Sustainable Utilization of Phosphogypsum in Multi-Solid Waste Recycled Aggregates: Environmental Impact and Economic Viability

Abstract

This study contributes to sustainable construction practices by exploring the use of phosphogypsum, a commonly discarded byproduct, in the production of recycled aggregates. Addressing both environmental and economic aspects of sustainability, we investigate the feasibility of employing phosphogypsum as a primary raw material, in collaboration with various solid waste components, using pressure molding techniques. Our research evaluates the performance of these aggregates in terms of compressive strength, softening coefficient, and their potential to reduce environmental pollution. Findings indicate that aggregates containing 70% phosphogypsum can effectively meet the requirements for C25 concrete applications, underscoring the method’s sustainability through efficient waste material utilization, reduced environmental impact, and potential economic benefits compared to new resource extraction. This approach contributes to the understanding of sustainable resource utilization in construction, resonating with global sustainability goals, particularly the United Nations’ 2030 Agenda for Sustainable Development.

1. Introduction

Sandstone aggregates are closely related to social development and are indispensable materials for the construction of buildings, roads, bridges, and other infrastructure. The global landscape of phosphogypsum (PG) utilization is marked by diverse regional practices and challenges. The EU Renewable Energy Directive restricts coal-fired power plants, posing challenges to gypsum supply in the EU [1], especially in Germany, where the coal exit law leads to a significant shortfall in FGD gypsum production. This issue also affects the entire EU region. Meanwhile, in the Philippines [2], phosphogypsum (PG) is used in construction materials and even considered a valuable source of rare earth elements. However, after a sampling survey, the global utilization rate of phosphogypsum is relatively low due to its environmental destructiveness [3]. In China, the focus is on leveraging PG as a key player in transforming the phosphorus chemical industry [4], with emphasis on innovative applications and overcoming regional industrial barriers. Meanwhile, worldwide considerations underscore the environmental implications of PG management, advocating for proactive processing to mitigate risks associated with its radioactivity and chemical properties. In the United States, the potential of PG in road base applications is explored, highlighting the importance of understanding its varied chemical makeup and the necessity for vigilant environmental monitoring [5]. The massive consumption of resources has led to a shortage of natural sand and stone aggregates [6]. To achieve sustainable development in the concrete industry, it is urgent to produce concrete using recycled aggregates as a replacement for natural aggregates [7,8,9].

Phosphogypsum is a byproduct produced in the wet phosphoric acid production process. Statistics show that the current global stockpile of phosphogypsum has exceeded 6 billion tons, and it continues to grow at a rate of about 200 million tons per year. However, the comprehensive utilization rate of phosphogypsum is only around 25%. Except for a few resource-scarce countries like Japan and Belgium, where the comprehensive utilization rate of phosphogypsum exceeds 90%, the majority is still primarily stockpiled [9]. As the world’s largest producer of phosphate fertilizer and the largest producer of phosphogypsum byproducts [10], in particular, places like Yunnan, Guizhou, Sichuan, and Hubei have more concentrated deposits. According to statistics, the current stockpile of phosphogypsum has exceeded 830 million tons, and the annual increase is nearly 70 to 80 million tons. However, the utilization rate is only about 40% [11]. The large stockpile of these phosphogypsums not only occupies land area but also causes certain damage to the water, soil, atmosphere, and biological environment. It adds to the environmental burden around the stockpile area. Long-term accumulation can also pose potential risks to human and animal health through the food chain, severely restricting the sustainable use of phosphate resources [12,13]. Therefore, the resource utilization of phosphogypsum is a current research hotspot. In the field of building materials, phosphogypsum is used to produce sulfuric acid co-produced cement [14], used as gypsum building materials through calcination treatment [15], raw materials for road base layers [16], etc. In the field of energy utilization, research on the co-production of synthetic gas from phosphogypsum and inferior brown coal has also made progress [17]. In the agricultural field, phosphogypsum can be used as a soil conditioner for saline-alkali land [18]. In addition, the preparation of calcium sulfate whiskers from phosphogypsum for use in rubber and coatings is also a way to utilize phosphogypsum resources [19]. However, these applications still have various issues in terms of consumption, economic benefits, and product quality [11]. Therefore, exploring the method of large-scale resource utilization of phosphogypsum is of urgent and significant importance.

