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Article

Modification and Stabilization of Collapsible Loess Using Diammonium Phosphate Solution

by
Chengjuan Ying
1,
Lingxia Huang
2,*,
Haiming Chen
3,*,
Yadong Zhang
3 and
Duoxi Yao
1
1
School of Earth and Environment, Anhui University of Science and Technology, Huainan 232001, China
2
College of Civil Engineering, Fuzhou University, Fuzhou 350025, China
3
Engineering Research Center of Underground Mine Construction, Ministry of Education, Anhui University of Science and Technology, Huainan 232001, China
*
Authors to whom correspondence should be addressed.
Crystals 2024, 14(5), 426; https://doi.org/10.3390/cryst14050426
Submission received: 5 April 2024 / Revised: 19 April 2024 / Accepted: 22 April 2024 / Published: 29 April 2024
(This article belongs to the Section Inorganic Crystalline Materials)
Browse Figures
Versions Notes

Abstract

:
The collapsible loess will rapidly soften and lose its bearing capacity when soaked in water. Under a mild condition (20 °C), the biomimetic inorganic agent, diammonium phosphate (DAP), reacts with calcite in the collapsible loess, producing a stronger bonding material, hydroxyapatite (HAP), to modify and stabilize the soil. Uniaxial compression, permeability tests, and morphological analysis using X-ray diffraction and scanning electron microscopy equipped with an energy X-ray dispersive system were used to assess the effectiveness of DAP stabilization on the collapsible loess. The results indicated that HAP improved the inter-particle bonding within the loess, filled the pores within particles, reduced the permeability, and consequently mitigated the collapsibility of the loess. The compressive strength of the DAP-treated loess increased as DAP concentration increased. Following 28 days of curing, the compressive strength of the loess treated with a 3.0 mol/L DAP solution was six times greater than that of the untreated group. DAP’s reinforcement effect on the loess was superior to that of cement. The compressive strength of the DAP-treated loess was about double that of the cement-treated loess and the permeability coefficient was reduced by more than 50% at equivalent solid content. Furthermore, DAP generated 82% fewer carbon emissions compared to Portland cement. Considering eco-friendly and sustainable development, DAP offers a more competitive alternative for modification and stabilization of loess.

