Research on the Performance of Phosphorus-Building-Gypsum-Based Foamed Lightweight Soil in Road Reconstruction

Abstract

Current research on foamed lightweight soil primarily focuses on mechanical properties and durability, with few studies addressing its hydraulic characteristics and internal pore structure in road reconstruction applications. However, the material’s high porosity and low bulk density may significantly alter its mechanical properties and durability under prolonged rainwater exposure, highlighting the importance of investigating its hydraulic characteristics and internal foam structure. Based on the analysis of water absorption and bulk density in phosphogypsum-based foamed lightweight soil, this study further discusses the material’s softening coefficient and internal pore structure through systematic data comparison. Experimental results demonstrate that the unconfined compressive strength (UCS) of both dry and water-soaked specimens increases linearly with dry density. Notably, soaked specimens with 0.5 g/cm³ dry density achieve compliant 7-day UCS values while displaying a steeper strength increase compared to dry specimens. A dry density of 0.64 g/cm³ ensures a softening coefficient exceeding 0.75, confirming the material’s suitability for humid environments. The material contains predominantly small pores (90% ≤ 0.2 mm diameter), with improved bubble distribution at the edges and higher upper porosity. Spherical pores (roundness 0.5–1) enhance mechanical properties, while phosphogypsum (optimal 10% dosage) effectively improves both strength and workability but requires corrosion control due to its hydration products.

1. Introduction

Despite significant progress in the comprehensive utilization of phosphogypsum globally, its overall utilization rate remains relatively low compared to the increasing discharge and accumulation of phosphogypsum every year. This discrepancy primarily stems from the inherent characteristics of phosphogypsum and limitations in product applications. Phosphogypsum contains impurities such as phosphorus and fluorine, which, when used directly, result in poor water resistance, low strength and high corrosivity, which severely restrict its wide application. An ideal phosphogypsum utilization strategy should involve large consumption, no secondary pollution as well as simple operation. However, current research on using phosphogypsum as the main raw material of foamed lightweight soil for road application has yet to fully address these challenges, hindering its widespread engineering implementation. In recent years, scholars all over the world have conducted extensive research on phosphogypsum-based foamed lightweight soil materials.

Traditional gypsum-based foam lightweight soil is desulfurized gypsum, whose main component is CaSO₄·2H₂O with a purity typically ≥ 93%. As a byproduct of flue gas desulfurization (FGD) processes, desulfurization gypsum exhibits a chemical composition similar to natural gypsum, rendering it a viable alternative to cement. Que et al. used desulfurized gypsum to replace a proportion of cement to vary the desulfurization gypsum content (0%–30%) in order to evaluate the fluidity, unconfined compressive strength, durability and morphological characteristics, focusing on its applicability for subgrade backfilling. The results revealed that while increasing desulfurized gypsum content led to reduced flowability (175~183 mm) and unconfined compressive strength (0.75~2.75 MPa), all mixtures satisfied embankment requirements. Notably, the addition of desulfurized gypsum enhances material plasticity and failure mode, thereby broadening its potential engineering applications [1].

The foamed phosphogypsum lightweight material was prepared through phosphogypsum dehydration and the addition of a retarder, water reducers and mixing water. Yang et al. studied the mechanical properties and thermal conductivity characteristics of the material [2]. Wang et al. developed a phosphogypsum lightweight material with a composition ratio of phosphogypsum/fly ash/cement/Ca(OH)₂ = 49:20:25:6, achieving a compressive strength of 1.7 MPa at a density of 521.7 kg/m³ [3]. Zheng et al. employed a mixture of gypsum hemihydrate and phosphogypsum at a ratio of 7:3 as the main raw material. They mixed with foam, cement, lime and silica fume to prepare a lightweight phosphogypsum material with a dry density of 809.1 kg/m. Its compressive strength was 3.15 MPa at 328 days [4]. Samson et al. conducted comprehensive studies on the physical and mechanical properties and durability of high-content phosphogypsum foamed lightweight soil. Through systematic mix proportion tests, they examined the effects of slag and fly ash on phosphogypsum foamed light soil and determined the best mixing ratio. Finally, they prepared phosphogypsum-cement-slag foamed lightweight soil and phosphogypsum-cement-fly ash foamed lightweight soil [4]. Zheng et al. established a base mixture ratio of m (cement):m (GBFS):m (phosphogypsum) = 20:65:15, the water–binder ratio was fixed at 0.32, 2.5% sodium silicate with modulus of 1.2 was added as the activator, and the dosage of water reducer was 0.5%, all of which were based on the mass of cementitious materials. Phosphogypsum was added into the foamed lightweight soil by replacing GBFS with equal mass [5]. Wang et al. enhanced phosphogypsum’s strength and water resistance by incorporating cementitious additives including fly ash, ground slag and Portland cement. Through the addition of varying foam volumes (up to 60% by volume), PG-based lightweight building materials with a compressive strength of 1.7 MPa, a bulk density of 521.7 kg/m³ and a thermal conductivity of 0.0724 w/(m k) can be prepared [6].

