
1239001002.65
Table 4. Mechanical test scheme of indoor unconfined compressive strength of solidified PG.
Table 4. Mechanical test scheme of indoor unconfined compressive strength of solidified PG.
Fiber Length (mm)Fiber Content (%)
00
60.1, 0.4, 0.7, 1.0
90.1, 0.4, 0.7, 1.0
120.1, 0.4, 0.7, 1.0
Table 5. Test factor level setting.
Table 5. Test factor level setting.
FactorCodingLevel
−101
Concentration (mol/L)A0.511.5
Time (h)B0.511.5
Temperature (°C)C253035
Table 6. Response surface design and results.
Table 6. Response surface design and results.
Test NumberAcid Concentration (mol/L)Acid Treatment Time (h)Acid Treatment Temperature (°C)Amount of Carbonate Precipitation (g)
10.50.5301.86
21.50.5301.84
30.51.5301.84
41.51.5301.84
50.51251.85
61.51251.83
70.51351.85
81.51351.82
910.5251.83
1011.5251.82
1110.5351.84
1211.5351.82
1311301.92
1411301.92
1511301.93
1611301.92
1711301.93
Table 7. Analysis of variance of response surface regression model.
Table 7. Analysis of variance of response surface regression model.
SourceSum of SquaresVarianceMean SquareFpSignificance
Model0.028690.003275.43<0.0001significant
A0.000610.000614.530.0066
B0.000310.00037.420.0296
C01001
AB0.000110.00012.370.1674
AC0.00002510.0000250.590.4664
BC0.00002510.0000250.590.4664
A20.00510.005118.92<0.0001
B20.008310.0083197.85<0.0001
C20.011410.0114270.16<0.0001
Residual0.000370.00004
Lack of fit0.000230.00011.940.2643not significant
Pure Error0.000140
Cor Total0.028916
Table 8. Results of heavy metal leaching concentration of PG.
Table 8. Results of heavy metal leaching concentration of PG.
ElementUntreated PG (mg/L)MBF-MICP Solidified PG (mg/L)Standard Value (mg/L)
Cd<0.003<0.0030.003
As<0.01<0.010.01
Co<0.005<0.0050.005
Zn0.188<0.0060.006
Se<0.01<0.010.01
Pb0.075<0.050.05
Ni<0.01<0.010.01
Ba1.583<0.0040.004
Hg<0.01<0.010.01
Cu<0.01<0.010.01
Ag<0.1<0.10.1
Cr<0.01<0.010.01
Mn<0.001<0.0010.001
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Nan, G.; Zhang, J.; Liu, K. Mechanical Behavior and Pollutant Stabilization of Modified Basalt Fiber-Reinforced Bio-Cemented Phosphogypsum. Buildings 2026, 16, 455. https://doi.org/10.3390/buildings16020455

AMA Style

Nan G, Zhang J, Liu K. Mechanical Behavior and Pollutant Stabilization of Modified Basalt Fiber-Reinforced Bio-Cemented Phosphogypsum. Buildings. 2026; 16(2):455. https://doi.org/10.3390/buildings16020455

Chicago/Turabian Style

Nan, Gan, Jiaming Zhang, and Kai Liu. 2026. "Mechanical Behavior and Pollutant Stabilization of Modified Basalt Fiber-Reinforced Bio-Cemented Phosphogypsum" Buildings 16, no. 2: 455. https://doi.org/10.3390/buildings16020455

APA Style

Nan, G., Zhang, J., & Liu, K. (2026). Mechanical Behavior and Pollutant Stabilization of Modified Basalt Fiber-Reinforced Bio-Cemented Phosphogypsum. Buildings, 16(2), 455. https://doi.org/10.3390/buildings16020455

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