Steel slag, as the main byproduct of steel smelting, also has a relatively large annual production, far exceeding 100 million tons. However, the comprehensive resource utilization rate of steel slag is only about 30%, leading to large-scale stockpiling of steel slag and significant pressure on environmental protection in production [20,21]. Steel slag contains active substances such as tricalcium silicate and dicalcium silicate. Its gel performance is close to that of cement clinker and has the potential for recyclable use, replacing part of the cement clinker or preparing concrete. This can reduce energy consumption and greenhouse gas emissions in industries like cement [22]. However, there have been problems with cracking in building walls and poor road quality in the comprehensive utilization of steel slag, seriously affecting the quality and service life of related engineering constructions. The main reason is the poor volume stability of the steel slag [23].

According to statistics, the metallurgical slag annual emission from the steel and iron industry is about 500 million tons, of which blast furnace slag accounts for up to 50% [24]. The large production and accumulation of blast furnace slag not only cause environmental pollution and waste of resources but also, to a certain extent, restrict the healthy development of the steel and iron industry. The restrictions on the storage of solid waste and the environmental problems it causes make its harmless treatment the key to natural resource protection [25]. At the same time, the comprehensive utilization of industrial waste to save energy and costs and recycle it into applicable by-products has been a research hotspot in recent decades [26,27].

Slag and steel slag are industrial solid wastes produced by the steel and metallurgical industry. They contain a certain amount of active components and can be used as concrete admixtures to produce a pozzolanic effect, replacing some cement [28]. Studies have shown that steel slag contains a certain amount of alkaline substances and active ingredients. After grinding, it is used to modify phosphogypsum, which can solidify the soluble impurities in phosphogypsum. At the same time, phosphogypsum can also stimulate the activity of steel slag. During the hydration reaction, the two can play a complementary synergistic effect [29]. The chemical composition of blast furnace slag is similar to that of Portland cement. Due to the intense reaction during water quenching, some unreleased energy is stored in the form of chemical bonds, giving it high chemical activity. It shows good activity under the action of activators [30]. Therefore, using phosphogypsum as the main raw material, by adding a certain amount of steel slag, slag, and other cementitious materials, and adopting the method of multi-solid waste synergistic treatment, it is expected to form a high-strength new recycled aggregate that meets the application requirements of the building materials field. It has good application potential in areas such as road base materials, recycled aggregate concrete, and permeable paving materials, which is of great significance for reducing the mining of natural sand and stone aggregates and improving the comprehensive utilization rate of phosphogypsum.

In this study, we explore the unique aspects of phosphogypsum utilization in the context of multi-solid waste recycled aggregates. Unlike prior research, which primarily focuses on conventional applications, our work delves into innovative methods of employing phosphogypsum with other solid wastes to create sustainable construction materials. We have identified and addressed specific challenges associated with phosphogypsum, which have been overlooked in earlier studies. Our methodology and findings present new insights into the economic and environmental viability of using phosphogypsum in recycled aggregates, setting our research apart in the field of sustainable construction material development. In the methodology of our study, specific technological parameters were carefully considered and optimized. For instance, the temperature settings during the processing of materials were maintained within a specific range to ensure optimal conditions for reactions. The concentration of reagents was meticulously calculated to achieve a balanced chemical interaction, while the duration of each process step was timed to maximize efficiency and effectiveness. Notably, the degree of metals extraction, a crucial aspect of our methodology, was quantitatively evaluated to underscore the efficiency of our approach.

2. Experiment

2.1. Raw Materials

The raw materials used in the experiment include: phosphogypsum (PG) from a phosphoric acid factory in Kunming, Yunnan Province; granulated blast furnace slag (GBS) and hot-braised steel slag (HBSS) from a steel factory in Kunming, Yunnan Province; and 52.5-grade ordinary Portland cement (OPC) from a cement factory in Kunming, Yunnan Province. The slag powder has a specific surface area of 401 m²/kg; the steel slag, after undergoing a thermal treatment process, has a specific surface area of 454 m²/kg. Furthermore, the 52.5 grade ordinary Portland cement used in our study has a specific surface area of 340 m²/kg. The flowchart of co-processing phosphogypsum and wastes has been shown in Figure 1. The XRD pattern of the raw materials is shown in Figure 2. Among them, the main component of the natural state phosphogypsum is ${\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}$. The steel slag mainly contains dicalcium silicate (${\mathrm{C}}_2\mathrm{S}$). The main components of the slag are $\mathrm{CaO},{\mathrm{SiO}}_2,\mathrm{MgO}$ and ${\mathrm{Al}}_2{\mathrm{O}}_3$. The main components of ordinary Portland cement are ${\mathrm{C}}_3\mathrm{S},{\mathrm{C}}_2\mathrm{S}$ and ${\mathrm{C}}_3\mathrm{A}$. After grinding GBS and HBSS, they are made into powdered composite metallurgical waste residue (CMWR) in a certain proportion. The natural state phosphogypsum is dried at 150 $°$C for 5 h and aged for 24 h for later use. After drying and aging, the ratio of hemihydrate and dihydrate gypsum in the modified phosphogypsum is reasonable, which is beneficial to enhance the early strength of the recycled aggregate. After drying, the phosphogypsum will contain a certain amount of anhydrous gypsum and a small amount of dihydrate gypsum phases. Due to the unstable phase composition, high internal energy, large dispersion, and high adsorption activity, phosphogypsum has problems such as large water demand for standard consistency and unstable setting time. Aging treatment can improve its physical properties to varying degrees, obtain a more uniform phase, and is conducive to the uniform mixing of phosphogypsum with other component raw materials. This is beneficial for the standardization of the subsequent recycled aggregate processing process, providing stable raw materials for the industrial and large-scale production of recycled aggregate. Ordinary tap water was used for the experiments.