1. Introduction

Loess, widely distributed in the Loess Plateau in Western China [1,2], forms as wind-deposited sediment under the arid and semi-arid climatic conditions of the Quaternary period [3,4,5]. Grain-size analysis indicates that the grain-size fraction of loess ranges from 0.02 to 0.05 mm. Silt-size particles are about 50 weight percent of loess. Clay-sized particles (less than 0.005 mm) make up another 5 to 10 percent. These silt-size particles compose the skeleton particles in loess and are loosely cemented by calcium carbonate. As a result, loess is loose and porous with strong permeability. The word “loess”, with connotations of origin by wind-deposited accumulation, comes from German word for “loose”.
Despite demonstrating high strength in arid conditions, loess exhibits extremely poor water stability and unique collapsibility [6,7,8]. When exposed to water, the bonding materials between the skeletons of loess particles dissolve and fail, leading to internal structural disintegration. Consequently, the strength rapidly diminishes, and uneven additional subsidence occurs due to external loads or self-weight. This phenomenon can result in unstable building foundations and structural damage in engineering projects. Although loess is widely used as filling material in foundations [9], its susceptibility to collapse poses significant challenges for construction.
Dynamic compaction and cement/lime modification are two traditional soil improvement methods of collapsible loess foundations. Dynamic compaction uses the dynamic effect of a heavy hammer’s impacts to enhance the loess’s stability. The process encourages a denser rearrangement of the loess particles’ skeleton, with gradual infilling of the pores. However, dynamic compaction cannot address the solubility of the bonding materials between loess particles. Even after compaction, the loess can still exhibit collapsibility once it is exposed to water for a period.
Cement or lime can effectively modify the structural properties of loess and enhance its mechanical properties [10]. This soil stabilization method involves mixing lime or cement into the soil to increase its strength. Cement stabilizes the collapsible loess through various chemical reactions, including ion exchange, cement hydration and hardening, flocculation and aggregation, etc. Horpibulsuk et al. [11] studied the changes in the microstructure of cement-modified silty clay after 7 days of curing. Their findings indicated that the bonding between soil particle aggregates was enhanced and the soil’s pores were diminished because of cement hydration, which consequently improved the soil’s strength and structural properties. Lemaire et al. [12] observed the microstructure of lime- and cement-modified plastic silty soil for a 28-day curing time. Following the addition of cement, the soil particles were completely covered and stronger cemented by a layer of bonding material, resulting in a significant enhancement of the modified soil’s strength and water stability [13].
Although cement modification can effectively mitigate the collapsibility of loess [14], the production of cement is energy-intensive and results in the substantial emission of carbon dioxide into the atmosphere. Cement, as a primary source of carbon emissions, is responsible for about 9% of the total industrial carbon emissions in China. The Chinese government has actively pledged to peak carbon dioxide emissions by 2030 and to realize carbon neutrality by 2060 [15]. To achieve these objectives, it is essential to explore innovative eco-friendly stabilization methods or materials for collapsible loess that could serve as a sustainable alternative to cement [16,17,18]. These methods also enable conserving resources, safeguarding the environment, diminishing expenditures, and shortening the construction timeline [19,20].
The loess’s engineering properties are significantly affected by the type and the interparticle bonds between the skeleton of loess particles and aggregates [21,22,23]. In loess, the carbonate cementing material mainly exists in the form of CaCO3 (calcite), significantly contributing to the soil’s strength [24,25,26]. Furthermore, calcium carbonate is a highly effective binder that can react with phosphates [27]: phosphate ions can interact with calcium ions released from calcite in aqueous solutions, resulting in the precipitation of Ca10(PO4)6(OH)2 (hydroxyapatite, HAP) crystals.
HAP, which possesses the chemical formula Ca10(PO4)6(OH)2, serves as an inorganic repair and modification material with broad application prospects [28]. HAP is hexagonal P63/m, and calcite is trigonal R3C. HAP has better chemical stability, higher strength and stiffness, and a lower solubility than calcite. In some ancient monuments and churches, it has been discovered that some oxalate and apatite coatings were added artificially to protect the carbonate substrates from environmental erosion [24,29]. Yang et al. [30] and Sena da Fonseca et al. [31] have treated marble by brushing, dripping, or immersing it in diammonium phosphate (DAP, with the chemical formula (NH4)2HPO4) solutions to prepare the precipitation of HAP. The generated protective HAP coatings have a good adhesion strength and effectively improve the acid and alkali corrosion resistance of marble. E. Sassoni et al. [32] have experimentally demonstrated that phosphates can also effectively stabilize soil materials, to prevent sand entering oil wells in unconsolidated or poorly consolidated reservoir formations during production. Christakis A. et al. [33] alternated between injection, mixing, and two aqueous solutions of calcium chloride and potassium phosphate in these formations. At 25 °C, the reaction of two solutions formed sufficiently uniform coatings of calcium phosphate crystals to bind sand particles. Iosif T. Hafez et al. [34] investigated the interactions of Polyelectrolytes (PEs) with sand particles or HAP crystals. In supersaturated solutions, PEs provide active sites for the nucleation composite of HAP through the adsorption onto the silicate surface. Eventually, the formed precipitation of composite HAP–PE binds together with sand particles to stabilize the loose formations.
Another commonly used consolidation or stabilization method of soil involves reacting DAP with either the soil itself or externally added carbonates to generate HAP for modifying and stabilizing the soil. Under mild temperature conditions (10–20 °C), the DAP solution can react with calcium ions to form HAP complexes [25], which improve the interparticle bonding strength in soil. This effectively increases the strength of the binder between silicate soil particles and fills the surrounding pores. The chemical reaction is as follows [25]:
10CaCO3 + 5(NH4)2HPO4 → Ca10(PO4,CO3)6(OH,CO3)2 + 5(NH4)2CO3 + 3CO2 + 2H2O
where PO43− and OH− can be partially replaced by CO32−.
The resulting HAP is typically non-stoichiometric, as it contains carbonate ions, and its precipitation is expected to be preceded by the formation of several intermediate metastable phases [25]. The formation process of HAP complexes is mild and non-exothermic, allowing it to fill pores without causing additional soil deformation. Loess, being a natural weakly alkaline saturated soil rich in calcium carbonate, can sufficiently react with DAP and generate HAP complexes without the necessity of adding an external calcium source. However, during the precipitation reaction, if a large amount of phosphates binds to the surfaces of carbonate-containing soil particles through adsorption and ion exchange, it may reduce the activity of surface calcium ions. This, in turn, may promote the formation of various amorphous calcium phosphate (CP) phases rather than HAP, diminishing the reinforcing effect on the soil. In the precipitation reactions of HAP complexes, it is generally believed that the calcium-to-phosphorus (Ca/P) molar ratio of approximately 1.18 is conducive to the formation of amorphous calcium phosphate [35,36]. Increasing the Ca/P molar ratio during the reaction can favor the formation of HAP, and at Ca/P molar ratio of 1.5, calcite crystals can be completely transformed into HAP [35].
In the present work, we aimed to assess the impact of DAP stabilization on the collapsible loess. To achieve this, the remolded samples were mixed with DAP solutions of varying concentrations, specifically 0.5, 1.0, 1.5, 2.0, and 3.0 mol/L. Additionally, cement-treated loess samples with different cement contents were also prepared under the same curing conditions for comparison. The stabilized performance of the treated loess samples was compared through uniaxial compression tests and permeability tests. Finally, by utilizing micro-analytical techniques such as X-ray diffraction (XRD) and scanning electron microscopy equipped with an energy X-ray dispersive system (SEM/EDX), the structural characteristics of the modified loess were examined to verify the effect and mechanism behind the reaction between DAP solution and calcium carbonate in loess to generate HAP for reinforcing the loess. The research findings offer a novel and competitive option for stabilizing collapsible loess grounds, characterized by efficiency, energy savings, and environmental friendliness.