Phosphogypsum can be classified into three forms according to the production method: anhydrous, semihydrate and dihydrate. In China, wet dihydrate phosphoric acid production accounts for 80% of the total output [7]. However, phosphogypsum exhibits distinct characteristics from desulfurized gypsum in forming foamy lightweight soils. The insoluble impurities in phosphogypsum are primarily composed of co-crystalline phosphorus pentoxide, quartz sand, phosphorus-lime and organic matter, while soluble impurities are soluble phosphorus pentoxide, soluble fluorine, sodium and potassium ions and some heavy metal elements, which seriously affect the performance of phosphogypsum-based materials. Through calcination at 180 °C, raw phosphogypsum can be converted into phosphorus building gypsum. During the calcination process, many impurities that are not conducive to foamy lightweight soil are removed, and the proportion of phosphate dihydrate is reduced. Consequently, phosphorus building gypsum is used to ensure the strength and performance requirements of foamed lightweight soil and to ensure the recycling of phosphogypsum.

In the production of foamed gypsum-based composites, the methods adopted include the chemical foaming method and the physical foaming method. The physical method involves the direct incorporation of pre-foamed air bubbles into the gypsum matrix, whereas the chemical method relies on gas generation through the chemical reaction of the added foaming agent to generate foam in the gypsum matrix [8]. The foamed light soil prepared in this paper adopts the physical foaming method. After hardening, there are still a lot of pores in the foamed light soil, but it is not accompanied by pore water pressure. The pore structure represents the most critical factor governing the material’s compressive strength. Typically, foamed lightweight fillers usually contain two types of pores: primary pores, which are small and formed during the curing process of cement materials, and artificial bubbles, which are large and produced by various foaming methods. While primary pores are small and inevitable, the mechanical properties of foamed lightweight fillers with the same porosity may vary greatly due to the different parameters such as the size, distribution and shape of artificial bubbles.

To comprehensively investigate the hydrological performance of this subgrade material, the deterioration pattern of the strength and performance of the foamed light soil was further revealed by measuring the water absorption, softening coefficient and pore structure. The present study employs phosphorus building gypsum as a partial cement replacement rather than phosphogypsum, because the use of phosphogypsum will cause the foamed lightweight soil to have a shapeless and serious phenomenon of defoaming, and the degree of this phenomenon occurring in phosphorus building gypsum is significantly less than that of phosphogypsum. This paper provides substantial theoretical support and practical references for implementing foamed lightweight soil in roadbed engineering applications. Furthermore, phosphorus building gypsum can also be studied and applied on a large scale [9].

2. Experimental Test

2.1. Experimental Materials

The experimental study mainly employed the following raw materials: cement, GBFS, phosphorus building gypsum, foam and water.

(1)

Cement: 42.5 sulfoaluminate cement.

(2)

GBFS: WISCO slag powder, S95 grade.

(3)

Phosphorus building gypsum: Juhai brand phosphorus building gypsum.

The foaming system consisted of the compound foaming agent [10], which was diluted with water at a ratio of 1:40 for later use. A polycarboxylate-based superplasticizer with a 20% and 18% water reduction rate was employed to enhance workability.

2.2. Experimental Ratio

The preparation process of foamed lightweight soil encompasses soil sample treatment, material mixing, foaming agent preparation, foam mixing and molding maintenance. Material mixing: Initially, the raw materials are combined in a cement mixer at a low speed for 3 min according to the mixture ratio designed in

Table 1. Test mix ratio of foamed lightweight soil.