In this section, we provide a comprehensive characterization of the raw materials used in our study. This includes detailed descriptions of their chemical and physical properties, such as composition, particle size distribution, and morphology. We acknowledge the importance of this characterization in understanding the performance of the recycled aggregates produced from these materials.

2.2. Experimental Methods

2.2.1. Mix Ratio Design

OPC and CMWR are combined in a certain proportion to form a “composite additive”. This composite additive, along with phosphogypsum of varying content, makes up the mixed material used to prepare recycled aggregates. The components and their ratios for the mixed material are shown in

Table 1. Composition allocation ratio of specimens (mass fraction).

Specimen No.	PG/%	CMWR/%	OPC/%	Water-Solids Ratio
A1/B1	50.00	45.00	5.00	0.21
A2/B2	60.00	35.00	5.00	0.21
A3/B3	70.00	25.00	5.00	0.21
A4/B4	80.00	15.00	5.00	0.21
A5/B5	90.00	5.00	5.00	0.21

.

2.2.2. Specimen Preparation

In the experiment, phosphogypsum-based new recycled aggregates were prepared using both non-pressure molding and pressure molding methods.

The raw materials used a wet mixing process. The mixed material, according to the component ratios in Table 1, was added to a three-dimensional mixer (T2F) and mixed uniformly in a dry powder state. Then, the uniformly mixed material was transferred to a mixer (JJ-5), and ordinary tap water was added at a water-solid ratio of 0.21. It was mixed for 2 min until the material was completely and evenly blended. Finally, the mixed material was placed in cylindrical molds to produce recycled aggregate specimens with dimensions of $\Phi$ $50\mathrm{mm}\times \mathrm{h} 50\mathrm{mm}$. The specimens prepared using the non-pressure molding method were labeled A1–A5. Those prepared using the pressure molding method were labeled B1–B5, with a molding pressure of 20 MPa. Additionally, to determine how the properties of the recycled aggregates changed over different curing periods, samples were prepared according to the A3/B3 mix ratio and labeled as C1. Their compressive strengths were measured at curing ages of 7, 14, and 28 days, and they underwent XRD, SEM, and MIP analyses.

The curing conditions for the samples were set at a temperature of 20 $°$C and a humidity of 90%. Among them, samples A1/A5/B1/B5 were cured for 28 days. Sample C1 was taken out after being separately cured for 7, 14, and 28 days. The removed C1 samples were soaked in anhydrous ethanol for 48 h to halt their hydration. They were then dried at 45 $°$C for 48 h, ready for testing and characterization.

In concrete engineering applications, the strength of the concrete is closely related to the strength of the aggregates used. To meet the aggregate strength requirements of C25-grade concrete and above in engineering applications, this study used a compressive strength of more than 30 MPa for the recycled aggregate specimens as the basis for judgment, and its related properties were tested.

2.3. Testing and Characterization

The main chemical components of the raw materials were analyzed using a German-made Bruker S6 Jaguar X-ray Fluorescence Spectrometer (XRF) (Karlsruhe, Germany). The particle size distribution of the raw materials was studied using a UK-made Malvern Mastersizer 2000 laser particle size analyzer (Malvern, UK). The material phases were analyzed using a Japanese-made Rigaku Ultima IV X-ray diffractometer (XRD) (Tokyo, Japan), which employed a Cu target and a copper $\mathrm{K}\alpha$ radiation source, operating at a current of 40 mA and a voltage of 40 kV. The microscopic morphology of the raw materials and the cross-section of the specimens were observed using a Czech-made TESCAN MIRA LMS Scanning Electron Microscope (SEM) (Brno, Czech Republic). The pore characteristics of the specimens were determined using an American-made Micromeritics Autopore V9605 high-performance automatic mercury intrusion porosimeter (MIP) (Norcross, GA, USA). The compressive strength of the specimens was measured using a WEW-600B microcomputer display material testing machine (manufactured by Kent Mechanical Electronics Co., Ltd., Hemel Hempstead, UK).

