1. Introduction
Global desert distribution is extensive and diverse, and is primarily concentrated in the tropical regions between the Tropic of Cancer and the Tropic of Capricorn and in specific cold climate zones. According to the United Nations Environment Programme, approximately one-third of the Earth’s land area is covered by deserts. Major deserts include the Sahara Desert, the Arabian Desert, the Gobi Desert, and the Victoria Desert (in Australia) [
1,
2]. Desertification in China is predominantly concentrated in the northwestern and northern regions of the country, encompassing a total area of approximately 186,300 km
2. Notable examples include the Taklamakan Desert, the Mu Us Desert, and the Kubuqi Desert [
3,
4,
5,
6]. Aeolian sand, prevalent in these desert regions, is a very peculiar material characterized by loose aggregation and uniform particle size distribution, with an inherent lack of cohesion. Therefore, before aeolian sand can be used as a subgrade for construction structures, it must undergo stabilization to ensure its integrity and stability [
7,
8]. The unique properties of aeolian sand pose both challenges and opportunities for stabilization. To enhance aeolian sand strength, physical methods like reinforcement, chemical methods, such as cement and alkali-activated material (AAM) stabilization, and biological methods, like microbially induced calcite precipitation (MICP), can be used [
9,
10,
11]. Physical methods are relatively costly and the high temperature and arid environment of the desert inhibits microbial activity, resulting in MICP not achieving optimal stabilization effects. Therefore, chemical methods are more suitable for stabilizing aeolian sand in desert environments. AAM has several distinct advantages, such as environmental friendliness, durability, and cost-effectiveness, as it typically uses waste or industrial by-products as raw materials, reducing environmental pollution and lowering production costs [
12]. However, there are also challenges and limitations associated with AAM technology. For example, its standardization and range of applications may be limited due to a lack of established guidelines [
13], and AAM may not achieve early strength development as quickly as traditional cement, requiring a longer time to reach design strength [
14]. This delayed strength development can be a significant drawback in projects where early strength is crucial. Compared with AAM, Portland cement excels in early strength development, reaching design strength in a shorter period and meeting the requirements for early strength in engineering projects [
15]; it is also simpler to handle and apply.
Therefore, using Portland cement to improve the strength of aeolian sand remains a considerable method in practical conditions. Studies have emphasized the influence of cement and water content on the unconfined compressive strength (UCS) of cemented specimens. Mixing aeolian sand with cement has been found to offer adequate bearing capacity for low- to moderate-height buildings and improve the compaction and load-bearing capacity of aeolian sand. The influence of curing time, cement dosage, and water dosage on the mechanical properties of aeolian sand stabilized with cement (ASC) has also been investigated [
16,
17,
18,
19,
20,
21,
22].
Despite extensive research on the stabilization of aeolian sand using cement, most previous studies have focused on examining two of the three factors (curing time, cement dosage, or water dosage). There remain gaps in understanding the optimal combination of curing time, cement dosage, and water dosage for maximizing the stabilization effect. To determine the impact of these three parameters and the optimal combination, a comprehensive analysis of all factor and level combinations is necessary. Traditional testing approaches would require a prohibitively large number of experiments. Thus, this study employs an orthogonal experimental design (a widely used multifactor experimental method that reduces the number of specimens while ensuring the accuracy of results).
Orthogonal experimental design allows for an efficient and systematic investigation of the effects of curing time, cement dosage, and water dosage on the UCS of ASC. The curing times selected for this study (7, 14, 28, 60, and 90 days) encompass the typical range used in engineering conditions, allowing for an assessment of both short-term and long-term strength development. The cement dosages (3%, 5%, 7%, 9%, and 11%) and water dosages (3%, 6%, 9%, 12%, and 15%) were chosen based on their relevance to practical applications and an existing study [
23].
In addition to macroscopic strength measurements, this study employs optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) to explore the microstructural changes in ASC [
8,
24,
25]. These techniques provide detailed insights into the bonding mechanisms, pore structure, and mineralogical transformations that occur during stabilization. The microscopic analysis elucidates how hydration products contribute to the increased mechanical properties of the ASC.
The specific research process of this paper is as follows: Initially, a series of specimens are prepared and cured according to the principles of orthogonal experimental design. Subsequently, these specimens undergo unconfined compression tests to measure their UCS, a critical indicator for assessing the macro-mechanical properties of ASC. Following this, the test data are subjected to an analysis to explore the specific effects of curing time, cement dosage, and water dosage on UCS, and to determine the optimal parameter combination design. Finally, the post-compression specimens are subjected to microstructural analysis using SEM and XRD to reveal the mechanism of cement action during the solidification of aeolian sand. The findings from the micro tests elucidate the changes in UCS observed in the macro-mechanical tests, and the correlation between macroscopic mechanical properties and microscopic changes has been established. The experimental flow chart can be seen in
Figure 1.
