1 Introduction
Phosphogypsum, a byproduct generated in large quantities during phosphoric acid production in the phosphate chemical industry, contains impurities such as fluorides, phosphorus pentoxide, and other compounds, imparting a strong acidity (with a minimum pH value as low as 1.9) [1]. This characteristic poses significant environmental pollution challenges. Long-term stockpiling of phosphogypsum can produce leachate containing fluoride ions and soluble phosphorus pentoxide, which can spread via rainwater or groundwater, contaminating surrounding soil and water bodies [2]. In regions with high rainfall, leachate can lead to groundwater pollution and ecosystem degradation. The land-intensive nature of phosphogypsum stockpiling further exacerbates the issue, as large storage sites remain idle, leading to inefficient land use [3]. Moreover, as phosphogypsum contains reusable calcium sulfate, direct stockpiling results in resource wastage. Dry phosphogypsum, particularly when exposed to wind, can generate dust pollution, with airborne particles potentially carrying toxic substances that endanger the health of nearby populations [4]. The fundamental solution to phosphogypsum-related pollution lies in its resource utilization. Current approaches to its reuse span three major sectors: construction materials, chemical processes, and agriculture. In the construction sector, phosphogypsum is primarily used as a cementitious material to replace cement or as a filler for road base layers. In the chemical sector, it is employed for extracting and recovering valuable metals [5] . In the agricultural sector, phosphogypsum is utilized to produce soil stabilizers or conditioners, which improve soil structure while supplying calcium and sulfur nutrients essential for plant growth.
Currently, the development of high-value utilization technology for phosphogypsum is focused on new building materials, such as paper-faced gypsum board, high-strength gypsum powder, gypsum-based dry-mixed mortar, and other innovative building materials [6]. Deng [7] investigated the hydration and hardening characteristics of phosphogypsum. The study observed that as the phosphogypsum content increased, there was a decline in reactivity, non-evaporable water content, and silicate levels. It was further noted that adding phosphogypsum in amounts not exceeding 30% enhanced mechanical properties, reduced porosity, refined pore size distribution, and densified the microstructure. Ma [8] explored the influence of slag and phosphogypsum content on the mechanical performance and microstructure of composite cementitious materials. The findings revealed that composite materials containing 10–20% phosphogypsum exhibited superior strength compared to those without phosphogypsum. This improvement was attributed to the sulfates in phosphogypsum, which promoted the formation of significant amounts of ettringite, accelerated the dissolution of red mud and slag, and increased the release of aluminates, silicates, and Ca2+ ions, resulting in the generation of more C-(A)-S-H gel and ettringite. Vallecillo [9] blended phosphogypsum sourced from Florida, USA, with limestone (LR) and recycled concrete aggregates (RCA) and evaluated the load-bearing capacity of the resulting matrix. The results indicated that the success of PG-modified particle matrices largely depended on the strength of the original aggregates. Rosales [10] analyzed the physicochemical properties, mechanical behavior, and environmental impact of phosphogypsum-based cement mortars prepared using phosphogypsum samples extracted from Huelva, Spain. The study highlighted that the mineralogical transformations of the mortar, influenced by phosphogypsum treatment and curing conditions, significantly impacted its mechanical performance. Raza [11] investigated the combined use of calcined red mud, dihydrate gypsum, and ground granulated blast furnace slag (GGBFS) in geopolymer concrete, along with their optimized proportions. The results demonstrated that partially substituting calcined red mud with dihydrate gypsum and GGBFS in geopolymer concrete effectively enhanced its mechanical properties, durability, and microstructural characteristics. This substitution was deemed a feasible alternative for improving the performance of geopolymer concrete.
Concurrently, China’s transportation infrastructure has experienced rapid development in recent years, with a notable increase in road construction projects [12]. The demand for curbstones, a critical component of road infrastructure, has consequently risen [13]. In light of the continuous growth in phosphogypsum production and the increasing environmental awareness among the population [14], it is pertinent to explore methods for maintaining the structural integrity of curbstones while incorporating industrial waste to partially replace cement or aggregates in cement concrete. This approach aims to reduce engineering and construction costs [15]. Curbstones must exhibit excellent compressive strength and durability to withstand the effects of prolonged vehicle loads and environmental changes associated with road use. Additionally, they must resist freeze-thaw cycles to ensure stability, particularly in the harsh climatic conditions of northeastern China. Given the extensive application and high demand for curbstones, the production materials must also be cost-effective.
Phosphogypsum, after modification, can effectively meet these requirements. When treated appropriately, it can partially replace cement, thereby satisfying the mechanical performance demands of curbstones. Furthermore, phosphogypsum is inexpensive and readily available, making it a cost-efficient and stable resource. Its application in curbstone production not only reduces manufacturing costs but also minimizes cement consumption, contributing to the conservation of natural resources. Current methods to enhance the activity of cementitious materials include ball milling modification, high-temperature treatment, and the addition of additives, with the latter being a more commonly used approach. Studies have shown that triethanolamine is typically employed as a grinding aid and surfactant in cement. Jiang et al. [16] found that TEA has a beneficial effect on the early strength of cement, and adding materials with high pozzolanic activity on this basis can increase the later strength. Yan et al. [17], in their study on the effects of TEA on cement, discovered that TEA can introduce the crystallization process of the CH phase, significantly altering the morphology of CH crystals. Xiang et al. [18] used various activators and found that TEA outperformed other activators in terms of effectiveness, with lower cost and wider application [19]. Building on previous studies, this paper explores the mechanism of TEA’s effects on phosphogypsum-based cementitious materials.