2. Materials and Methods

2.1. Materials

In this study, analytical-grade diammonium phosphate (DAP) was used. It appears as colorless, transparent monoclinic crystals. It has a solubility of 58 g at 10 °C, and the resulting aqueous solution is alkaline with a pH of 8.0. The cement used in the experiment was P.O 42.5, whose density was 1910 kg/m3 and specific surface area was 1.447 m2/g. It exhibits low shrinkage during the hardening process, effectively mitigating the occurrence of shrinkage cracks and ensuring reliable test outcomes.
The loess utilized in the experiments was sourced from Lanzhou, Gansu, in Western China, and was classified as silty clay. It falls under the category of fine-grained clay, characterized by its loose appearance without stratification and well-developed pores within its matrix. The soil’s bulk unit weight is 1.43, with a specific gravity of 2.71. It has a plastic limit (wp) of 15.1%, a liquid limit (wl) of 25.2%, and a plasticity index (Ip) of 10.1. The distribution of grain sizes for this loess is detailed in Table 1. The loess’s natural porosity ranges from 52% to 55%, indicating a high potential for collapsibility.
The primary chemical composition of the loess, as illustrated in Table 2, includes calcite/dolomite and clay minerals, primarily illite and montmorillonite, along with minor amounts of kaolinite. These components loosely bond the silicate particles together, forming various aggregates of loess. The original soil sample underwent crushing and sieving through a 0.65 mm square-hole sieve to eliminate impurities such as stones and roots. It was then mixed with the stabilization agents and remolded by compaction to prepare the loess samples for further experimental research.

2.2. Methods

To assess the stabilization effects of various agents on the loess, experiments were designed in accordance with the “Standard for Geotechnical Testing Method (GB/T 50123-2019)” [37]. The experimental design included the following groups:
  • The blank control group (UT) served as the reference without any form of stabilization;
  • The cement-treated groups (CM-4, CM-6, and CM-8) employed various cement contents to modify and stabilize the loess;
  • The DAP-treated groups (DT-0.5, DT-1.0, DT-1.5, DT-2.0, and DT-3.0) employed different concentrations of DAP solution.
Table 3 provides the specific mix proportions of stabilization agents for each experimental group. DAP treated the loess with its aqueous solution. With a DAP concentration of 3.0 mol/L, the solid content of DAP within the treated loess samples approximated 6.3% by weight. During the reaction between DAP and the soil’s calcium carbonate, a Ca/P molar ratio of 4.8 was maintained, theoretically allowing for complete conversion to HAP. Considering the time required for the release of calcium ions from calcite, curing is essential to enhance the reaction and improve the stabilization effectiveness of the loess samples. The study involved evaluating unconfined uniaxial compressive strength, measuring changes in permeability, and analyzing carbon emissions and costs of stabilization agents. These tests and analyses were conducted to assess the stabilization effects of DAP and cement, as well as their environmental and economic benefits.

2.2.1. Unconfined Uniaxial Compressive Test

Cylindrical specimens were utilized to conduct the uniaxial compressive strength test on the loess samples. By compacting the remolded loess in a steel mold and then demolding, loess specimens required for unconfined compressive tests were prepared. The unconfined compressive strength test was performed under strain control employing an electro-hydraulic servo uniaxial compression machine, maintaining a loading rate of 1% per minute. In this test, each group requires the simultaneous preparation of 5 specimens as parallel controls. The numerical value of the ultimate compressive strength was obtained for each specimen, ensuring that their differences did not exceed 20%. The average result for a group of specimens was recorded as the uniaxial compressive strength of that group. The specific procedures for the preparing and curing of loess specimens for compressive testing are as follows:
  • The sieved loess was subsequently dried in an oven over 24 h at a temperature of 105 °C. After allowing the loess to cool to room temperature, an appropriate amount of cement and water, or an equal volume of DAP solution, was added into the loess according to the optimal moisture content ratio and the mix proportions in Table 3. The mixture was thoroughly blended with water for at least 5–10 min to ensure a uniform blend.
  • The mixed loess was then placed into a steel mold with dimensions of 50 mm by 100 mm. The loess within the mold was compacted using an electric compactor, and the specimens were removed after their top and bottom surfaces were leveled. The number of compactions for the remaining groups of loess specimens was based on this standard to maintain the degree of compaction and ensure uniformity in the preparation of the specimens. After compaction, the dry density of the untreated loess specimens should achieve 1.72 g/cm3.
  • To simulate the curing environment of a roadbed, the demolded cylindrical loess specimens were kept inside a curing chamber set to 23 degrees Celsius and 96% humidity. Five specimens were cured as a group for curing periods of 3 d, 7 d, 14 d, and 28 d. After curing, they were transferred to an oven and heated at 105 degrees Celsius until the weight remained constant before removal.
Based on the compaction test results of the loess obtained using an electric compactor, the best moisture level for the remolded loess was measured to be 16 ± 0.5%, with the maximum dry density of the untreated loess specimens reaching 1.72 g/cm3. The void ratio, measured by saturation and drying methods, was found to be within the range of 0.54–0.55.

