2.1 Analyzing Unconfined Compressive Strength
To explore the influence of stabilizing agents on the unconfined compressive strength (UCS) of stabilized pipe-jacking waste soil, unconfined compressive strength tests were conducted under varying proportions of different stabilizing agents and stabilizing agent ratios.
Based on Figure 4, under fixed ratios of sodium-based bentonite (Na-Ben) solution and polyacrylamide (PAM) solution, and with other conditions unchanged, increasing cement content leads to a moderate increase in the unconfined compressive strength (UCS) of cement-stabilized soil. However, within the analyzed ratio range, adding PAM solution and sodium-based bentonite (Na-Ben) leads to a decrease in the strength of cement-stabilized soil. The specific reasons are analyzed as follows:
Impact of PAM: PAM, a high molecular weight polymer, exhibits strong water-absorption capabilities [11]. Within the studied ratio range, the polymer absorbs moisture, reducing the available water for cement-hydration reactions, thereby limiting hydration and reducing strength.
Impact of sodium-based bentonite (Na-Ben): sodium-based bentonite (Na-Ben) is a clay mineral with significant water-absorbing and swelling capacities. Within the studied ratio range, its water-absorbing and swelling properties reduce the available moisture for cement-hydration reactions [12], thus affecting the hydration process and structural formation of cementitious slurry, which leads to reduced strength.
Changes in pore structure: The addition of PAM and sodium-based bentonite (Na-Ben) alters the pore structure of the cementitious slurry. The particles of sodium-based bentonite (Na-Ben) and molecules of PAM form larger pores or micro-cracks within the slurry [12], weakening the overall structure of cement, thus reducing the material’s strength.
Despite the reduction in strength due to the addition of PAM and sodium-based bentonite (Na-Ben), samples C5 and C10 still meet the strength requirements for sub-base materials of secondary highway pavements.
Based on Figure 5, under constant cement ratios, the unconfined compressive strength of stabilized slag soil at 7 days and 28 days remains nearly consistent with increasing PAM solution concentrations. Therefore, the effect of PAM solution on cement stabilization is minimal, which can primarily be explained as follows: within the studied ratio range, polyacrylamide (PAM) does not promote the formation of hydration products and only mildly affects cement hydration due to its water-absorbing properties. However, it forms a viscous gel-like substance in water, which provides certain viscosity and adhesion [11], thereby contributing to a modest increase in strength. Consequently, the material undergoes no significant macroscopic changes in strength.
According to Figure 6, under constant proportions of the stabilizer, the unconfined compressive strength initially increases and subsequently decreases with increasing sodium-based bentonite concentration. This trend is primarily attributed to the characteristics of sodium-based bentonite, which, at lower concentrations, absorbs less water and fills some voids within the specimens, thereby enhancing their density and compressive strength. Additionally, after absorbing water, sodium-based bentonite forms colloids that possess adsorption capabilities [12], thus stabilizing the structure by filling remaining voids post-cement hydration [13]. However, at higher concentrations, sodium-based bentonite induces uneven particle packing and the formation of more micro-pores within the specimens, thus reducing their density and strength [14]. Moreover, high concentrations of sodium-based bentonite absorb excessive moisture, leading to incomplete or less favorable cement hydration reactions, thereby impacting the formation of hydration products and subsequently affecting the specimens’ strength.
2.2 Analyzing Wet–Dry Cycling
To investigate the impact of wet–dry cycles on the stabilized soil, dry–wet cycle tests were conducted using mixtures C4 and C9 and selected based on unconfined compressive strength experiments. CFM1 denotes the dry–wet cycle specimens for mixture C4, whereas CFM2 represents those for mixture C9.
Based on Figure 8 and Figure7, the mass-loss rate gradually increases with the number of cycles. During the cycles, significant mass loss occurs, and an observation of the samples reveals that the top and bottom layers begin to spall after immersion, leading to considerable height reduction.
Combining Figure 9 with Figure 4, it is evident that the compressive strength of the 28-day samples decreases as the number of cycles increases[15], which is primarily occasioned by the influence of the wet–dry cycles on the hydration products in cement. Compounds such as ettringite and calcium aluminate hydrates, formed during cement hydration, undergo dissolution and precipitation cycles in the wet–dry process, gradually loosening the material’s structure and reducing its strength[16].
The resistance to wet–dry cycles is superior in sample CFM1 (mix C4) compared to CFM2 (mix C9). Although the polyacrylamide gel enhances the density and strength of CFM2 samples, during cement curing, the addition of PAM and sodium bentonite within the mixing range absorbs more moisture[17]. Moreover, sodium bentonite, within the mixing range, creates pores that affect strength formation. Therefore, CFM1 samples demonstrate more optimal performance in wet–dry cycle resistance.
From the wet–dry cycle experiments, it is evident that the cement-stabilized slag exhibits slightly reduced resistance to wet–dry cycles compared to unstabilized slag within the mixing range.
2.3 Analyzing Freeze–Thaw Cycling
Based on the compressive strength tests, CFM1 (representing mix C4) and CFM2 (representing mix C9) were selected for freeze–thaw cycle experiments.
Figure 10 indicates that during the cycling process, CFM2 samples exhibited surface cracking and initial spalling. With increasing freeze–thaw cycles, the spalling increased gradually, eventually leading to substantial fragment detachment, making it difficult to maintain overall integrity. Contrastingly, CFM1 samples also experienced cracks and spalling, however, the detachment of small fragments was minimal, which enabled the overall shape to remain relatively intact. This difference is primarily attributed to the addition of PAM and sodium bentonite solutions in the CFM2 samples, which reduced the overall strength and diminished their resistance to freeze–thaw cycles.
Based on Figure 11, it can be observed that the strength of mixes C4 and C9 decreases with an increase in freeze–thaw cycles at the age of 28 days. Sample C4 exhibits slightly more optimal freeze–thaw performance compared to C9, which is primarily explained as follows: when the moisture in cement freezes, it expands to occupy a region approximating its volume, exerting significant internal pressure within the concrete, thus leading to the formation or expansion of cracks. With repeated freeze–thaw cycles, these cracks multiply and expand, leading to structural damage and strength reduction. Within the specified range of ratios, the addition of PAM and sodium bentonite solutions during cement curing affects the hydration reaction of cement, with sodium bentonite increasing the number of cement structure cracks, leading to lower strength compared to specimens without added PAM.
Figure 12 indicates that as the number of cycles increases, the mass loss gradually increases. This observation is mainly attributable to the freeze–thaw cycles that damage the microstructure of the concrete, thereby altering the structure of hydration products (such as ettringite and calcium hydroxide) in the cement paste, which further reduces its bonding capacity and overall strength. Additionally, freezing and thawing alter the in-concrete chemical environment [18]. For instance, during freezing, moisture from the cement paste is expelled from capillaries and re-enters during thawing, thus disrupting the cement hydration reaction through repeated processes.[19]
From the freeze–thaw cycle experiments, it is evident that the frost resistance performance of cement-stabilized modified muck within the specified ratios is lower compared to the requirements of road-base materials.