Therefore, to further address the current challenges in directly utilizing phosphogypsum and to overcome its limitations in curb stone production, this thesis systematically explores the application of phosphogypsum in curb stone manufacturing. To compensate for the performance deficiencies of phosphogypsum in this application, different amounts of triethanolamine (TEA) were added to modify the phosphogypsum, and the effects of TEA on the macroscopic properties of phosphogypsum were studied. In addition, microscopic analysis methods were employed to examine the hydration process of phosphogypsum, the mechanism of TEA’s influence, and the impact on the properties of the cement-sand mixture, leading to the construction of a TEA-influenced hydration reaction model for phosphogypsum. To validate the practical applicability of this model, various curb stone mix designs were developed and subjected to performance testing. The study systematically analyzed the effects of phosphogypsum replacement rates and TEA content on the macroscopic properties of the curb stones, resulting in the development of a modified phosphogypsum-based curb stone system with excellent mechanical properties, freeze resistance, and environmental benefits. This system significantly reduces carbon emissions and provides fundamental theoretical support for the production of low-carbon, eco-friendly curb stones using phosphogypsum.
2 Materials and Methods
2.1. Raw Materials
Aggregate (Naturally extracted to provide structural support in construction projects), cement (P.O 42.5), phosphogypsum (CaSO4·2H2O>90%), TEA (Triethanolamine promotes hydration and is used to enhance the early strength of curbs. The molecular weight is 149, and the relative density is 1.12.), standard sand (Filling role in curb preparation), polycarboxylic acid water reducing agent (water reduction rate is 30%.), water.
2.2. Basic Physical and Chemical Properties of Phosphogypsum
The phosphogypsum used in this experiment is produced in a factory in Chongqing. A planetary ball mill was used to grind 96% phosphogypsum with 4% cement, sampled after 5 min of grinding, and analyzed for chemical composition using XRF and the test results are shown in Table 1.
The chemical composition of phosphogypsum primarily includes SO3, CaO, Al2O3, and SiO2, with SO3 content exceeding 50% and CaO content exceeding 30%. Additionally, small amounts of BaO, P2O5, and Al2O3 are detected. The coagulation performance and pozzolanic activity are influenced by CaO, while SiO2 and Al2O3 primarily affect the material’s activity. However, the contents of SiO2 and Al2O3 are relatively low, at 5.57% and 1.27%, respectively. Therefore, it is necessary to modify phosphogypsum to enhance its performance. Phosphogypsum contains a certain amount of F and P2O5, both of which not only pose risks to environmental and biological safety but also adversely affect the performance of materials. Fluoride inhibits the normal growth of calcium sulfate crystals, leading to prolonged setting times or reduced cementitious properties. P2O5, being an acidic substance, disrupts the normal hydration process during its reaction with cement, thereby reducing early-stage strength. To mitigate the effects of these harmful substances, alkali activators such as TEA can be employed to enhance the reactivity of phosphogypsum, thereby improving the performance of cementitious materials.
2.3. Mixing Ratio Design of Mastic Sand
According to “Test Method of Cementitious Sand Strength (ISO Method)” (GB/T 17671-2021) [20], phosphogypsum cementitious sand specimens measuring 40 mm × 40 mm × 160 mm were prepared [21]. The compressive and flexural strengths were utilized to evaluate the performance of the cementitious sand specimens, with higher values indicating better performance. The mass of standard sand used was 1350 g. The amount of TEA are 0.1–0.4%.
Three phosphogypsum mortar specimens of 40mm × 40mm × 160mm were prepared for compression and bending tests. The best TEA was 0.3%.
To ascertain the water demand for achieving standard consistency in phosphogypsum modified by TEA, wherein fixed cement constitutes 4% of the total cementitious material and phosphogypsum constitutes 96%, experimental determination was conducted using varying TEA concentrations (0.1%, 0.2%, 0.3%, and 0.4% of the mass of the cementitious material). The experimental approach adhered to the guidelines outlined in “Standard Consistency of Cement: Water Consumption, Setting Time, Stability Test Method” (GB/T 1346-2011) [22].
2.4. Determination of Water Consumption for Standard Consistency
The samples were prepared for microscopic analysis of the cementitious material by quantifying the water demand of phosphogypsum under varying TEA concentrations. The experimental findings are presented in Table 2.
According to Table 2, as the TEA content increased from 0.1% to 0.2%, there was a gradual increase in the water requirement for preparing phosphogypsum. Subsequently, increasing the TEA content from 0.2% to 0.3% resulted in a reduction in the water required to achieve standard consistency for phosphogypsum [23]. However, further increasing the triolamine dosage from 0.3% to 0.4% led to an increase in the required dosage for achieving standard consistency.
2.5. Mixture Ratio Design of Curb Stone
According to 3.2.1 in the thesis, design the curbstone mix ratio with different phosphogypsum dosage. The content of phosphogypsum is 20%, 30%, 40%, 50%.
Fig. 1 shows the process for the preparation of phosphogypsum-based curbs.
2.6. Experimental Methods
Standard consistency water consumption experiment. According to “Cement standard consistency water consumption, setting time, stability test method” (GB/T 1346-2011) [22], set the TEA dosage of 0.1%, 0.2%, 0.3%, 0.4%, and carry out the standard consistency water consumption experiment.
Strength of mortar. According to the “Cementitious Sand Strength Test Method” (GB/T 17671-2021) [20], prepare cementitious sand samples, and carry out the compressive and flexural strength test of cementitious sand after 14d and 28d of maintenance.