2.2.2. Permeability Test

To investigate the permeability coefficient changes of modified and remolded loess, variable head permeability tests were conducted. Based on the optimal moisture content, loess flow specimens were prepared:
  • Both untreated loess (control group) and loess treated with cement/DAP were compressed using an electric compactor to achieve maximum dry density. Subsequently, ring samples were extracted from the compressed specimens. These ring samples were subsequently stored in a conditioning room for a specified age.
  • The ring samples were inserted into the container of the saturation permeameter, sealed, and connected to a water head. The water was drained until no bubbles were observed in the overflow water. The permeability coefficient test was conducted when the sample saturation exceeded 0.95.
  • At ambient room temperature, the time interval was recorded for the water level’s descent from 90 cm to 70 cm. After each measurement, the water head was raised back to the specified height for another measurement. This process was repeated no less than five times. When the inflow and outflow rates stabilized and showed consistency, the calculated permeability coefficient was determined to represent the saturated permeability coefficient.
The formula for calculating the loess’s saturated permeability coefficient kT is the following:
k T = 2.3 a L A t 1 − t 2 l g H 1 H 2
In the formula, kT represents the saturated permeability coefficient (cm/s); a denotes the cross-sectional area of the tube (cm2); A symbolizes the cross-sectional area of the permeameter (cm2); L is the distance traveled by the water, equivalent to the specimen’s height (cm); t1 and t2 are the initial and final times of the water head readings, respectively (s); and H1 and H2 are the water heads at the start and end times, respectively (m).
After completing the permeability test, the ring samples were removed from the container and wiped clean of any surface moisture before being weighed. They were subsequently transferred to an oven at 105 °C for over 24 h to achieve complete dryness. Once the mass no longer changed, the samples were taken out and reweighed. The mass loss of the samples was recorded to calculate the volume loss of water after drying the saturated samples, which in turn allowed for the determination of the porosity ratio of the ring samples.

2.2.3. Characterization Analysis

After the completion of the above experiments, soil samples were obtained for subsequent characterization analysis. The loess’s primary chemical composition was determined through X-ray fluorescence spectroscopy (XRF) (Rigaku Co., Ltd. Akishima, Japan). Its crystalline phases were analyzed using a TTR-III θ/θ type high-power XRD instrument with Cu-Kα radiation at 40 kV and 200 mA. The analysis employed a scanning rate of 8°/min, covering an angular range from 2θ = 20° to 60°. Samples for analysis were collected from different internal regions of the loess specimens. Additionally, a JSM-IT100 SEM (Japan Electronics Co., Ltd., Tokyo, Japan), functioning at 15 kV with a resolution of 10–20 μm, was employed to observe the microsurface morphology of various soil samples.
Additionally, an X-ray scanning (EDX) (EDAX Inc., Warrendale, PA, USA) probe was utilized to analyze the elemental distribution within the samples. Prior to the analysis, the samples were finely milled into powder and uniformly spread on SEM stubs coated with a conductive adhesive. The EDX detector was then used to assess the distribution of various elements on the surface of soil particles. This allowed for an in-depth investigation into how DAP treatment affects the microscopic morphology of the loess particles.