Type	Phosphorus Building Gypsum (%)	Cement (%)	GBFS (%)	Water (%)	Water Reducing Agent (%)	Foam (%)	Mobility (cm)
First group	6.8	6.8	55.2	28.3	0.4	2.5	220
Second group	6.7	9.1	53.8	27.6	0.4	2.4	210
Third group	6.4	12.8	51.6	26.5	0.4	2.3	190

[6]. This step ensures thorough blending of the components. Subsequently, once the foam is generated, it is gradually introduced into the previously stirred mud. The mixture is then stirred at a low speed for an additional 3 min to ensure uniform distribution of the foam within the slurry. Then, a liquidity cylinder (80 mm × 80 mm) foamed lightweight soil test mold is used, in conjunction with a ruler to measure the fluidity of the foamed lightweight soil. Finally, the prepared mixture is injected into the pre-arranged mold. After curing the sample at room temperature for 28 days, it is demolded and subsequently subjected to various performance tests.

According to the current specification DB41/T 2280-2022 [11], the fluidity of foamed lightweight soil is stipulated to be within the range of (180 ± 20) mm. As evidenced by the flow rate data obtained from the three experimental groups presented Table 1, it is apparent that an increase in the incorporation ratio of phosphorus building gypsum leads to a continuous decline in flowability. Specifically, when the incorporation ratio of phosphorus building gypsum reaches 6.4%, the fluidity of the sample is 190 mm, thereby meeting the requirements set forth in the aforementioned specification. Consequently, all samples utilized in the following article for 28 days of hardening are all in the third set of ratios.

2.3. Test Content and Equipment

2.3.1. Water Absorption Test

The specimens used in this test are cubic in shape, with dimensions of 70 mm × 70 mm × 70 mm. Initially, the mass of each specimen is recorded. Subsequently, the specimens are placed in a water tank such that they are fully submerged, with the water depth exceeding the height of the specimens. After a predetermined soaking period, the specimens are removed from the water tank, and moisture on the surface of the specimen is wiped off. The mass of each specimen following water absorption is recorded. After soaking until the seventh day, the test is stopped. The mass water absorption is calculated using the following formula:

$$\omega =({\mathrm{m}}_1-{\mathrm{m}}_0)/{\mathrm{m}}_0$$

(1)

where $\omega$ represents the mass water absorption rate (%), ${\mathrm{m}}_1$ represents the mass after water absorption (g) and ${\mathrm{m}}_0$ represents the constant weight mass of the specimen (g).

2.3.2. Test of Unconfined Compressive Strength of the Test Block

According to the current specification DB41/T 2280-2022, the unconfined compression meter must not only comply with the provisions outlined in GB/T 2611 [12] but also exhibit an accuracy of no less than ±2%. Furthermore, the selected measurement range should ensure that the expected maximum failure load of the specimen is within the range of 20%~80% of the full range. In this experiment, the Tianchang Tongda brand unconfined compressor (Beijing TianchangTongda Instrument Co., Ltd., Beijing, China), as depicted in Figure 1, was employed to meet the requirements for the unconfined compressor meter. According to DB41/T 2280-2022, the unconfined compressive strength of foamed lightweight soil should exceed 0.3 MPa.

The test steps of the compressive strength test are detailed as follows: (a) Inspect the appearance of each specimen, ensuring that the surface of the specimen is flat and free from cracks or noticeable defects. (b) Measure the dimensions of each specimen with an accuracy of 1 mm and calculate the pressure-bearing area accordingly. (c) Position one specimen at the center of the lower platen of the material testing machine, ensuring that the pressure-bearing surface of the specimen is perpendicular to the top surface of the mold during formation. (d) Activate the unconfined pressure meter; when the upper pressure plate is close to the specimen, the base should be adjusted to make the specimen contact evenly. (e) Apply a continuous and uniform load at a speed of 0.15 kN/s until the specimen fails and record the failure load. (f) Repeat steps a to e for each specimen, measure and record the compression area and failure load of each specimen and calculate the unconfined compressive strength.

