*3.5. Zeta Potential after Pretreatment*

The supernatants of all sedimentation experiments were used to test the zeta potential, and the test results are shown in Figure 10. The initial zeta potentials of Type I slurry and Type II slurry were quite different, which were −15.4 mV and −31.6 mV, respectively. Figure 10a,b show the zeta potentials of the slurry pretreated with CPAM and APAM, respectively. After adding CPAM or APAM, the zeta potential of the slurry did not change significantly, which indicated that the four types of polymer flocculants used in the experiment did not basically change the electrical properties of the particles in the slurry. Especially when APAM was added, the zeta potential of the slurry was basically unchanged. Combining with Table 2, this could be because APAM has a relatively low charge density, and the electrical properties of the particles in the slurry basically do not change.

Figure 10c shows the results of the zeta potential of the waste pipe jacking slurry treated with FeCl3·6H2O and PAM. With the addition of FeCl3·6H2O, the zeta potential of the slurry changed significantly. The zeta potential gradually approached 0 from −31.6 mV, and when the addition of FeCl3·6H2O reached 5%, the zeta potential changed to about 10 mV.

**Figure 10.** Zeta potential after pretreatment. (**a**) CPAM. (**b**) APAM. (**c**) FeCl3 + PAM.

#### **4. Discussion**

#### *4.1. Flocculation-Settling Characteristics of Pipe Jacking Waste Slurry*

If the waste slurry from pipe jacking is not pretreated, it can be seen from Figure 2 that the sedimentation effect is very poor, and it is difficult to separate soil and water. When PAM is used as a pretreatment agent, all four flocculants can significantly improve the efficiency of slurry self-weight separation from Figures 3–6. The water content of slurry can be rapidly reduced from 300% to 159–208%. For Type I slurry, the optimal flocculant is APAM 7126, and the optimal addition is 0.06%. For Type II slurry, APAM 7126 is also the optimal flocculant, but the optimal addition is 0.25%. The properties of waste slurry produced in different periods of the same pipe jacking project are different, resulting in great differences in the optimal dosage of flocculant and sedimentation results.

From the test results of zeta potential in Section 3.5, the zeta potential of Type II slurry after PAM pretreatment is about −30 mV, and the corresponding optimal dosage of PAM is high. The zeta potential of Type I slurry after PAM pretreatment is about −15 mV, and the optimal additional amount of PAM is low.

The zeta potential of the slurry changes dramatically when the slurry is pretreated with compound conditioners (Figure 10c), and the optimal additional amount of PAM decreases. It can be inferred that the zeta potential of the waste slurry from pipe jacking is closely related to the sedimentation and flocculation results of the slurry. Therefore, the above experiments pretreated with PAM are analyzed, and the relationship between the water content of sediment and zeta potential after sedimentation is discussed, as shown in Figure 11.

**Figure 11.** Relationship between water content of sediment and zeta potential.

Figure 11 clearly reveals that the water content of sediment is significantly correlated with zeta potential. After flocculant pretreatment, the closer the zeta potential is to 0, the lower the water content of the sediment is. The initial zeta potential of Type II slurry is high. After PAM pretreatment, the zeta potential is still about −30 mV, so the water content of the sediment is high. The initial zeta potential of Type I slurry is relatively low. After PAM pretreatment, the water content of sediment obtained by sedimentation is relatively low. Therefore, it is speculated that Type II slurry contains more bentonite, which increases the zeta potential of the slurry, making it difficult to flocculate and separate. In other studies [9,25], slurries rich in montmorillonite were also found to be less effective at flocculation, explained by lower zeta potential, which confirmed the findings of this paper. According to these test results, it can be inferred that the clay minerals contained in the waste slurry produced by the construction of different formations is different, and it is necessary to further study the influence of clay minerals on flocculation in detail in the future.

It is worth noting that, for Type II slurry, after pretreatment with compound conditioners, the zeta potential of the slurry changes rapidly and the water content of the corresponding sediment decreases rapidly.