Material characterization experiments. Set the dosage of TEA as 0.1%, 0.2%, 0.3%, 0.4%, prepare the net slurry samples, and place the samples in a conservation box at 25°C, 99% humidity for 3d; take out the above samples through the first stage of jaw crusher crushing, and then through the ball mill grinding for 5min and through the square-hole sieve of 0.075mm to get the samples with particle sizes less than 0.075mm, drying. Initially crushed samples were used for SEM testing and samples less than 0.075mm were used for XRD & FTIR testing. SEM was performed on a Zeiss Sigma 300 by sticking the sample to a sample stage with conductive adhesive. FTIR was performed on a Nexus 670 by taking a sample visible to the naked eye and adding a suitable amount of dry potassium bromide powder to a mortar and pestle, grinding the sample thoroughly several times, and then placing it in a tablet press to press a tablet. XRD was performed on a D8 by entering the system after the XRD had stabilised for about two minutes. After the XRD has been stabilised for about two minutes, enter the system and place the sample under test on the test stand.
Frost resistance test. In accordance with the test procedure described in “Concrete Quality Control Standard” (GB50164-2011) [24], the specimens with the size of 100mm×100mm×100mm, and 15 specimens for each group of mixing ratios according to the mixing ratios shown in Table 4.
Compressive strength test. By the “Test method stipulated in the code for the design of concrete structures” (GB 50010-2010) [25], the compressive strength test of cube was carried out with specimens of 100mm × 100mm × 100mm in size, according to Tables 4, the 14d or 28d compressive strength tests were carried out on three specimens prepared from each mixture ratio of the curb stones as shown in Fig. 2.
3 Results and Discussion
3.1. Modification of Phosphogypsum Cementitious Material by TEA
3.1.1. Study on mechanical properties of cementitious material
According to Table 5, three 40mm × 40 mm × 160mm phosphogypsum mortar specimens were prepared for compression and flexural tests. And the experiment was repeated three times. The final data were obtained in strict accordance with the provisions of “Cement Mortar Strength Test Method (ISO Method)” (GBT 17671-2021). The experimental results are shown in Table 3, Fig. 2.
According to Fig. 2(a) and Fig. 2(b), when the TEA dosage is 0.2%, the 14d strength of phosphogypsum mortar is at its lowest, whereas at a dosage of 0.3%, the 14d strength reaches its maximum. To verify whether the 0.3% TEA dosage yields the optimal performance, 28d strength tests were conducted. As shown in Fig. 2(c) and Fig. 2(d), the experimental results align with expectations, indicating that the phosphogypsum mortar achieves its highest strength when the TEA dosage is 0.3%. The 28d compressive strength is 6.7 MPa, and the 28-day flexural strength is 1.8 MPa. Standard deviation calculations for the experimental results reveal that, across the four strength tests, the standard deviation remains consistently below 0.16. The highest observed coefficient of variation is 7%, which is well within the 10% limit stipulated by the “Cement Mortar Strength Test Method (ISO Method)” (GBT 17671-2021). This indicates that the data are reliable and meet the required experimental precision standards.
The figure demonstrates that an excessive or insufficient dosage of TEA adversely affects the strength of the cement mortar. When the TEA dosage is too low, its effect is minimal, failing to provide the cementitious material with adequate fluidity and thereby inhibiting the hydration process. Conversely, an excessive TEA dosage significantly increases the OH− concentration, inducing the precipitation of Ca2+ and Al3+ ions. This leads to the formation of substantial amounts of Aft in the hydration products. Ettringite is prone to expansion, which negatively impacts the long-term strength of the cement mortar. Consequently, the 28-day strength differences among the mortar specimens are more pronounced compared to the 14-day strength variations. This highlights the importance of optimizing the TEA dosage to balance early and late-stage hydration and ensure consistent strength development.
3.1.2. Micro-analysis of phosphogypsum cementitious material
To examine the impact of TEA on the microscopic components of phosphogypsum, the experimental determination of TEA to the mass of the gelling material of 0.1%, 0.2%, 0.3%, 0.4% for the dosage of the net slurry were produced by the characterization method described in 2.6 for the test, the water consumption is the water used for the standard consistency of the dosage of the water.
(1) XRF
The net slurry specimens of phosphogypsum were prepared according to the ratio of 96% phosphogypsum, 4% cement, and TEA, which were crushed after 3 d of maintenance and ground using ball mill, and the phosphogypsum fractions.
According to Fig. 3, the principal components of phosphogypsum after 3 days of hydration include SO3, comprising 47%–49.5% of the total composition, and CaO, comprising 40.7%–41.7%. Additionally, there are minor amounts of Al2O3, constituting 1.1%–1.4%, and SiO2, constituting 4.9%–5.8% of the total fraction. Comparison with unhydrated phosphogypsum reveals that in phosphogypsum modified and hydrated with TEA, the content of CaO increased while the content of sulfur trioxide (SO3) decreased. This suggests that TEA modification enhanced the hydration process of phosphogypsum, thereby improving its reactivity.
The combined content of CaO + Al2O3 + SiO2 was 48.8% at 0.1% TEA and 46.7% at 0.2% TEA. Subsequently, the content of these three compounds stabilized. When TEA was introduced to phosphogypsum, it initially increased the activity of phosphogypsum, but this effect diminished as TEA content increased further. This indicates that TEA initiates modification of phosphogypsum, and its effectiveness increases with higher TEA concentrations. The incorporation of TEA facilitates the dissolution of Ca2+ ions, which actively participate in the hydration reactions. This process promotes the formation of C-S-H and ettringite, two key hydration products that contribute to the material’s strength development. As a result, the consumption of dissolved calcium ions during these reactions effectively reduces the f-CaO content in the system. This reduction in CaO not only enhances the hydration process but also minimizes the potential for undesirable reactions that could compromise the performance of the cementitious material [26].