3. Results and Discussion

3.1. Compressive Strength

Figure 1 illustrates the remolded loess specimens treated with different concentrations of DAP. Following the drying process, the density of the DAP-treated loess increased by 1–4%, with the increment dependent on the concentration of DAP. The white crystalline material, such as HAP and other apatite components, observed in the loess specimens, was generated by the ion exchange and adsorption precipitation reactions between DAP and calcium carbonate found both on the surface and embedded within the loess matrix. Consequently, the DAP-treated loess specimens exhibited a whitish appearance. The formation of HAP can enhance the bonding between particles and aggregates within the loess. The efficacy of DAP treatment increased with increasing concentrations of the agent. This resulted in a whiter coloration of the specimen and a notable enhancement in surface hardness.
Figure 2 illustrates the compressive strength of the cement-treated loess. The untreated group (UT) exhibited minimal change in compressive strength over the course of curing. However, the compressive strength of the cement-treated group (CM) increased with prolonged curing periods. This improvement is attributed to the hydration reactions of the cement, which progressively develop a bonding matrix between soil particles. Cement-modified loess requires an extended curing time (28 d) to maximize its stabilization potential.
As the curing age advanced, the hydration reaction became more complete, resulting in increased cohesion and friction between loess particles. Moreover, the process yielded an increased amount of hydration products, which filled the pore and decreased the soil’s porosity. Owing to these effects, the cement-treated loess displayed a substantial improvement in compressive strength.
For any specific curing age, the compressive strength of the cement-treated loess rose correspondingly with a higher dosage of cement. This is because a higher cement content results in a greater quantity of hydration products formed during the hydration process, which consequently leads to higher compressive strength. For instance, after 28 days, the compressive strengths recorded for the CM-4, CM-6, and CM-8 groups were measured to be 2.73 MPa, 2.99 MPa, and 3.08 MPa, respectively. These values represent increases of 173%, 199%, and 208% compared to the untreated group (UT).
Figure 3 depicts the compressive strength results for the DAP-treated loess specimens. According to Figure 3, aside from the control group (UT) with minimal variation, the compressive strength of the DAP-treated loess specimens progressively increased as the curing age extended. For instance, the DT-3.0 group reached a strength of 5.68 MPa at 28 days, marking a 43.07% increase from its strength at 3 days. The specimens exhibited relatively high early-stage strength, with rapid development initially; however, the rate of strength development diminished after 14 days. For example, the compressive strength of the DT-2.0 group at 28 days showed only a 14% increment compared to its compressive strength at 14 days.
The concentration of DAP is a primary factor that influences the compressive strength of DAP-treated loess. Increasing the DAP concentration, that is, increasing the amount of DAP added, significantly enhanced the uniaxial compressive strength of the specimens. For loess with a high calcium content, appropriately raising the concentration of DAP can hasten the development of early-stage strength of the treated soil. For instance, at a curing age of 3 days, in comparison to the UT group, the compressive strengths measurements for the DT-0.5, DT-1.0, DT-1.5, DT-2.0, and DT-3.0 groups were 1.22 MPa, 1.49 MPa, 2.06 MPa, 3.42 MPa, and 3.97 MPa, respectively, corresponding to increases of 22%, 49%, 106%, 242%, and 297%.
In comparison to cement-treated loess, when the admixture content is comparable, DAP-treated samples exhibited significantly higher compressive strength. For example, the compressive strength at 28 days for the DT-2.0 was 14% greater than that of the CM-4, and the DT-3.0 group’s compressive strength surpassed that of the CM-4 group by 29%. As the concentration of DAP rose, there was a considerable impact on the effectiveness of DAP stabilization; more HAP complexes were formed with the calcium in the soil. These complexes not only strengthened the bonding between particles but also continuously filled the pores in the loess, rendering the soil sample denser. Consequently, significant enhancement was observed in the compressive strength of the loess.
The most significant increase in compressive strength occurred when the DAP solution concentration was within the range of 1.5–2.0 mol/L (corresponding to a solid content of 3–4%) after curing. Although cement also enhanced bonding during curing, its hydration process involved some shrinkage, which constrained its pore-filling capacity. As a result, this resulted in a less pronounced increase in the sample’s compressive strength compared to DAP-treated loess specimens.

3.2. Permeability and Porosity

Table 4 shows the void ratios and permeability coefficients of loess treated with different amounts of cement/DAP after 14 days. The experiments measured the time intervals (t1 − t2) required for the water head to drop by 20 cm, as well as the calculation of the permeability coefficient kT. The results indicated that both cement and DAP treatment significantly reduced the loess’s permeability coefficient. After 14 days, the permeability coefficient of untreated loess was 2.42 × 10−4 cm/s; it decreased to 0.59 × 10−4 cm/s for the CM-6 group and to 0.28 × 10−4 cm/s for the DT-3.0 group. Under the same admixture content conditions, the void ratio of DAP-treated loess was reduced by 0.029 compared to cement-treated loess, and there was a 52.5% decrease in the permeability coefficient, indicating that the HAP complexes generated significantly reduced the interconnected pores in the collapsible loess.
Table 5 presents the void ratios and permeability coefficients of loess treated with the same amount of cement and DAP at different curing ages. The data indicated that the void ratio of the DAP-treated loess continuously diminished as curing time increased, whereas the change in the void ratio for the cement-treated soil was relatively small during the curing process. This is attributed to the ongoing reaction between phosphate ions from the DAP solution and the calcite present in the loess, which requires an extended period. Insufficient curing time can result in the dissolution and migration of the formed amorphous, unstable calcium phosphate, and unreacted phosphate ions. Therefore, as the curing age progressed, more HAP was generated within the DAP-treated loess, leading to a reduction in both the void ratio and the permeability coefficient of the specimens. For instance, the permeability coefficient of the DT-3.0 group was 0.31 × 10−4 cm/s at 3 days, and it decreased to 0.13 × 10−4 cm/s at 28 days, representing a reduction of 58%.