2.3.3. Observation of Pore Structure Inside the Test Block

Given that the test block is a cube of 70 mm × 70 mm × 70 mm, its relatively small size renders it unsuitable for sectioning using a mechanical cutting device. Consequently, the test block is manually sectioned into a flat plane using a handsaw. The resulting cross-section is then examined using a Keyence VK-X3000 Series Instruments, Shanghai, China, as illustrated in Figure 2. The displayed images are observed and recorded for further analysis.

3. Result Analysis

3.1. Trends and Limits of Water Absorption

To investigate the water absorption characteristics of foamed lightweight soil, this study examines the variation in water absorption over time for foamed lightweight soil specimens with different proportions and dry densities, as depicted in Figure 3. The results reveal that water absorption initially increases with time, followed by a plateau phase where the rate of increase diminishes. Notably, a significant rise in water absorption occurs within the first day of immersion, while a steady increase is observed after two days. When the immersion duration reaches three days or longer, the water absorption rate stabilizes and remains essentially constant. According to the test data, it can be concluded that the water absorption of the test block is closely related to their dry density. Specimens with lower dry density exhibit higher water absorption due to the presence of a greater number of artificial pores. Specifically, when the dry density reaches about 0.85 g/cm³, the water absorption can reach 40% in seven days.

3.2. The Size of Unconfined Compressive Strength of Dry Test Block

A series of dry test blocks were selected, and their corresponding masses were recorded prior to conducting unconfined compressive strength tests. During these tests, the specimens were subjected to a uniform loading rate of 0.15 kN/s until failure occurred. Observations revealed that the foamed lightweight soil exhibited distinct cracking upon failure, as illustrated in Figure 4 and Figure 5. The peak load applied at this time of failure was recorded, and then the unconfined compressive strength was calculated.

Based on the experimental data, a discernible relationship exists between the unconfined compressive strength of the test block and their corresponding dry densities. To elucidate this relationship, the dry density and strength data were plotted on a coordinate system, as shown in Figure 6. The results indicate an approximately linear correlation satisfying $\mathrm{y}=0.946955+1.11451\mathrm{x}$, where $\mathrm{y}$ is the unconfined compressive strength and $\mathrm{x}$ is the dry density. It is evident that a linear relationship exists between the unconfined compressive strength and the dry density of the test block. Specifically, the test block with the lowest dry density among all tested mix proportions exhibits an unconfined compressive strength of 1.3 MPa, which satisfies the specification requirement of being greater than 1 MPa. This result confirms that the strength of the experimentally prepared test blocks complies with the specification, indicating the feasibility of the adopted mix ratio under dry conditions. However, the foamed lightweight soil developed in this study is primarily intended for pavement reconstruction projects with substantial material demands. Given that some projects are located in regions characterized by high humidity, frequent precipitation and low-lying terrain, the material cannot be expected to remain in a dry condition for extended periods. Therefore, it is necessary to calculate the softening coefficient of the foamed lightweight soil to ensure that it is a water-resistant material.

3.3. The Size of Unconfined Compressive Strength of Soaked Test Block

A series of test blocks were subjected to a 7-day water immersion period, as depicted in Figure 7 and Figure 8. The greenish surface discoloration observed is attributable to the hydration reaction of cement. Prior to conducting unconfined compressive strength tests, the corresponding dry densities of these specimens were recorded. During testing, each specimen was uniformly loaded at a rate of 0.15 kN/s until failure occurred. Upon failure, the foamed lightweight soil specimens exhibited slight cracking, but the crack did not fully penetrate the component. Instead, only partial localized spalling was observed, accompanied by pronounced flattening of the specimens, as illustrated in Figure 9 and Figure 10. The peak applied load value at the point of failure was recorded for each specimen, enabling subsequent calculation of the unconfined compressive strength.