The novel contributions of this research lie in its integrated approach, combining macroscopic and microscopic analyses to provide a holistic understanding of the stabilization mechanism of ASC. This study not only clarifies the influence of key factors on UCS but also provides practical recommendations for optimizing the stabilization process. The findings have significant implications for guiding infrastructure construction in desert areas, ensuring the durability and safety of engineering projects, and ultimately promoting economic and social development in these regions. By addressing the identified knowledge gaps and offering innovative insights, this research contributes to the advancement of techniques for stabilizing aeolian sand, making it a valuable resource for both academic and practical applications.
4. Discussion
4.1. Key Findings of This Study
This study systematically evaluated the effects of curing time, cement dosage, and water dosage on the UCS of ASC, resulting in several key findings as follows:
- (1)
Effect of curing time. Prolonging the curing time significantly increased the UCS of ASC. Specifically, UCS increased substantially at curing times of 60 and 90 days, indicating the importance of long-term curing for ASC performance.
- (2)
Effect of cement dosage. With increasing cement dosage, the UCS of ASC improved significantly. Higher cement dosages provided more binder, filling the voids between sand particles and enhancing the overall strength and stability of the material.
- (3)
The UCS of ASC reached its maximum at a water dosage of 9% and then decreased with further increases in water dosage, indicating that there was an optimal dosage for the strength. These research findings aligned well with existing studies [
46,
47,
48].
Through microstructural analysis, we observed that a higher cement content and moderate water content led to the formation of more hydration products. Over time, these products became denser, filling the pores in the material and thereby increasing its strength and density [
49]. This finding further supports the results of macroscopic mechanical performance tests.
4.2. Influence of Water/Cement Ratio (w/c)
Although the orthogonal experimental design in this study did not specifically include the water/cement ratio (
w/
c) as a variable, the results and the existing literature indicate that
w/
c plays a crucial role in determining the UCS of cement-stabilized materials [
46,
49]. SEM images and XRD patterns showed that adequate water promoted sufficient hydration reactions, but excessive water increased porosity, weakening the structural integrity of the material. This strongly demonstrated that the
w/
c significantly affected the strength of ASC.
The w/c of the optimal combination in this study was 0.82, which was higher than the optimal w/c for conventional sand. Several factors contributed to this phenomenon. Firstly, the significantly smaller particle size of aeolian sand resulted in a larger specific surface area. This increased surface area required relatively more water to ensure adequate hydration and coating of the particles. The high surface area created a higher demand for water to cover the additional surface area and facilitate the necessary chemical reactions for cement hydration. Secondly, the natural moisture content of aeolian sand was typically lower compared with that of conventional sand. Lower initial water content required more additional water to reach the same hydration state.
By acknowledging the influence of w/c on UCS, even though it was not a primary variable in this study, we can suggest that future research should include w/c as a critical parameter. This would help in better understanding and optimizing the mechanical properties of ASC.
4.3. Standard Deviation Analysis
In this study, both range analysis and ANOVA were used to assess the influence of curing time, cement dosage, and water dosage on the UCS of ASC. The results from these analyses were consistent and provided a clear ranking of the factors based on their impact on UCS.
From the range analysis, RA = 0.36, RB = 1.13, and RC = 0.27, indicating that cement dosage had the most significant influence on UCS, followed by curing time and water dosage. From ANOVA, FA = 4.7840 (significant), FB = 49.4695 (significant), and FC = 3.0657 (not significant), confirming that cement dosage was the most significant factor affecting UCS, followed by curing time, while the impact of water dosage was not statistically significant. From the SD analysis, SDA ranged from 0.303 to 0.700, indicating considerable variability. SDB ranged from 0.028 to 0.380, showing relatively low variability. SDC ranged from 0.323 to 0.624, indicating significant variability.
The consistency between range analysis and ANOVA results reinforced the conclusion that cement dosage was the primary factor influencing UCS, with curing time also playing a significant role. The SD analysis highlighted the data’s stability and consistency. Cement dosage showed a significant impact with low variability, while curing time and water dosage exhibited higher variability. This could be explained by the variations in the mixing and compaction processes, which could result in a non-uniform distribution of cement and water within the sand matrix. Such non-uniformity could cause differences in the strength development of the material, thereby contributing to higher variability in UCS.