3.3. Micro-Mechanism of DAP Stabilization

In addition to the considerable amount of quartz that constitutes the large particles, the loess also contains a wealth of ultra-fine carbonates, mainly existing as calcite (CaCO3) and a minor amount of dolomite (CaMg(CO3)2), as shown in Table 1. The clay minerals containing Al and Fe within the loess can also react with DAP [36]. During these reactions, the bonding between the particles was enhanced and the pores were filled, which consequently increased the compressive strength of the loess.
Figure 4 illustrates the XRD pattern for the untreated loess. Apart from the peaks of quartz, analysis using standard PDF cards confirmed that the loess contained a substantial amount of calcite. This calcite can react with the added DAP solution to form HAP complexes, which served to stabilize the soil. Figure 5 depicts SEM images of the untreated remolded loess sample. Figure 6 illustrates the three predominant pore types found within the loess.
Figure 5 reveals that the particle surfaces are also relatively smooth. The loess particles exhibited aggregate envelopes on their surfaces, to which small amounts of needle-shaped or plate-shaped debris of clay minerals were adhered. In Figure 6, the scaffold pores are large and interconnected, exhibiting pronounced instability within which various types of debris of clay materials are partially filled (Figure 6a). The inlaid pores are smaller in size and more stable, and they are less prone to failure after soaking (Figure 6b). Furthermore, various micro-pores are also commonly found in bonding materials, impacting the water stability of soil (Figure 6c). The pores of collapsible loess are primarily composed of scaffold pores. After water immersion, seeping flow will erode and destroy these interconnected structures in soil, when subjected to its own weight or certain external pressures. The clay debris that was adhered to the pore walls also became dispersed and migrated into the surrounding pores, resulting in the collapse and size reduction of the large interconnected pores [6]. The inherent strength and low water stability of the loess contribute to the phenomenon of wet collapse in the loess. From the foregoing discussion, it can be deduced that the microstructure of untreated loess exhibits characteristics of large interconnected scaffold pores with minimal bonding, and the pores surrounding the soil particles are clearly visible. Within the loess, the interaction between particles primarily occurred through point-to-point or point-to-edge connections. The limited contact areas between particles, with only scattered bonding agents, contributed to the lower strength of the inter-particle bonding.
Figure 7 illustrates the SEM images and EDX results of loess samples treated with DAP concentrations of 1.0, 2.0, and 3.0 mol/L, following a curing period of 3 days, at a magnification of 20 μm. It was indicated that calcium ions in the loess can effectively adsorb and combine with phosphate ions from the DAP solution, leading to co-precipitation reactions that yield HAP complexes. The amount of phosphorus absorbed by calcite in the loess increased as the DAP concentration rose. The samples that underwent treatment using 3.0 mol/L DAP exhibited the highest phosphorus adsorption. In the SEM images accompanying the EDX results, the relative volume of the aggregates was small. The integrity of the substantial inter-particle bonding material was compromised when the loess sample was pulverized into powder, leaving behind the relatively smaller minerals adhering to the particle surfaces. The increased strength of the loess specimens is due to the enhanced bonding between particles resulting from the reaction of DAP with calcite, which produces HAP within the loess.
The SEM observations revealing the absorption of phosphorus and its precipitation on the surface of soil particles, along with the formation of calcium phosphate precipitates, are presented in Figure 8 and Figure 9. Specifically, Figure 8 depicts the results after 3 days of curing for different DAP concentrations. At a lower DAP concentration (0.5 mol/L), it was difficult to observe HAP complexes on the soil particle surfaces (Figure 8a). The comparatively lower Ca/P molar ratio resulted in the formation of the precursor and amorphous phase, distinct from HAP, with poor abilities of bonding and higher solubility [37]. Comparatively, at a higher DAP concentration (2.0 mol/L) and Ca/P molar ratio, the surface of the soil particles clearly exhibited the presence of precipitated HAP complexes with a distinct nanoscale spatial structure [26] and strong bonding strength [14] (Figure 8d). Figure 8 also reveals the appearance of many new small aggregates on the surface of the particles in the loess. This is because the various debris materials around the particles were bonded or even enveloped by the HAP complexes in the soil. As the concentration of DAP increased, more HAP complexes were generated, leading to an improvement in interparticle bonding and the adhesiveness of the clay debris. Additionally, the debris also helped to fill the pores between the loess particles, thereby augmenting both the loess’s strength and its resistance to water.
Figure 9 shows the SEM images of loess treated with 3.0 mol/L DAP at different curing ages. The longer curing age significantly promoted the accumulation of HAP complexes within the treated loess. A large amount of HAP complexes with special reticular structures precipitated between the soil particles. These precipitates bonded the soil particles together into large aggregates, thus enhancing the loess’s compressive strength. In Figure 9d, the pores’ structure has changed to inlaid contact or direct contact, as seen in Figure 6b, in the loess treated with 3 mol/L DAP after 28 days of curing. Soil particles were bonded by a large amount of HAP complexes, leading to a notable enhancement in the loess’s compressive strength. A further analysis of the SEM images was conducted from groups with shorter curing ages, such as those shown in Figure 9a,b. It was pictorially evident that the HAP complex’s web-like structure around and atop the particle surfaces was clearer in the groups with longer curing ages.
The precipitated HAP complexes not only exhibited lower solubility and better water stability than calcite [25] but also more debris clay minerals were adhered in the loess [36]. These complexes filled the pores between soil particles or acted as binding agents, improving the pore structure within the loess. This process enhanced the loess’s compressive strength and reduced its permeability coefficient, rendering it denser and more stable. Consequently, the collapsibility of the loess was significantly mitigated.