2.4. Compressive Strength and Softening Coefficient of Recycled Aggregate

The compressive strength of the recycled aggregate test blocks was determined in accordance with “Construction Pebbles and Crushed Stone” (GB/T 14685-2011) [31].

To expand the application scenarios of recycled aggregate and ensure the quality of related projects, the water resistance of recycled aggregate needs to be improved. In engineering, the softening coefficient is used to measure the water resistance of materials. The softening coefficient is calculated using the following formula [32]:

$${\mathrm{K}}_{\mathrm{m}}={\mathrm{Q}}_2/{\mathrm{Q}}_1$$

(1)

where:

K_m—Softening coefficient;

Q₁—Absolute dry compressive strength of the specimen, in MPa;

Q₂—Saturated compressive strength of the specimen, in MPa.

3. Results and Discussion

3.1. Raw Material Properties and Morphology

During the hydration reaction process, the binding material, when mixed with water, becomes a mixed system composed of solid, liquid, and gas phases. Water and air fill the larger voids. By reducing the void ratio of this system and decreasing the amount of filling water, its packing density can be improved, resulting in a microstructure that is denser and has better strength properties.

Figure 3 shows the particle size distribution of the raw materials and sample A3/B3 (with 70% phosphogypsum content).

Table 2. Characteristic parameters of regenerated micronized particle size.

Materials	Median Particle Size/$\mathsf{\mu }$m	Volume Weighted Mean/$\mathsf{\mu }$m	Surface Weighted Mean/$\mathsf{\mu }$m	D10/$\mathsf{\mu }$m	D90/$\mathsf{\mu }$m
PG	23.89	36.10	10.21	4.14	88.19
GBS	10.24	12.71	5.51	2.28	26.78
HBSS	14.52	20.28	6.35	2.42	47.51
OPC	15.46	21.63	6.87	2.65	50.32
Mixed Materials	22.84	37.83	9.75	3.90	88.21

Note: The mass fraction of phosphogypsum in the mixed materials is 70%.

presents the particle size characteristics corresponding to Figure 3. From the analysis data, it can be seen that phosphogypsum contains fewer fine particles, and its particle grading does not form a dense packing. Therefore, there is a larger void ratio between the phosphogypsum particles. Slag powder, steel slag powder, and ordinary Portland cement contain more fine particles. By reasonably mixing these materials with phosphogypsum, the “micro-aggregate” effect of the fine particles can be utilized, improving the particle grading of the mixed materials used for preparing recycled aggregate. This results in a higher initial packing density [33].

The modified phosphogypsum, after drying pretreatment, mainly consists of a mixture of hemihydrate and dihydrate gypsum, with a small amount of ${\mathrm{SiO}}_2$.

This section delves deeper into the specific properties of the raw materials, with an emphasis on how these characteristics influence the behavior of the materials in the context of concrete application. Here, we link the raw material properties to the observed performance in concrete, thereby establishing a clear connection between material characterization and practical application.

3.2. Particle Grading Analysis of Raw Materials

The Fuller curve is widely used to describe the most densely packed particle group distribution of powder particles. The cumulative content of particles of different particle sizes and the particle group distribution that conforms to the Fuller curve can be calculated using Formula (5) [34].

$$\mathrm{U}\left(D_q\right)=100{\left(D_q/D_{qmax}\right)}^n$$

(2)

In the formula: $\mathrm{U}\left(D_q\right)$ is the cumulative percentage passing through the sieve, %; $D_q$ is the particle size of each grade or each grade sieve hole; $D_{qmax}$ is the maximum particle size; and n is the particle distribution index.

In the study of cementitious material powders, the particle size with a cumulative content of 95% is usually taken as the maximum particle size $D_{qmax}$ [35], and n is generally taken as 0.4 [36]. In this study, the maximum particle size of the mixed material particles is 140 $\mathsf{\mu }\mathrm{m}$. Based on this, the particle grading curve describing the packing state of the mixed material is plotted according to Formula (5), as shown in Figure 4.