(2) XRD
Fig. 4(a) displays the X-ray diffraction (XRD) patterns of phosphogypsum treated with various concentrations of TEA. The primary components identified in the four samples are C-S-H and CASO4. Compared to the control group without TEA, the addition of TEA did not introduce new diffraction peaks in phosphogypsum but altered the peak intensities of existing products. Specifically, the peaks corresponding to CSH increased with TEA addition, indicating a gradual rise in their concentrations. Therefore, the strength of mortar specimens is also increasing. In contrast, the peaks of CaSO4 became less distinct, suggesting a decrease in their concentrations. This observation suggests that TEA disrupts the original composition of phosphogypsum, facilitating the dissociation of Ca2+ ions from phosphogypsum and promoting its hydration process [27].
(3) FTIR
Fig. 4(b) presents the FTIR spectra of phosphogypsum at various TEA doping levels. The bending vibrational peak at 456 cm−1 corresponds to the stretching of Si-O bonds, while the peak at 596 cm−1 indicates the presence of Al-O bonds in the samples, suggesting some volcanic ash activity. As the TEA content increases, the intensities of the Si-O and Al-O waveforms gradually strengthen. When combined with XRF analysis of the samples, this indicates that TEA addition increases the proportions of SO3, CaO, Al2O3, and SiO2, thereby enhancing substance purity. Furthermore, the bending vibration peak at 3399 cm-1 signifies O-H bond stretching, revealing that TEA modifies phosphogypsum. This modification increases the Ca2+ content in the system, elevates pH levels, boosts OH− content, and ultimately promotes the hydration reaction within the phosphogypsum system [28]. The peak of Al-O observed in the figure is highest when the TEA content reaches 4%, indicating that an excessive amount of TEA promotes the hydration reaction between phosphogypsum and cement, leading to the generation of AFt. The overproduction of AFt contributes to internal expansion within the matrix, which adversely affects the long-term strength of the mortar specimens. This phenomenon highlights the need for optimizing TEA dosage to balance hydration enhancement and minimize the negative impact on late-stage mechanical properties.
(4) SEM
Fig. S1 show the SEM images of phosphogypsum with different TEA doping. As can be seen from the figure, when TEA doping is 0.1%, the surface of phosphogypsum has a small amount of rod-needle AFt, and C-S-H and other hydrides surrounded by the surface of the particles are relatively smooth, and it has more compact structure. When the dosage of TEA is increased to 0.2%, a large number of rods and needles of AFt and flocculated C-S-H and other hydrides appear, and the surface is rougher than in Fig. S1(a), and the TEA may start to locally corrode the surface of phosphogypsum, and the surface of the particles has obvious pits; when the dosage of TEA is increased to 0.3%, the rods and needles of AFt and flocculated C-S-H and other hydrides on the surface are more rough than in Fig. 8, and the surface of the particles is more smooth and compact. Compared with Fig.S1, the pits on the surface of the particles were less, probably because the surface of the phosphogypsum was corroded more seriously by TEA, and the surface of the particles generated a reticulated C-S-H and Ca(OH)2 interspersed with each other to make the structure denser. When the dosage of TEA was increased to 0.4%, a large number of pits and there were surface cracks in the particles due to the deepening of the corrosion [29]. The presence of pits and cracks results in an unstable internal structure of the cementitious material, increasing the irregularity of particle surfaces and disrupting the continuity and integrity of the particles. This, in turn, adversely affects the formation and distribution of hydration products. The propagation of cracks leads to the formation of internal microcracks, which makes the material more susceptible to structural degradation under changes in external environmental conditions. Consequently, the mechanical properties and freeze-thaw resistance of the material are compromised.
3.1.3. Hydration mechanism of modified phosphogypsum-based ementitious materials
Through the microscopic analysis of phosphogypsum composite cementitious material systems with various ratios, this part will be a mechanistic analysis in terms of hydration product structure and hydration effect.
From a structural perspective of hydration products, the initial stages of phosphogypsum hydration involve ionization to produce Ca2+ [30] shown in Eq. (1). Simultaneously, cement in the system also ionizes to generate [SiO4]4− ions. Some of these [SiO4]4− ions combine with the Ca2+ ions from phosphogypsum ionization, while others, under alkaline aqueous conditions, break covalent bonds such as Si-O-Si and Ca-O-Ca [31]. The covalent bond is broken in the alkaline water environment, at this time, TEA intervenes in the reaction, as shown in the Fig. S2, TEA continuously corrodes the surface of phosphogypsum, and complexes with the Ca2+ ions in the system, occurring in the reaction as shown in Eq. (2), the formation of covalent bonding, complexes are soluble in water, the formation of soluble points on the surface of the gelling material system, so that the Ca(OH)2 in the liquid phase mediates the increase in the degree of supersaturation of the stabilization, and the reaction as shown in Eq. (3) occurs [32]. To obtain a large amount of OH−. At the same time, there are cement hydration broken Al-O, Si-O, Si-O-Si, and Al-O-Al in the hydration reaction, which combine with each other and make the charge redistribution to form new ions; in the middle and late stages of hydration, OH− is involved in the repolymerisation reaction and reacts with Ca-O-Ca, Al-O-Al, Si-O-Si to form the Ca-O-Al-O-Si- structure and thus C-S-H mesh gel. Towards the end of hydration, the C-S-H gel continues to polymerize to form C-(A)-S-H, i.e. as Table -Si-O-Ca-O-O-Al-O-hexahedral mesh structure [33].