Based on the above discussion, the flocculation mechanism of different PAM can be further discussed. All four PAM shown in Figures 3–6 can accelerate the settlement of slurry. However, the addition of PAM has a very small change in the zeta potential of the slurry in Figure 10. Therefore, it can be speculated that electrical neutralization is not the main mechanism of the four PAM. The initial zeta potential of the slurry is negative, but both APAM can make slurry flocculate and settle rapidly, indicating that bridging plays a major role in flocculation. APAM 7126 achieves a better sedimentation effect than APAM 720VJ; the higher molecular weight of 7126 could be the main reason, which confirms that bridging is the main flocculation mechanism. Two CPAM have a certain charge density, and compared with the two APAMs, they have a better performance in adjusting the zeta potential as shown in Figure 10. However, the optimal amount of flocculant added is much higher than that of APAM, and its lower molecular weight may be the main reason. These results confirm that bridging is the main mechanism for PAM to flocculate particles.

The zeta potential of the slurry decreases rapidly after the addition of FeCl3·6H2O, which indicates that the FeCl3·6H2O significantly increases the strength of the ions in the slurry, thereby reducing the electrostatic repulsion between soil particles. After the subsequent addition of PAM, the PAM can be better interacted with soil particles in the slurry, thereby improving the effect of flocculation and separation. Therefore, under the action of compound conditioning, the electric neutralization and bridging mechanism work together, which greatly reduces the amount of PAM added. It can be seen from Figure 10c that when excess FeCl3·6H2O is added, the zeta potential begins to increase again, and the electrostatic repulsion between particles increases, and the effect of flocculation begins to decrease again.

In summary, in the case of slurry with difficulty in separation, compound conditioning can be used for pretreatment to reduce zeta potential and improve separation efficiency.

#### *4.2. New Method for Rapid Dewatering*

Aiming at the problem that a large amount of waste pipe jacking slurry with high water content needs to be treated and utilized, a new method based on the combination of flocculation–sedimentation and solidification is proposed; the schematic diagram is shown in Figure 12.

According to the amount of bentonite added to the slurry during the construction of pipe jacking projects, FeCl3·6H2O is added appropriately to adjust the zeta potential of slurry. Then, the waste slurry is pretreated with APAM (7126), and after a short period of sedimentation (600 s or more), the water content of the sediment would be less than 165%. It is estimated that, after 600 s of sedimentation and separation, the volume of slurry is reduced to 60% of the original, which greatly reduces the occupation of the storage site by the waste slurry. The separated sediment is cured by adding a solidification agent. According to Section 3.4, adding 20–30% SAC can achieve an unconfined compressive strength of more than 30 kPa within 3 days, which meets the requirements of walking and

vehicle transportation. The cured sediment can be transported to other areas for disposal and utilization.

**Figure 12.** Schematic diagram of flocculation-sedimentation and solidification combined method.

The cost of flocculant and solidification agent by this method can be estimated. The cost depends on the nature of the waste slurry. If more bentonite remains in the waste slurry, the dewatering and reduction of the slurry will become difficult, and the addition of flocculant and curing agent will increase, resulting in the increase of the cost. Considering the most unfavorable conditions, such as Type II slurry, FeCl3·6H2O and APAM 7126 are selected as conditioners and SAC as a solidification agent. The price of conditioners and solidification agent per cubic slurry is 1.19 € and 3.02 € respectively, and the total is 4.21 €. It can be seen that the cost of adding SAC is slightly higher, which is mainly because the water content of the sediment after sedimentation is still high, which leads to the need to add more SAC to ensure the strength of solidified sediment. Therefore, if the slurry storage site in the project allows, the sedimentation time could be extended, thereby further reducing the water content of the sediment, which can reduce the amount of SAC added and reduce the construction cost.

It is worth noting that the addition of FeCl3·6H2O results in the presence of some chloride ions in the solidified sediment, and chloride ion erosion can lead to the deterioration of cement-based materials and reduce the strength of the soil [26]. The effect of chloride ions on solidified sediments, as well as finding other agents to replace ferric chloride, will be the subject of future research.