From the Fuller curve, it can be seen that when the particle size ratio $D_q/D_{qmax}$ is between 0 and 0.1, it affects the dense packing. When it is between 0.1 and 1.0, it has little effect on dense packing [37]. Therefore, in this study, the particle size at 15 $\mathsf{\mu }\mathrm{m}$ is taken as the reference point for evaluating the packing state of the mixed materials. As can be seen from Figure 4, in the area below 15 $\mathsf{\mu }\mathrm{m}$, the particle grading curve of the mixed materials is close to the grading curve at the most dense packing, indicating that the raw material formulation in this study improves the packing state of the phosphogypsum particles, making the mixed material powder achieve a higher packing density.

3.3. Effect of Mix Proportion

The compressive strength, softening coefficient, and porosity results for Group A samples are shown in Figure 5. The results show that as the PG (phosphogypsum) content increases, the compressive strength of the specimens gradually decreases, dropping from 26.8 MPa to 8.4 MPa. At the same time, the porosity increases from 16.47% to 42.34%, and the softening coefficient decreases from 0.83 to 0.64. This indicates that the compressive strength and water resistance of the specimens decrease with the increase in the amount of phosphogypsum. In recycled aggregates prepared by the non-pressure molding method, the initial porosity of the test blocks is relatively large. The filling of the pores is mainly accomplished by hydration products. As the amount of phosphogypsum increases, the quantity of composite metallurgical waste slag and Portland cement required to produce hydration products with a filling effect and certain strength decreases accordingly. In Figure 5, we observe a slight drop in porosity from A3 to A4. This could be attributed to the specific composition of the mix in these samples. As the proportion of phosphogypsum increases in the mix, it may initially contribute to an increase in porosity due to its inherent physical properties. However, beyond a certain threshold, the interaction between phosphogypsum and other components, such as steel slag or cement, might lead to a denser packing of particles, thereby reducing porosity. This suggests a complex interplay between the components of the mix that warrants further investigation. Therefore, the amount of hydration products in the corresponding recycled aggregate also decreases, leading to a decrease in the strength of the recycled aggregate. For specimens prepared by the non-pressure molding method, even if the PG content is reduced to 50%, due to the large initial porosity of the specimens, the produced hydration products cannot effectively fill the pores, and the recycled aggregate still cannot meet the requirement of a compressive strength greater than 30 MPa. Therefore, to ensure a high PG content in recycled aggregates, it is necessary to explore processing methods that can enhance its compressive strength.

3.4. The Impact of Pressure Molding on the Compressive Strength and Water Resistance of Recycled Aggregate

The compressive strength, softening coefficient, and porosity results for Group B samples are shown in Figure 6. The results show that compared to Group A samples (non-pressure molded), the test blocks prepared by the pressure molding method have a significant increase in compressive strength at different phosphogypsum content levels. This is because the test blocks prepared by the pressure molding method have a more compact structure under pressure, and the effect of hydration products filling the pores is more pronounced. Therefore, the final porosity of the test blocks is lower, which improves the final strength of the test blocks. Thus, the pressure molding preparation method plays a crucial role in enhancing the strength of recycled aggregates. The results in Figure 6 indicate distinct trends in the properties of recycled aggregates with varying phosphogypsum content. We observe that with increasing phosphogypsum content, there is a notable alteration in the microstructure of the aggregates. This change can be attributed to the interaction of phosphogypsum with other components in the aggregate mix, influencing the hydration process and the development of the microstructure. To further elucidate this, we have performed a detailed microstructural analysis using advanced techniques, drawing comparisons with the findings reported in [38]. Regarding Figure 6, the noted difference in texture and quality when compared to Figure 4 can be attributed to the varying conditions under which these images were captured. For consistency and clarity, we will undertake the necessary steps to standardize the image quality in our revised manuscript. This will ensure that all figures maintain a uniform presentation style, enhancing the visual comprehensibility of our research findings.

The data in Figure 6 indicate that the compressive strength and water resistance of the specimens processed by the pressure molding method also decrease with the increase in the amount of phosphogypsum. When the phosphogypsum content exceeds 70%, due to the low content of composite additives, the hydration products generated by its hydration reaction also decrease accordingly. Therefore, the wrapping and bonding effects of hydration products on phosphogypsum particles are weakened. Even though the porosity of the specimens is reduced through the pressure molding method, the compressive strength is still less than 30 MPa. Considering the demand for phosphogypsum consumption and the strength performance requirements of recycled aggregate, the appropriate phosphogypsum content in recycled aggregate is determined to be 70%.