Studied from the perspective of hydration synergistic effect, when the reaction starts to proceed, the hydration of raw materials produces ions such as [SiO4]4−, Ca2+, [CaSiO4]2−, [CaSiO4]2−, OH−, etc., and each chemical bond breaks and reaggregates to generate a new reactant, which is glued on the surface of the solid particles, and gradually forms an irregular surface layer and pore structure. In addition, when TEA is dissolved in the system, it will continuously corrode the surface of phosphogypsum particles, making the surface of phosphogypsum rough and uneven, thus destroying the stability of phosphogypsum particles and increasing the activity of phosphogypsum. And TEA complexes with Ca2+ ions in the system, so that the Ca(OH)2 in the liquid phase mediates the increase in the degree of supersaturation of stability, thus obtaining a large number of OH−. During the second hydration phase, the products within the hydrated slurry undergo complete reaction with free ions. This process results in the formation of C-S-H gels and irregular crystals, which amalgamate to create a layered and reticulated structure. These structures bond together to form a new silicate skeleton. Some of the initial reactant crystals and unreacted particles are embedded within this skeleton and within the pores, contributing to the overall structure of the hardened slurry and providing it with a certain level of strength. This process of secondary hydration further solidifies the material, enhancing its durability and structural integrity [31]. With the continuous hydration of the slurry in the insufficiently reacted Ca2+, OH− and other ions and part of the C-S-H gel repolymerisation reaction occurs to form C-A-S-H gel, and with the C-S-H gel, new mineral crystals and other hydrates again polymerization, and gradual curing to form a TEA modified phosphogypsum-based composite cementitious material system.
3.2. Research on Performance of Phosphogypsum-based Curbstone
In this section, the effect of the amount of phosphogypsum on the basic performance of Curb is studied. Including mechanical properties and frost resistance. The experiment was repeated three times. The final data were obtained in strict accordance with the provisions of “Concrete Curbstone” (JC/T 899-2016).
3.2.1. Determine the mixture ratio of phosphogypsum roadbed curb stone
(1) Determine the water-binder ratio
Different water-binder ratios were set for preliminary experiments, and the experimental ratios and results are shown in Table 4.
It is observed that without the addition of phosphogypsum, the compressive strength of the curbstone initially decreases as the water-binder ratio increases. However, at a water-binder ratio of 0.46, the compressive strength begins to increase, reaching its peak of 30.8 MPa at a ratio of 0.49. Beyond this point, as the water-binder ratio continues to increase, the compressive strength starts to decline again. Based on these findings, further experiments were conducted to refine the water-binder ratio around 0.49 while incorporating phosphogypsum into the mix. This aimed to determine the optimal water-binder ratio that balances the hydration characteristics and mechanical properties of the curbstone with the addition of phosphogypsum.
Each group were produced three 100mm×100mm×100mm cubic soil mix specimens for compressive strength experiments, and the experimental ratios and results are shown in Table 4.
When phosphogypsum and TEA are added to the curb mix, the compressive strength of the curb shows a gradual increase with the water-binder ratio. At a ratio of 0.49, the compressive strength begins to decrease, reaching its lowest point of 30.1 MPa at a ratio of 0.5. Subsequently, the compressive strength starts to rise again, reaching its peak of 41.5 MPa at a ratio of 0.52. By combining the results from both water-binder ratio experiments (Fig. 13 and Fig. 14), it is evident that phosphogypsum and TEA play a significant role in enhancing the compressive strength of the curb. Considering these findings, the optimal water-binder ratio is determined to be 0.52 for achieving the highest compressive strength when phosphogypsum and TEA are included in the mix.
(2) Determine the sand rate
Each group were produced three 100mm×100mm×100mm cubic soil mix specimens for compressive strength experiments, the ratios and results of the experiments are shown in Table S1.
The 3-day compressive strength of the specimens shows a notable increase when the sand ratio is increased from 35.6% to 36.6%. The compressive strength peaks at 29.1 MPa when the sand ratio is 36.6%. Beyond this point, as the sand ratio increases further, the compressive strength begins to decline gradually. Overall, the compressive strength of the specimens exhibits an initial increase followed by a decrease with varying sand ratios. It is evident from the data that the specimens prepared under the condition of a 36.6% sand ratio exhibit the best performance, achieving a compressive strength of 29.1 MPa. This indicates that the sand ratio of 36.6% optimizes the mechanical properties of the specimens at the 3-day testing period.
(3) Determine the mixing ratio
According to the results obtained from the previous experiments, the water-binder ratio was fixed at 0.52, the sand rate was 36.6%, and different phosphogypsum substitution rates for cement were adjusted so as to select the optimum phosphogypsum substitution rate, and the mix ratio design is shown in Table 4.
Phosphogypsum, cement, fine aggregates and coarse and additives, etc. According to Table 4 weighing materials, the materials stirred and mixed well. Pour it into the mould smeared with lubricating oil, and then place it on the vibrating Table for full vibration, and then demould the specimen after vibration and drying in the room. Finally, following “Standard Test Methods for Physical and Mechanical Properties of Concrete”(GB/T50081-2019), the specimens were set in a curing environment with a temperature of 25±2°C and a relative humidity of 100%, and waited for them to be cured in batches until 14d and 28d.
3.2.2. Effect of phosphogypsum dosing on mechanical properties of phosphogypsum-based curbstones
The 100mm × 100mm × 100mm cubic non-standard specimens made by Table 4, three parallel samples were prepared for each group of fit ratio. Due to the slow hydration rate of phosphogypsum, 14d and 28d strength experiments were conducted. After 14d and 28d of maintenance under standard conditions, the compressive strength test was carried out in accordance wibye “Mixed Soil Curbstone” (JC/T899-2016) using the microcomputer-controlled oil-electric hybrid servo pressure tester for the test, and the experimental results are shown in Table S2.