The experimental results reveal a significant correlation between the unconfined compressive strength of the test block and its mass. To further elucidate this relationship, the dry density and corresponding strength values were plotted on a coordinate system, as illustrated in Figure 11. The relationship between the two is approximately expressed by $\mathrm{y}=0.212772+1.98234\mathrm{x}$ where $\mathrm{y}$, $\mathrm{x}$ is the dry density. A linear relationship is evident between the unconfined compressive strength and the dry density of the test block. However, a substantial disparity in strength values is observed, with the measured strengths significantly exceeding those predicted by the functional relationship established for dry test blocks. Notably, the unconfined compressive strength of test blocks with the lowest dry density across all proportions is only 0.8 MPa < 1 MPa after soaking in water for seven days, which falls short of the requisite strength standards. Consequently, it is imperative to impose dry density specifications for all the test blocks prepared based on the proportion. Through the function fitted by the coordinate system, a dry density of approximately 0.4 g/cm³ is determined. To ensure that the unconfined compressive strength is still 1 MPa after soaking in water for seven days, considering that the strength of the foamed lightweight soil prepared by the experiment is only 0.8 MPa at a dry density of 0.4 g/cm³, the dry density should be increased to 0.5 g/cm³ to meet the strength requirements. Preliminary observations reveal that the strength of the specimens increases with rising dry density following water immersion. This phenomenon can be attributed to the fact that specimens with high dry density contain fewer artificial bubbles and are less dispersed. Consequently, the pores are less prone to fusion and penetration, resulting in a more uniform matrix skeleton. This uniformity mitigates the formation of internal stress concentration under external load, thereby enhancing the bearing capacity and leading to a significant increase in unconfined compressive strength with increasing dry density.

3.4. Calculation of Softening Coefficient of Foamed Light Soil

3.4.1. Definition and Formula of Softening Coefficient

The softening coefficient serves as an indicator of the properties of water resistance and is expressed as $K=\mathrm{f}/F$, where $K$ represents the softening coefficient of the material; $\mathrm{f}$ denotes the unconfined compressive strength of the material in the water-saturated state, measured in MPa; and $F$ signifies unconfined compressive strength of the material in the dry state, also measured in MPa.

3.4.2. Requirements for Softening Coefficient of Foamed Lightweight Soil

According to the current specification of foamed lightweight soil, the unconfined compressive strength following a three-day water immersion period is typically considered representative of the material’s strength in the water-saturated state. However, in this study, the softening coefficient is calculated with the unconfined compressive strength after soaking in water for seven days, to ensure a more rigorous assessment of the material’s water resistance. When determining the softening coefficient, it is essential to compare the unconfined strength before and after immersion, selecting those of the highest quality for analysis. Given the inherent variability in component quality, which cannot be controlled to remain almost constant, comparative analysis using coordinate axis values is employed. In particular, when analyzing the unconfined strength values post-immersion, it is observed that the greater the mass, the lesser the impact of component softening due to water absorption. Therefore, focusing on the softening coefficient at the minimum mass analyzed, the softening coefficient can be guaranteed to meet the standard. For instance, when the unconfined compressive strength of the dry test block of the 140 g specimen is 1.38 MPa, the unconfined compressive strength of the soaked test block is 0.867 MPa, and the softening coefficient is merely 0.628. To further refine this analysis, the unconfined compressive strength of both dry and soaked test blocks is represented by two fitting linear functions, and the equation can be obtained. Solving this equation for a dry density of 0.80 g/cm³ reveals that when the softening coefficient exceeds 0.8, the material can be considered as water-resistant. Such materials are suitable for applications in environments characterized by high humidity, rain, low terrain and conditions where prolonged dryness cannot be maintained.

3.5. Analysis of Internal Pore Structure

Following the processing and analysis of the captured images using MATLAB 2024, the subsequent charts are obtained, as depicted in Figure 12, Figure 13 and Figure 14, which, respectively, represent the three sections of the lower, middle and upper parts of a test block.

3.5.1. Pore Size Distribution

As illustrated in the figure, the pore size distribution of the specimen exhibits a predominantly decreasing trend, with a higher prevalence of smaller pores and a gradual reduction in large pore size. Notably, pores with diameters below 0.2 mm constitute the majority of the total porosity. The parameter $D90$, representing the pore diameter at which the cumulative pore size reaches 90%, serves as a critical indicator. This metric signifies that 90% of the pores in the phosphorus-building-gypsum-based cement foamed lightweight soil have diameters smaller than the D90 value. Analysis reveals that the D90 values for the lower, middle and upper sections of the specimen are approximately 0.2 mm. However, non-uniform pore size distribution can lead to structural instability. The presence of interconnected pores and a thin pore wall, resulting from heterogeneous pore sizes, may compromise the mechanical integrity of the material. This phenomenon ultimately contributes to reduced compressive strength and overall structural instability in the material system.