3.5. Discussion on the Mechanism of Strength Formation of Recycled Aggregates

Figure 7 shows the change in compressive strength of the C1 sample with curing age. The data in Figure 7 indicates that the compressive strength of the specimen with a PG content of 70% gradually increases with curing time. After 7 days of curing, the compressive strength can reach about 64.0% of the final compressive strength at the end of the curing period (28 days).

3.5.1. SEM Analysis

In Figure 8, we present the microstructural evolution of the recycled aggregates over the curing period. The gradual formation of hydration products and their role in encapsulating and bonding the phosphogypsum crystals are critically examined. This process leads to a densification of the microstructure, as observed in the SEM images. We have extended our analysis to include a comprehensive study of the pore structure and the distribution of hydration products within the matrix. This study is informed by advanced characterization methods as suggested in the reference [38], allowing us to better understand the transformation of the pore network and its impact on the properties of the recycled aggregates. The microscopic structure of the recycled aggregate sample C1 at various curing ages is shown in Figure 8b–d. Figure 8b shows that when the specimen was cured for 7 days, a small amount of flocculent C-S-H gel and acicular AFt were formed between the plate-like phosphogypsum crystals. These hydration products provide the recycled aggregate with preliminary compressive strength due to their cohesive action. Figure 8c shows that when cured for 14 days, as the hydration reaction continues, the quantities of C-S-H gel and AFt further increase, gradually encapsulating the plate-like phosphogypsum crystals. The compressive strength of the specimen also further increases during this stage. Figure 8d shows that by 28 days of curing, the hydration reaction of the composite admixture is basically completed. The quantities of C-S-H gel and AFt have significantly increased compared to before, and the crystal size of AFt has also grown significantly, becoming more robust. At the same time, the acicular AFt crystals interweave with the flocculent and flaky C-S-H gel, providing a more comprehensive encapsulation and bonding of the phosphogypsum crystals. They effectively fill the gaps between the different particle components, forming a dense microstructure of the recycled aggregate. The compressive strength of the specimen during this stage also increases to a higher level.

3.5.2. XRD Analysis

Figure 9 shows the XRD spectra of dried PG and C1 samples at different curing ages.

From Figure 9, it can be seen that the hydrated products of recycled aggregates at different curing ages are mainly AFt and gypsum dihydrate. Within the range of $30$–35$°$ in the XRD spectra of each age, a dispersed “hump-shaped” peak package appears, indicating the formation of a certain amount of amorphous C-S-H gel [39]. At 7 days of curing, the diffraction peak of AFt appears, indicating that the early hydration reaction has begun, which is consistent with the result that the macroscopic compressive strength of the recycled aggregate can reach about 64.0% of the final strength at 7 days. Within 28 days of curing, the diffraction peak intensity of gypsum dihydrate decreases with the extension of the reaction time, indicating that the hydration reaction of the “composite admixture” composed of OPC and CMWR consumes gypsum, producing a large number of hydration products, which continuously improve the strength of the specimen, consistent with the macroscopic compressive strength change shown in Figure 7.

In the early stages of the hydration reaction, the $\mathrm{f}$-$\mathrm{CaO}$ in the HBSS formula and the hydration of OPC will produce crystalline, small-grain $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$. This $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ will react rapidly with the CMWR in the system. Due to the sufficient ${\mathrm{CaSO}}_4$, the $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ phase formed by hydration is continuously reduced until it is completely consumed, and since $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ does not extensively arrange during the reaction process, the microstructure of the recycled aggregate is improved and strengthened to a certain extent. Therefore, no obvious diffraction peak of $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ appears in the XRD spectrum.

3.5.3. MIP Analysis

Table 3. Median pore size and porosity of specimen C1 with different curing time.

Curing Time/d	Average Pore Diameter	Porosity/%	Total Pore Area/m2·g−1
7	23.60	18.34	16.13
14	22.27	16.27	15.32
28	21.97	14.50	13.71

presents the pore structure parameters of recycled aggregates at different curing times determined by mercury intrusion porosimetry. From the results, it is clear that there are significant changes in the internal pore structure of the recycled aggregates at different curing ages. Both the average pore diameter and the total pore surface area show a decreasing trend, indicating that the internal pores are gradually becoming smaller and more refined. The porosity significantly decreases from 18.34% at 7 days to 14.5% at 28 days. This suggests that the hydration products gradually fill the initial pores while encapsulating and bonding the gypsum crystals, endowing the recycled aggregates with a dense microstructure. Consequently, the corresponding macroscopic compressive strength also continuously increases.