3.4. Carbon Emissions and Cost Analysis

The production and transportation of cement are associated with substantial carbon emissions, which can cause a warming effect on the environment [38]. China’s government developed alternative industrial byproducts and fuels to reduce the CO2 emissions of the cement industry [39]. In the phosphorus chemical industry, China also developed and grasped various sustainable DAP production technologies with low energy consumption [40]. Table 6 lists the CO2 emissions and costs associated with the use of cement and DAP for modifying and reinforcing loess.
A comparative analysis revealed that the carbon emissions resulting from cement were significantly higher than those from DAP, while the material cost of DAP is relatively higher. For instance, to increase the 28-day compressive strength of 1 kg loess to exceed 3 MPa, CM-6 required 60 g of cement, resulting in a carbon emission of 49.2 g and a cost of 0.025 RMB; in contrast, DT-1.5 required 33.7 g of DAP, with a significantly lower carbon emission of 8.8 g and a cost of 0.132 RMB. While the use of DAP for reinforcing loess does indeed increase the cost, it significantly reduced the carbon emission by 24.7 g per kilogram of loess stabilized, representing an 82% decrease in carbon emissions compared to cement modification. These findings suggest that despite the higher material cost of DAP, using it to reinforce loess offers a significant advantage in terms of reducing carbon emissions, presenting a more competitive alternative for eco-friendly and sustainable development.

4. Conclusions

As an innovative, eco-friendly soil modification agent, DAP releases phosphate ions that react with the carbonates in the collapsible loess, generating HAP complex precipitates around the soil particles. The generated HAP complex has more stable chemical properties and lower solubility than the carbonates that contribute to the bonding between soil particles in untreated loess. Through steam curing, HAP complexes continuously precipitate in humid conditions and effectively fill the abundant pores in loess, preventing the collapse of soil structure after water immersion. Therefore, DAP can effectively enhance the strength of the collapsible loess and improve its water stability. Compared to traditional cement, the use of DAP for soil stabilization yields a more notable improvement in strength, a greater reduction in the coefficient of permeability, and a more significant decrease in carbon emissions. From the outcomes and analysis, the following conclusions can be formulated.
Unconfined compressive tests suggest that DAP has a better stabilization effect on loess than Portland cement. Loess treated with a 3.0 mol/L DAP solution showed a significant increasement in maximum compressive strength by 297% after a 28-day curing period. Holding the solid content constant, the DAP-treated loess exhibited a 14–29% greater compressive strength than that of the cement-treated loess.
HAP complexes are also more effective than cement in reducing interconnected pores within loess. The permeability coefficient of DT-3.0 was 0.31 × 10−4 cm/s at 3 days, and it decreased to 0.13 × 10−4 cm/s at 28 days. As the curing age increases, there is a notable decrease of 58% in the permeability coefficient of the DAP-treated loess. Under the same solid content, the permeability coefficient of the DAP-treated loess is 52.5% lower than that of cement-treated loess.
SEM/EDX analysis showed that DAP reacted with the carbonates in loess, leading to the formation of HAP during curing. This reaction improved the interparticle bonding and filled the pores within the loess, strengthening its structure and significantly increasing its compressive strength. The curing time is pivotal for effectively promoting the development and bonding strength of HAP complexes within the treated loess. After 28 days, a large amount of distinct nanoscale reticular structures of HAP complexes can be observed between the soil particles, enveloping both the particles and clay debris.
As an innovative agent for soil stabilization, DAP holds the promise to replace traditional cement and lime in the stabilization of collapsible loess. According to carbon emission and cost analysis, DAP offers superior environmental advantages over cement, with a significant reduction in carbon emissions by 82%. Considering the solubility of DAP in water, it is recommended to employ 3.0 mol/L of DAP solution for the effective stabilization of loess.

Author Contributions

Conceptualization, L.H. and H.C.; methodology, L.H.; validation, C.Y., L.H. and Y.Z.; formal analysis, C.Y. and L.H.; investigation, C.Y. and L.H.; resources, H.C. and D.Y.; data curation, L.H. and Y.Z.; writing—original draft preparation, C.Y., L.H. and Y.Z.; writing—review and editing, C.Y. and H.C.; visualization, L.H. and Y.Z.; supervision, D.Y.; project administration, H.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Project Nos 41440018 and 41672278).

Data Availability Statement

The data presented in this study are available on request from the corresponding author due to confidentiality reasons.

Acknowledgments

The authors gratefully acknowledge the support from the National Natural Science Foundation of China (Project Nos 41440018 and 41672278).