3.5.2. Pore Connectivity

As depicted in the figure, the porosity of the specimen exhibits an increasing trend from the bottom to the top. Additionally, pore connectivity improves towards the edge of the specimen. This phenomenon can be attributed to the inverted demolding process, which causes the pores to be displaced and redistributed. During the evaporation of water, some pores undergo defoaming, with a portion being carried upwards to the upper regions of the specimen. Consequently, this results in a higher overall porosity in the upper part and a lower overall porosity in a lower part. Conversely, the overall porosity remains approximately 10%. As a result, the lower part is denser and the stronger is slightly greater than the upper section.

Currently, mainstream visual analysis software available in the market lacks a dedicated module for quantitative analysis of pore connectivity. Therefore, to quantitatively characterize the connectivity features among the microscopic pores within foamed lightweight soil, this study draws on a pore connectivity evaluation methodology. This approach classifies the internal pores of foamed lightweight soil into four grades, each corresponding to different connectivity degrees: Grade 0 represents isolated pores (isolated closed pores inside foamed lightweight soil), Grade 1 represents connected pores (connecting pores with inlet and outlet on the same surface), Grade 2 represents connected pores (connecting pores with inlet and outlet on two adjacent surfaces) and Grade 3 represents connected pores (connecting pores of the inlet and outlet on two opposite surfaces) [13]. Notably, the pore structure of the sample exhibits a near absence of Grade 0 isolated pores, indicating a high level of pore connectivity.

3.5.3. Roundness of Pores

The pore morphology significantly influences the properties of foamed lightweight soil. By defining the elongation index, the pore morphology of gypsum-based foamed light soils can be quantitatively analyzed. Elongation (I/L) is defined as the ratio of pore width to length. According to the elongation index, the pores of steel slag micronized cement foamed light soil can be categorized into four types: (1) sheet satisfaction (0 < I/L < 0.33), (2) cake satisfaction (0.33 < I/L < 0.67), (3) elliptical satisfaction (0.67 < I/L < 1) and (4) spherical satisfaction (I/L = 1). When the pore elongation approaches 1 and the pore shape is closer to a sphere, the pore morphology of foamed lightweight soil under varying influencing factors can be further characterized by defining the sphericity value. The sphericity value is defined as the ratio of the pore surface area to the spherical surface area corresponding to the pore volume. The closer the sphericity value is to 1, the more closely the pore shape resembles a sphere, resulting in more uniform the force distribution and superior load-bearing capacity.

As is evident from the figure, the elongation index of the aperture is concentrated around 2, with roundness values ranging between 0.5 and 1. Notably, pores with larger size have a greater contact area with the surrounding slurry, making them more susceptible to deformation under stress. Phosphorus-building-gypsum-based foamed lightweight soil with roundness between 0.5 and 1 demonstrates superior pore morphology, with the majority of pores approximating a spherical shape. Spherical pores have good mechanical properties, which contribute significantly to enhancing the mechanical properties of foamed lightweight soil. However, it is important to note that an excessive increase in pore roundness leads to greater exposure to air, which in turn can result in pore deformation when subjected to stress.

4. Effect of Phosphorus Building Gypsum on Foamed Light Soil

The insoluble impurities in phosphogypsum primarily consist of co-crystalline phosphorus pentoxide, quartz sand, phosphorus-lime and organic matter, whereas the soluble impurities include soluble phosphorus pentoxide, soluble fluorine, sodium and potassium ions, and heavy metal elements. Phosphorus building gypsum undergoes calcination at 180 °C, followed by sieving to eliminate large-particle impurities and reduce phosphorus and fluorine content. This process also combines bound water and free water, leading to a reduction in both the particle size and elemental distribution density of phosphogypsum [14].

Phosphogypsum is predominantly employed as a retarder and gelling material in cement production, offering a dual benefit of effectively reducing production costs and environmental pollution. As a raw material for cement and cemented materials, lime primarily serves a consolidating function in foamy light soil, enabling colloidal materials to achieve tight bonding [15]. Quartz sand contributes to the formulation by filling voids and improving the initial synergy of slurry. Additionally, it reacts with cement to form a gel, thereby improving strength and durability, improving the flow of the slurry.