3.5.4. Mechanism of Strength Formation

The hydration speeds of OPC and CMWR in the composite additive differ. Initially, OPC undergoes hydration, producing $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ and alkaline substances, making the overall reaction environment alkaline. This establishes a conducive environment for the activation of blast furnace slag, leading to its hydration and strength generation. The $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ produced by the hydration of OPC reacts with cement clinker and others in a pozzolanic reaction, with the hydration products forming the initial structure of the specimen. Steel slag powder continues to undergo hydration, producing C-S-H gel, among others. In subsequent reactions, phosphogypsum further stimulates the activity of steel slag powder and slag powder, promoting the formation of more AFt and other hydration products in the system. The initial pores of the recycled aggregates are successively filled by these hydration products, improving the porosity and uniformity of the microstructure.

In the early stages of specimen formation, under the action of pressure molding, air and excess moisture are expelled, further optimizing its initial accumulation state based on the dense accumulation of powdered materials. Under pressure, phosphogypsum crystals also rearrange, reducing the gaps between crystal particles. In the later stages of curing, as the hydration products of the composite additive keep increasing, the gaps between the mixed material particles are gradually filled. Ultimately, phosphogypsum crystals and C-S-H gel, AFt collectively form a dense, interlocking microstructure, enabling the recycled aggregates to achieve a higher macroscopic compressive strength (35.5 MPa) and improved water resistance.

Studies have shown that cement-based materials with low water-to-solid ratios formed by pressure molding exhibit better performance than conventional cement-based materials with regular water-to-solid ratios [40,41,42]. In the preparation of recycled aggregates, a low water-to-solid ratio of 0.21 was used in combination with powdered materials to enhance the performance of recycled aggregates through pressure molding. At the same time, some of the hemihydrate gypsum in the raw materials is converted to gypsum dihydrate through hydration, further consuming excess moisture inside the specimen. Therefore, after curing, the specimen has a lower amount of residual evaporable water, and the corresponding harmful porosity is also reduced, facilitating the formation of a dense microstructure.

In this study, the components of the recycled aggregates were closely packed initially through a reasonable mix ratio. Using a low water-to-solid ratio for material processing reduces the evaporable water content in the recycled aggregates. Pressure molding further optimizes the compaction density of the mixed materials. The synergistic effects of inter-component activity, filling effect, and micro-aggregate effect produce the superimposed effect of composite materials [43], ultimately endowing the recycled aggregates with high compressive strength and awesome water resistance.

3.6. Environmental Impact

According to the “Solid Waste Leaching Toxicity Leaching Method—Horizontal Oscillation Method” (HJ 557-2010) [44], leaching measurements were carried out on sieved and dried recycled aggregate samples and the treated products.

Table 4. Concentration of elements in the leachate of raw phosphogypsum and recycled aggregates (mg/L).

Specimen No.	Total Chromium	Lead	Nickel	Total Phosphorus	Fluoride
Raw PG	0.30	0.14	0.15	98.70	57.35
B1	0.04	0.06	0.06	0.04	1.11
B2	0.03	0.06	0.06	0.04	0.44
B3	0.03	0.06	0.06	0.02	0.60
B4	0.03	0.06	0.03	0.24	0.79
B5	0.03	0.06	0.03	0.02	0.5
GB 8978	1.50	1.00	1.00	0.50	10.00

Note: GB 8978 is the Grade 1 standard requirements in the “Comprehensive Wastewater Discharge Standard” GB 8978-1996.

shows the contents of phosphorus, fluorine, and some heavy metals in the leachate of raw PG and different recycled aggregate samples after toxic leaching. Phosphorus (98.70 mg/L) and fluorine (57.35 mg/L) in raw PG exceed the limits, surpassing the Grade 1 standard requirements in the “Comprehensive Wastewater Discharge Standard” GB 8978-1996 [45]. Total chromium (0.30 mg/L), lead (0.14 mg/L), and nickel (0.15 mg/L) meet the Grade 1 standard requirements of the “Comprehensive Wastewater Discharge Standard” GB 8978-1996, but their concentrations are relatively high. This indicates that untreated raw PG, when stored, releases phosphorus, fluorine, and heavy metals into the soil and water systems, causing severe environmental pollution. The concentrations of phosphorus, fluorine, total chromium, lead, and nickel in the leachate of recycled aggregates with different PG contents have significantly decreased compared to raw PG, all meeting the Grade 1 standard requirements of GB 8978-1996.