As the phosphogypsum content increases, both the 14-day and 28-day compressive strength of the curb stones initially increase and then decrease. Specifically, as the phosphogypsum content in the curb stones gradually increases, the compressive strength rises slowly, reaching its peak when the phosphogypsum content is at 30%, with a 14-day compressive strength of 25.6 MPa and a 28-day compressive strength of 31.4 MPa. This indicates that an appropriate amount of phosphogypsum can enhance the strength of curb stones. However, the compressive strength begins to decline as the phosphogypsum content increases further. When the phosphogypsum content reaches 50%, the compressive strength is at its lowest, with a 14-day compressive strength of 13.6 MPa and a 28-day compressive strength of 18.1 MPa. This demonstrates that excessive addition of phosphogypsum can severely impact the mechanical properties of curb stones. The results of the compressive strength of the combined curbstone, when the phosphogypsum substitution rate is 30%, the strength is 31.4 MPa, and it meets the 30 MPa standard specified in “Mixed Soil Curbstone” (JC/T 899-2016) [34]. The standard deviation of the experimental results was calculated to be generally within 0.8, with the maximum coefficient of variation reaching 6.5%, which is below the 15% threshold specified in the “Concrete Strength Inspection and Evaluation Standard” (GB/T 50107-2010). This indicates minimal data variability, meeting the required experimental accuracy standards.
3.2.3. Effect of phosphogypsum dosing on frost resistance of phosphogypsum-based curbstones
This section focuses on evaluating the frost resistance of phosphogypsum-based curbstones, a critical consideration in regions prone to freezing and thawing cycles. Frost resistance significantly influences the longevity and maintenance costs of curbstones and road infrastructure. Phosphogypsum-based curbstones offer advantages in resource utilization and environmental sustainability. The study aims to analyze how varying dosages of phosphogypsum impact the frost resistance of curbstones under simulated freeze-thaw conditions. Through comprehensive testing and analysis, this research seeks to establish a scientific basis for the application of phosphogypsum-based curbstones in cold climates. The findings will inform strategies to optimize material formulations and construction techniques, thereby enhancing the durability and performance of curbstones under freezing conditions.
(1) Loss of quality
The 100mm × 100mm × 100mm cubic non-standard specimens were made in accordance with Table 4, three parallel samples were prepared for each group of fitment ratio, and the experiment was carried out by the steps in “Mixed Soil Curbstone” (JC/T899-2016) [34] after 21d of curing in standard conditions. Specific results are shown in Table S3.
There is a notable trend of increasing specimen mass during the freeze-thaw process, resulting in a specified rate of mass loss of 0, as outlined in the experimental methodology. The curb stone with a phosphogypsum content of 20% exhibited the largest increase in mass, reaching 3.5%, while the curb stone with a phosphogypsum content of 30% showed the smallest increase in mass, at 1.6%. The most significant mass increase occurred during the initial five freeze-thaw cycles. This phenomenon primarily stems from the dissolution of concrete in a water environment, where aggregates are exposed to water. In phosphogypsum-based concrete specimens, freezing causes numerous internal cracks to form. These cracks enlarge with successive freeze-thaw cycles, allowing more water to penetrate the interior of the specimen, thereby increasing water absorption and overall mass.
It can also be observed that despite the varying replacement rates of phosphogypsum, the exhibited properties are consistent. The figure shows that the mass of curb stones with phosphogypsum contents of 30%, 40%, and 50% increases steadily with the number of freeze-thaw cycles. However, the mass of the curb stone with a 20% phosphogypsum content rises significantly compared to the control group after five freeze-thaw cycles. Subsequently, its mass continues to increase steadily with additional freeze-thaw cycles.
(2) Loss of strength
According to the experimental method in Chapter 2, the compressive strength experiments were carried out on the specimens before and after freezing and thawing respectively, and the strength loss rate of the specimens was calculated, and the specific results are shown in Table S4.
Observing the data, when the phosphogypsum substitution rate is 20%, the mass loss rate is recorded at 17.4%. Increasing the phosphogypsum substitution rate from 20% to 30% reduces the strength loss rate slightly to 16.8%. However, when the substitution rate is further increased to 40%, the strength loss rate rises to 21.1%, and it escalates significantly to 32.7% at a substitution rate of 50%. The trend indicates that initially, with higher phosphogypsum substitution rates, the strength loss of specimens after freeze-thaw cycles decreases, but beyond a 30% substitution rate, the strength loss rate begins to rise again, and at a faster rate. This suggests that phosphogypsum has a pronounced effect on the frost resistance of curbstone concrete. During freeze-thaw cycles, the surface mortar gradually peels off, and the internal phosphogypsum undergoes transformation into powdery slag after prolonged exposure to water and freezing conditions. This transformation significantly diminishes the mechanical properties, leading to a decline in compressive strength of the specimens. Therefore, increasing the phosphogypsum substitution rate exacerbates the strength loss rate due to these detrimental effects on material integrity over successive freeze-thaw cycles. When phosphogypsum particles undergo hydration reactions in cement-based materials, they produce hydration products such as C-S-H. However, when its replacement ratio is too high, the surface of the particles experiences increased corrosion due to hydration. Excessive phosphogypsum content also leads to the presence of a large amount of unhydrated phosphogypsum, which forms micro-pores. During freeze-thaw cycles, water continuously freezes and thaws within the cementitious material. When water in the pores freezes, it expands in volume and generates internal stresses, especially in regions with larger pores, causing microcracks to rapidly propagate on the surface or within the material. As the phosphogypsum content increases, the number of micro-pores also increases. Liquid water enters these micro-pores and expands during the freezing process, thereby damaging the internal structure. This damage leads to an unstable internal structure of the curb stone, thereby reducing its compressive strength.
3.3. Phosphogypsum and Curbstone Visualisation Analysis
To explore the feasibility of the current research area and results on phosphogypsum and curbstones. Bibliometric methods were used to visualize and analyze 104 publications on phosphogypsum and curbstones on the Web of Science between 2010–2024 with the help of cite space.
