4.1. Microstructural Analysis
To further analyze the mechanism of GO modification on the cemented soil, SEM tests were conducted on specimens with different GO contents, as shown in
Figure 13. In the absence of GO admixture (
Figure 13a), the soil particle surfaces were coated with hydrates formed during cement hydration. However, there was a lower amount of cement filling the gaps between the particles, resulting in an overall loose skeleton characterized by a structure of large particles and large pores. From
Figure 13b, it is evident that at a GO content of 0.03%, there was an increase in the quantity of spherical and needle-like small particles present in the sample. These particles were distributed on the larger particles, effectively filling the pores and establishing surface-to-surface contact. However, due to the limited quantity, the pores cannot be completely filled, showing a large particle–small particle–micropore–soil particle structure. When the GO content further increased to 0.06% (
Figure 13c), the number of small particles increased significantly and then aggregated to form aggregates that filled pores and compacted the soil. This forms a structure composed of large particles–aggregate particles and soil particles. As shown in
Figure 13d, when the GO dose is 0.09%, the number of small particles continues to increase, pores are effectively filled, and particles are tightly combined to form a dense structure.
Figure 13e demonstrates that a GO content of 0.12% resulted in the reemergence of larger pores within the microstructure, accompanied by a decrease in inter-particle cements, resulting in an overall porous structure.
Based on the aforementioned analysis, there exists a critical threshold for the microstructural density of GO-modified specimens in relation to the amount of GO admixture. Excessive or insufficient admixture is found to adversely affect the structural compactness. When mixed solely with cement, the hydration products are limited to coating the surface of larger particles, resulting in a weak cementing effect and the presence of a significant pore structure [
32]. The hydration process of Portland cement involves a multi-phase and intricate chemical reaction. This reaction facilitates the transformation of dispersed cement powder particles into a binding cement slurry, generating a variety of hydration products and effectively amalgamating particles of varying sizes [
33]. Particularly during the initial stages of hydration, the prevalent hydration products include tetracalcium aluminoferrite (4CaO·Al
2O
3·Fe
2O
3) and tricalcium aluminate (3CaO·Al
2O
3). However, during this period, the resulting hydrate exhibits negligible effects on the enhancement of the cemented soil and displays low density [
34].
Figure 14 presents a schematic diagram illustrating the mechanism analysis of GO-enhanced cemented soil. It is shown that the increase in GO dose has two different effects. First, GO can effectively fill micropores and microcracks in the soil, so as to improve the compactness of the structure. In addition, GO also acts as a bridging agent to further enhance structural integrity. Finally, the surface of GO is rich in active functional groups, including hydroxyl, carboxyl, and epoxy groups. These functional groups have the ability to stimulate the hydration reaction of cement, facilitating the interweaving of calcium silicate hydrate (C–S–H) and other products, resulting in the formation of a network-like structure. Consequently, this process enhanced the integrity and compactness of the structure [
3].
The primary constituent of cement hydration products is the calcium silicate hydrate gel, which comprises approximately 50% to 60% of the total volume of hardened cement [
35]. The C–S–H gel is essential for providing gelling strength and influencing significant engineering properties, including strength, shrinkage, and permeability resistance. It can exist in various forms, such as mesh-like, colloidal, or flocculent materials. The structure and properties of C–S–H gels play a crucial role in determining the fundamental engineering properties of gelled materials [
36].
A 0.09% GO content resulted in increased hydrophilic and dispersive qualities within the soil–cement matrix after 28 days of curing, attributed to the presence of C, H, and O functional groups [
31]. The cement with GO, when cured for 28 days, adapted to micropores and microcracks in the soil–cement matrix. The high surface area, two-dimensional properties, and pore-filling characteristics of GO contribute to the nucleation of hydration products and enhance the cement hydration process [
37]. In this way, GO maintained a uniform structure and controlled the hydration process at the nanoscale [
38]. The uniform distribution of GO prevented the agglomeration or clustering of cement in the soil–cement matrix. Consequently, the soil sample exhibited a positive effect by increasing the density and the number of hydration products, resulting in a polyhedral structure.