Calcium diphosphate, when utilized as an additive in cement, serves to regulate the setting time and enhance the compressive strength of the material. It effectively adjusts the hardening rate and performance of cement by reacting with minerals constituents within the cement to form stable compounds. This reaction contributes to the improvement of cement in the early and late stages. By incorporating an appropriate amount of dicalcium phosphate, the workability of concrete is enhanced, while improving its crack resistance and durability.

Phosphorus construction gypsum requires up to 60% water to reach the standard consistency, in contrast to cement, which requires only 30%. This indicates that phosphorus building gypsum, to a certain extent, increases the water consumption of foamed lightweight soil. Notably, phosphorus building gypsum itself exhibits a loose porous plate-like structure with slits, which provides space for the mixing of artificial bubbles and protects them from cracking. It has been observed that when water consumption is insufficient, the slurry fails to form properly, causing all materials to aggregate directly into flocs. Conversely, excessive water consumption leads to an overabundance of moisture, allowing water to penetrate the bubbles before they are fully formed. This premature water ingress results in bubbles dissipating and rupturing, causing defoaming.

The influence of soluble phosphorus and fluorine on cement setting time mainly stems from their reaction with calcium hydroxide generated during cement hydration, resulting in the formation of insoluble phosphorus and fluorine salts. These salts adhere to the surface of cement clinker particles, thereby retarding the hydration rate of cement and prolonging its setting time. It can be seen that phosphorus building gypsum can play a positive role in foamed lightweight soil to a certain extent. By delaying solidification, it enhances the material’s strength, integrity and stability, while also optimizing water consumption and providing space for bubble mixing. However, excessive use of phosphorus building gypsum can lead to over-fluidity of the mixture. Therefore, controlling the proportion of phosphorus-containing building gypsum is of paramount importance [16].

5. Conclusions

(1)

After a seven-day water immersion period, foamed lightweight soil with varying dry densities exhibits distinct water absorption rates. Specifically, lower dry density corresponds to higher water absorption rate, attributed to the prevalence and dense distribution of artificial bubbles that facilitate pore merging and connectivity.

(2)

Overall, the unconfined compressive strength of dry specimens demonstrates a linear correlation with dry density, with an increase in dry density corresponding to a rise in unconfined compressive strength.

(3)

Similarly, the unconfined compressive strength of water-soaked specimens is linearly related to dry density, showing an upward trend with increasing dry density. Notably, when the dry density reaches 0.5 g/cm³, the unconfined compressive strength meets the requirements after seven days of immersion. Furthermore, the rate of increase in unconfined compressive strength of soaked blocks with the change in dry density is much greater than that of dry blocks.

(4)

When the dry density is 0.64 g/cm³, the softening coefficient remains above 0.75, satisfying the criteria for water-resistant materials. This characteristic renders the material suitable for deployment in high-humidity, rainy and low-terrain environments where prolonged dry conditions cannot be maintained.

(5)

Artificial bubbles with diameters of 0.2 mm or less constitute approximately 90% of the total pores. The connectivity and distribution of these bubbles are superior around the test block periphery compared to the central region, with porosity being higher above the test block than below. Pore roundness ranges from 0.5 to 1, with closer approximation to a spherical shape yielding superior stress distribution and mechanical properties.

(6)

Phosphorous building gypsum generally benefits foamed light soil by enhancing its integrity and strength, delaying the cement setting time when added and providing a porous, plate-like structure that supports foam existence. However, the hydration products of phosphorus building gypsum can corrode foamed lightweight soil, necessitating a controlled dosage of approximately 10%.

(7)

Despite the valuable insights gained from this study, several limitations persist, including the lack of a comparative analysis between building gypsum and phosphogypsum at equivalent dosages, the absence of large-scale preparation and application to obtain errors in practical application, and the unexplored effects of different superplasticizers on experimental outcomes. Future research endeavors should aim to address these shortcomings, focusing on increasing the incorporation rate while reducing cement usage and conducting more in-depth and nuanced investigations.