(1) Keyword co-occurrence
Fig.S3(a) shows the frequency of keywords in the 14 years of research, the larger the node the greater the frequency, and the more connected the research is the more relevant [35]. The keywords in descending order of frequency are “blast furnace slag”, “compressive strength”, “performance “, “concrete”, and “building materials”, indicating that these five aspects are the most widely researched.
(2) Keyword highlighting
Fig. S3(b) shows the yearly distribution of common frequency terms in the last 14 years. From the figure, it can be seen that the research on cementitious materials was dominated by fly ash and slag between 2010 and 2020, while the research on phosphogypsum gradually increased after 2020, indicating that phosphogypsum is gradually becoming the mainstream direction of today’s research on cementitious materials. Combining all the keywords, it can be seen that blast furnace slag (2.01) has the strongest keyword explosion, followed by binder (1.89), natural radioactivity (1.87), waste phosphogypsum (1.64), Portland cement (1.58). It shows that there is more research on gelling materials and environmental safety.
(3) Keyword Clustering
Fig. S3(c) shows the interconnected network clusters formed by keywords with similar research topics in the research field from 2010–2024. In the research related to phosphogypsum and curbstones, a total of 8 clusters were obtained, of which the largest cluster is the hydration product. There are a total of 233 nodes and 864 connecting lines in these clusters. Usually, when Q is greater than 0.3 it indicates that the structure of this grid graph is significant [36]. The Q of this grid structure graph is obtained as 0.58 through analysis, which indicates that the keyword clustering of phosphogypsum and curbstone in the field of civil engineering and building materials is clearly defined. Silhouette is used to determine the network’s average homogeneity, and the closer the score is to 1, the higher the homogeneity of the network is [37]. The Mean Silhouette for this network is 0.8094, indicating that the clustering results are reasonable.
3.4. Phosphogypsum Carbon Emission Reduction Calculations
By using phosphogypsum instead of cement in the preparation of curbstones, not only can phosphogypsum be effectively utilized, but also the production costs and carbon emissions can be reduced. This section analyses the data based on relevant articles and information to investigate the carbon emission of phosphogypsum and the ways of carbon reduction.
3.4.1. Stockpiling and transport of phosphogypsum
(1) Transport
Phosphogypsum is usually produced with the upgrading of production processes and is mainly generated in industrial areas, while the pile abandonment and landfill areas are usually in the suburbs, and the average distance from the phosphogypsum to the pile abandonment is 5km. Taking a 30t vehicle as an example, according to the “Standard for Calculation of Carbon Emissions from Buildings” (GB/T 51366-2019) [38], the carbon emission factor of vehicle is 0.078 (kgCO2/(t-km)). Therefore, the amount of carbon dioxide released during the vehicle transportation of phosphogypsum is 5 × 0.078 = 0.39 (kgCO2/t).
(2) Stacking
Fuel consumption will be elaborated when stacking, according to the “Building Carbon Emission Calculation Standard” (GB/T 51366-2019), the CO2 emission factor of diesel fuel combustion is 0.0741 (tCO2/GJ), and the low-level heat of diesel fuel is 42.7 (GJ/t). Then the unit emission of vehicle can be expressed as 3.2 (tCO2/t), and the CO2 emission of diesel fuel consumption for direct stacking of phosphogypsum is 0.73 (kgCO2/t).
This gives a total CO2 emission of 0.73 + 0.39 = 1.12 (kgCO2/t) from the transport and landfill of waste concrete.
3.4.2. Production of phosphogypsum
(1) Transport
Setting the average transportation distance of phosphogypsum to the plant to be about 50 km and that the transport method is the same as that of landfill, the carbon emission of the transport phase of the phosphogypsum recycling process is 50 × 0.078 = 3.9 (kgCO2/t).
(2) Production
The electricity consumption of the phosphogypsum production process is about 6.37 ((kW-h)/t). The average emission of electricity is about 0.94 (kgCO2/(kW-h)) [39]. From this, the CO2 emission per tonne of the phosphogypsum production process is about 6.37 × 0.94 = 5.98 (kg/t). Therefore, CO2 emission from phosphogypsum is 1.12 + 9.88 = 11 (kgCO2/t).
3.4.4. Production and transport of natural aggregates and cement
(1) Mineral mining
According to “Iron ore open pit mining unit product energy consumption quota” (GB 31335-2014) [40]. The energy consumption in the ore mining stage is about 1.5kgce/t, and the standard coal unit emission is 2.54kgCO2/kg. Therefore, the carbon emission is about 1.5 × 2.54 = 3.81 (kgCO2/t).
(2) Production process
According to “Evaluation Methods and Requirements for Low-Carbon Products of Ready-Mixed Concrete” (T/CBMF 27-2018) [41], the natural sand production process and cement’s unit carbon dioxide emissions are 732 (kgCO2/kg) and 3.98 (kgCO2/kg), respectively
Therefore, the carbon emissions from the sand and cement stages are about 732 + 3.98 = 735.98 (kgCO2/t). Therefore, the CO2 emission from the mining and production process is about 3.81 + 735.98 = 739.79 (kgCO2/t).
(3) Transport
According to the “Standard for Calculating Carbon Emissions from Buildings” (GB/T 51366-2019) [36], the carbon emission factor of vehicle is 0.078 (kgCO2/(t-km)), and then the carbon emission during transport is 3.9 (kgCO2/t). Therefore, the total CO2 emission of natural ore aggregate is 739.79 + 3.9 = 743.69 (kgCO2/t).
The carbon emissions generated during transport and production are compared between phosphogypsum and natural sand and cement. In this paper, it is found that the use of re-phosphogypsum significantly reduces the CO2 emissions generated during transport and production.