When the content of GO exceeds the critical value, the remaining GO flakes will adsorb a significant amount of free water. Consequently, this would reduce the availability of water for cement hydration, leading to inadequate hydration. On the other hand, the extensive specific surface area of GO resulted in a pronounced agglomeration effect. This hindered the uniform dispersion of GO within the cemented soil, thereby limiting its interaction with soil particles and reducing its effectiveness. Consequently, this led to a decrease in the formation of C–S–H [
11].
4.2. Ion Exchange Effect
In addition, the soil particle colloids are negatively charged and have substantial amounts of adsorbed K
+ and Na
+ on their surfaces. The addition of cement resulted in the production of Ca
2+ during the hydration process. The Ca
2+ ions undergo cation exchange with the K
+ and Na
+ ions present on the surface of soil particles. This exchange leads to a decrease in the thickness of the double electric layer and an increase in the bonding force between the soil particles. As a result, the microstructure of the soil experienced some improvement. Furthermore, the addition of GO greatly stimulates the cement hydration reaction, resulting in the generation of a substantial quantity of hydration products and Ca
2+. This further enhances the exchange of Ca
2+ with K
+ and Na
+ on the surface of the soil particles, resulting in a more pronounced thinning of the soil particles’ double electric layer and a significant increase in the bonding force between them (see
Figure 15). This results in a stronger and denser overall structure, leading to a significant improvement in water resistance and mechanical properties [
39].
4.3. Analysis of the Correlation between Microstructure and Macroscopic Properties
Although SEM images can demonstrate the differences between cement-treated soil modified with different amounts of GO from the perspective of micromorphology, it is difficult to quantitatively characterize microstructural indicators, such as apparent pore ratio, as well as the relationship between microstructure and macroscopic properties.
Particle (pore) and fracture image recognition and analysis systems for automatic identification, quantification, and statistical analysis of particles, pores, and fractures are based on optical microscopy and electron microscopy images [
40]. The fundamental principle entails automating the removal of stray points through binarization, segmenting, and identifying particles and pores based on their respective grayscale values and conducting quantitative statistical analysis of microstructural parameters, including the apparent pore ratio and fractional dimensional values of porosity. Quantitative characterization of microstructures can be achieved through the above steps and test methods [
41].
The apparent pore ratio is an essential indicator of soil density degree. This approach indirectly addresses the diverse characteristics of the pore ratio in three-dimensional space by utilizing two-dimensional parameters. The apparent pore ratio can be computationally characterized by calculating the ratio of pore area to soil particle area in the binarized image [
42]. The apparent pore ratio is determined as follows:
where
N is the apparent pore ratio;
Ap is the area of pores in the SEM images (μm
2); and
As is the area of soil particles in the SEM images (μm
2).
Figure 16 displays the changes in the apparent pore ratio across varying contents of GO. It illustrates that the apparent pore ratio of the specimens initially decreased and then increased as the GO content increased. At a content of 0.09% GO, the apparent pore ratio reached its minimum value, indicating the highest level of compaction in the cemented soil.
This analysis reveals a clear correlation between the UCS and the apparent pore ratio.
Figure 17 illustrates the relationship between UCS and apparent pore ratio for cemented soils incorporating different levels of GO admixtures.
The relationship between the UCS and the apparent pore ratio
qu (see
Figure 17) can be fitted as follows:
where
x is the apparent pore ratio.
Equation (5) demonstrates a quadratic polynomial relationship between the UCS and the apparent pore ratio qu. The synergistic effect of an optimum amount of GO and cement stimulated the hydration process, resulting in the production of more abundant hydration products and the formation of numerous regular structures in the soil. Additionally, the hydration products have the ability to fill the voids between soil particles, whereas GO can penetrate even the smaller cracks. The synergistic combination of these two processes significantly enhanced the structural strength and compactness of the soil.