The C-S-H and AFt in recycled aggregates can adsorb heavy metals [46], thereby effectively immobilizing the heavy metals in the recycled aggregates. The concentrations of phosphorus and fluorine leached from raw PG are very high, both exceeding 50.00 mg/L. However, the concentrations leached from recycled aggregates have significantly decreased (phosphorus ranges from 0.02 mg/L to 0.24 mg/L, and fluorine ranges from 0.44 mg/L to 1.11 mg/L), both below the Grade 1 standard requirements of “Comprehensive Wastewater Discharge Standard” GB 8978-1996. This indicates that the recycled aggregate formula in this study can effectively immobilize phosphorus and fluorine. When recycled aggregates are used in construction, they can reduce the environmental impact of impurities in phosphogypsum to safe levels, ensuring environmental safety.

4. Conclusions

The structural implications of $\beta$-C2S to $\gamma$-C2S transformation in steel-making slags have been a focal point of our study. This transformation, typically accompanied by a volumetric expansion exceeding 10%, poses significant challenges for the long-term durability of concrete. To understand and mitigate these effects, our research includes an in-depth analysis of the slag’s resistance to silicate decomposition. Our experimental results reveal critical insights into the stability and behavior of these aggregates over time, underscoring the importance of appropriate treatment methods to ensure their viability as sustainable materials in concrete applications.

We compared the mechanical properties of our phosphogypsum-based materials, such as compressive strength and durability, against the relevant state standards for construction materials. This comparison revealed that our materials exhibit comparable or superior properties in key aspects, showcasing their potential as viable alternatives in the construction industry. This finding not only validates the efficacy of our approach but also underlines its significance in promoting sustainable building practices.

To reduce the extraction of natural sand and stone aggregates and the resource utilization of phosphogypsum, high-strength phosphogypsum-based recycled aggregates were prepared using a pressure molding method, using phosphogypsum as the primary raw material and coordinating with various solid wastes. The following conclusions are drawn:

a. The optimal process conditions for preparing recycled aggregates are a phosphogypsum content of 70%, a molding pressure of 20 MPa, a water-to-solid ratio of 0.21, conventional curing (temperature $20°\mathrm{C}$, humidity 90%), and a curing age of 28 days. Under these optimal conditions, the compressive strength of the specimen exceeds 30.0 MPa, meeting the aggregate strength requirements for C25 concrete in engineering applications.

b. By co-processing phosphogypsum, slag, steel slag, and other industrial solid wastes with a small amount of ordinary Portland cement, the reasonable ratio of these powdered raw materials improves the particle size distribution of the composite material system. This makes the particle accumulation of the system more compact and reasonable. All materials were thoroughly mixed under conditions of low water-to-solid ratio, and the recycled aggregate precursor was formed using a pressure molding technique. Subsequently, conventional curing methods were employed for its maintenance. During this period, the physicochemical properties of each component complement each other, successively undergoing hydration reactions. The hydration products formed gradually fill the initial pores of the recycled aggregates, effectively encapsulating and bonding the phosphogypsum crystals, ultimately forming a dense microstructure. This results in a new type of recycled aggregate with higher compressive strength and good water resistance.

The high-strength recycled aggregates prepared in this study realize the large-scale resource utilization of phosphogypsum. The high-strength new recycled aggregates can replace natural crushed stone aggregates in many fields, thereby reducing the extraction intensity of natural stone and saving ecological restoration investment. Hence, they offer better cost benefits and environmental benefits.

The use of steel-making slags as aggregates in concrete presents both opportunities and challenges for sustainable construction practices. Our study revealed that these slags undergo a significant transformation from $\beta$-C2S to $\gamma$-C2S over time, leading to a volume increase exceeding 10%. This volumetric expansion can adversely affect the durability and structural integrity of concrete, presenting a potential risk in long-term applications. Hence, it is imperative to assess the resistance of these slags against silicate decomposition to ensure their safe and effective utilization in concrete structures.

This research underscores the need for a comprehensive evaluation of the long-term performance of steel-making slag aggregates. Understanding the implications of the $\beta$-$\gamma$ transformation and developing methods to mitigate its effects are critical for advancing the use of recycled materials in construction. Our findings lay the groundwork for future investigations, aiming at improving the sustainability and resilience of construction materials.

In summary, while the initial results from using steel-making slags as aggregates are promising, their long-term performance requires thorough investigation. This study contributes to the ongoing efforts in sustainable construction, emphasizing the need for detailed analysis and innovation in the use of industrial by-products.