3.5. Speculative Application Scenarios for Curb Stones
Currently, the primary method for preventing vehicles from veering off the road is the installation of guardrails. However, according to national regulations, guardrails are only mandated on roads with speed limits exceeding 40 km/h, and upon vehicle impact, surrounding sections of the guardrail often collapse. To innovatively apply curb stones in practical scenarios, this paper proposes a conceptual design for a phosphogypsum-based curb stone that prevents vehicles from leaving the road. This curb stone is designed to deploy embedded steel bars upon a vehicle’s uncontrolled exit from the road, anchoring into the vehicle to reduce speed and hook the vehicle, providing an added safety measure for roads with potentially hazardous surroundings. Additionally, the curb stone utilizes waste materials as its raw material, offering technical support for the recycling and reuse of waste resources and contributing to ecological restoration efforts.
Fig. S4(a) illustrates the basic structure of the curb stone, which primarily consists of a phosphogypsum-based curb stone shell and an anti-collision device housed within the shell. Fig. S4 (b), Fig. S4(c), Fig. S4(d) show the side view, top view (external), and top view (internal) of the anti-collision device, respectively. The device includes a left steel plate (2), right steel plate (3), top steel plate (4), bottom steel plate (5), first rebar (6), second rebar (7), first spring (8), second spring (9), hooked rebar (10), support steel plate (11), and limiting steel plate (12). The top steel plate (4) is equipped with a through-hole (13) that allows the hooked rebar (10) to pass through. The height of the limiting steel plate (12) is greater than that of the support steel plate (11).
The left and right ends of the bottom steel plate (5) are connected to the left steel plate (2) and right steel plate (3), respectively. The left end of the top steel plate (4) is connected to the left steel plate (2), while the right end of the top steel plate (4) is connected to the left end of the first rebar (6). The right end of the first rebar (6) is connected to the right steel plate (3). Additionally, the left and right ends of the first spring (8) are connected to the top steel plate (4) and the right steel plate (3), respectively, and are arranged parallel to the first rebar (6). The second spring (9), support steel plate (11), and limiting steel plate (12) are vertically arranged in sequence from left to right on the upper surface of the bottom steel plate (5). The top of the second spring (9) is connected to the non-hooked end of the hooked rebar (10).
Fig. S4(e) illustrates the structure of the curb stone shell, which includes a main shell housing the anti-collision device and a cover shell. Once the anti-collision device is placed inside the main shell, the main shell and cover shell are joined together using cement slurry.
The operating principle of this curb stone is as follows: When a vehicle collides with the phosphogypsum-based curb stone from the left, the curb stone shell provides the first layer of cushioning. Subsequently, the left steel plate (2) is displaced to the right due to the impact, causing the top steel plate (4) to also move rightward. At this point, the first rebar (6) provides a second layer of cushioning. When the first rebar (6) is compressed and fails, the top steel plate (4) moves to the limiting steel plate (12). This action causes the hooked rebar (10) to be ejected through the through-hole (13), embedding into the underside of the vehicle. This mechanism helps to slow down and secure the vehicle, preventing it from veering off the road.
4 Conclusions
This study takes phosphogypsum as the research object, modifies phosphogypsum by triethanolamine, researches the influence of phosphogypsum on the mechanical properties and anti-freezing performance of curbstone, sums up the change rule of mechanical properties and anti-freezing performance of phosphogypsum-based curbstone and analyses the micro-mechanism of phosphogypsum’s influence on the performance of curbstone. The main conclusions are as follows:
Through the test of the grit prepared by different TEA doping of phosphogypsum cementitious materials, we found that: when the triethanolamine doping is 0.3%, the phosphogypsum grit has better Mechanical properties, its flexural strength is about 0.9 MPa, compressive strength is 1.9 MPa. Through the phosphogypsum cementitious material of the net pulp of microscopic test we found that: three tests concluded that triethanolamine doping has a promoting effect on the hydration reaction of phosphogypsum.
Through the compressive strength test of specimens with different phosphogypsum substitution rates, it can be seen that when the substitution rate of phosphogypsum is 30%, the compressive performance of the specimen reaches the maximum, and its 14d compressive strength is 25.6 MPa, and 28d compressive strength is 31.4 MPa, meeting the standard of greater than 30 MPa as specified in “Concrete Curbs” (JC/T 899-2016).
After the freeze-thaw cycle tests on the specimens with different phosphogypsum substitution rates, it can be seen that there is a tendency for the mass of each specimen to increase during the freeze-thaw process, so that the rate of loss of mass, according to the experimental methodology, is 0. The greatest increase in mass was observed during the first 5 freeze-thaw cycles. It can also be seen that the different substitution rate of phosphogypsum shows the same properties, indicating that phosphogypsum does not have much effect on the loss of mass of the frost resistance of the kerbstone. At the same time, with the increase of the substitution rate of phosphogypsum, the strength loss rate at the end of the freeze-thaw cycle also increases gradually and the increasing trend is faster and faster after the experiment shows that when the substitution rate of phosphogypsum is 30%, the strength loss rate is 16.8% at the lowest. The specimen met the requirements specified in the “Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete” (GB/T 50082-2009), which stipulates that after 25 freeze-thaw cycles, the strength loss rate should not exceed 25%, and the mass loss rate should not exceed 5%.
By calculating the carbon emission reduction of phosphogypsum, it is found that the total CO2 emission of the production and transport process of phosphogypsum is 11 (kgCO2/t) far less than that of natural sand and cement, which is 743.69 (kgCO2/t). By visualizing and analyzing phosphogypsum and kerbstone, it was found that before 2020, research on cementitious materials was dominated by fly ash and slag, while after 2020, phosphogypsum gradually became the mainstream of research. In terms of research directions for curb stones, this paper presents a conceptual design for a phosphogypsum-based curb stone that prevents vehicles from veering off the road.
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