Conflicts of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Remolded loess specimens treated with different concentrations of DAP. (a) Untreated; (b) 0.5 mol/L DAP; (c) 1.0 mol/L DAP; (d) 1.5 mol/L DAP; (e) 2.0 mol/L DAP; (f) 3.0 mol/L DAP.
Figure 1. Remolded loess specimens treated with different concentrations of DAP. (a) Untreated; (b) 0.5 mol/L DAP; (c) 1.0 mol/L DAP; (d) 1.5 mol/L DAP; (e) 2.0 mol/L DAP; (f) 3.0 mol/L DAP.
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Figure 2. Compressive strength of cement-treated loess specimens.
Figure 2. Compressive strength of cement-treated loess specimens.
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Figure 3. Compressive strength of DAP-treated loess specimens.
Figure 3. Compressive strength of DAP-treated loess specimens.
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Figure 4. XRD patterns of untreated loess.
Figure 4. XRD patterns of untreated loess.
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Figure 5. SEM images of untreated loess. (a) Pore structure in loess; (b) surface morphology of loess particles.
Figure 5. SEM images of untreated loess. (a) Pore structure in loess; (b) surface morphology of loess particles.
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Figure 6. Pore types of the loess. (a) Scaffold pores; (b) inlaid pores; (c) pores in bonding.
Figure 6. Pore types of the loess. (a) Scaffold pores; (b) inlaid pores; (c) pores in bonding.
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Figure 7. EDX results and SEM images (insets) of loess specimens treated with different contents of DAP. (a) 1.0 mol/L; (b) 2.0 mol/L; (c) 3.0 mol/L. The yellow and red crosses is other scanning location for EDX in the insert images, the red blocks is the correspond-ing scanning location of EDX.
Figure 7. EDX results and SEM images (insets) of loess specimens treated with different contents of DAP. (a) 1.0 mol/L; (b) 2.0 mol/L; (c) 3.0 mol/L. The yellow and red crosses is other scanning location for EDX in the insert images, the red blocks is the correspond-ing scanning location of EDX.
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Figure 8. SEM images of loess specimens treated with different contents of DAP for 3 d of curing. (a) 0.5 mol/L; (b) 1.0 mol/L; (c) 1.5 mol/L; (d) 2.0 mol/L.
Figure 8. SEM images of loess specimens treated with different contents of DAP for 3 d of curing. (a) 0.5 mol/L; (b) 1.0 mol/L; (c) 1.5 mol/L; (d) 2.0 mol/L.
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Figure 9. SEM images of DAP-treated loess specimens treated with 3.0 mol/L DAP at different curing ages. (a) 3 d; (b) 7 d; (c) 14 d; (d) 28 d.
Figure 9. SEM images of DAP-treated loess specimens treated with 3.0 mol/L DAP at different curing ages. (a) 3 d; (b) 7 d; (c) 14 d; (d) 28 d.
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Table 1. The distribution of the loess’s grain size.
Table 1. The distribution of the loess’s grain size.
Grain-Size FractionSilty-Fine SandCoarse SiltFine SiltClay
Grain diameter (mm)>0.050.01–0.050.005–0.01<0.005
Content (%)12.461.917.37.9
Table 2. The loess’s primary chemical composition.
Table 2. The loess’s primary chemical composition.
CompositionCaOMgOAl2O3SiO2CO2Fe2O3
Content (%)13.02.9312.948.010.36.23
Table 3. The mix proportions of stabilization agents.
Table 3. The mix proportions of stabilization agents.
GroupsCement Content (wt%)Molar Concentration of DAP (mol/L)Water Content (wt%)
UT0-16
CM-44-
CM-66-
CM-88-
DT-0.5-0.5
DT-1.0-1.0
DT-1.5-1.5
DT-2.0-2.0
DT-3.0-3.0
Table 4. Results of the Loess Permeability Test (14 d curing).
Table 4. Results of the Loess Permeability Test (14 d curing).
GroupsUTCM-4CM-6CM-8DT-1.5DT-2.0DT-3.0
Void ratio 0.5420.5250.5180.5050.5020.4930.489
Time (s)18407410598126189
Permeability coefficient (10−4 cm/s)2.421.080.590.410.440.340.28
Table 5. Results of the Loess Permeability Test (different curing ages).
Table 5. Results of the Loess Permeability Test (different curing ages).
GroupsCM-6 3 dCM-6 14 dCM-6 28 dDT-3.0 3 dDT-3.0 14 dDT-3.0 28 d
Void ratio 0.5230.5180.5190.4960.4890.485
Time (s)787484143189327
Permeability coefficient (10−4 cm/s)0.560.590.520.310.280.13
Table 6. Carbon emissions of cement and DAP production and their market price.
Table 6. Carbon emissions of cement and DAP production and their market price.
Stabilization AgentCarbon Emission Factor (kg CO2/kg)Market Price (RMB/kg)
OPC0.82 [39]0.42 [41]
DAP0.26 [42]3.92 [43]
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Ying, C.; Huang, L.; Chen, H.; Zhang, Y.; Yao, D. Modification and Stabilization of Collapsible Loess Using Diammonium Phosphate Solution. Crystals 2024, 14, 426. https://doi.org/10.3390/cryst14050426

AMA Style

Ying C, Huang L, Chen H, Zhang Y, Yao D. Modification and Stabilization of Collapsible Loess Using Diammonium Phosphate Solution. Crystals. 2024; 14(5):426. https://doi.org/10.3390/cryst14050426

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Ying, Chengjuan, Lingxia Huang, Haiming Chen, Yadong Zhang, and Duoxi Yao. 2024. "Modification and Stabilization of Collapsible Loess Using Diammonium Phosphate Solution" Crystals 14, no. 5: 426. https://doi.org/10.3390/cryst14050426

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